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6-Channel, Low Noise, Low Power, 24-Bit - ADC with On-Chip In-Amp and Reference
AD7794
FEATURES
Up to 22.5 effective bits RMS noise: 40 nV @ 4.17 Hz 85 nV @ 16.7 Hz Current: 400 A typ Power-down: 1 A max Low noise programmable gain instrumentation-amp Band gap reference with 4 ppm/C drift typ Update rate: 4.17 Hz to 500 Hz Six differential analog inputs Internal clock oscillator Simultaneous 50 Hz/60 Hz rejection Reference detect Programmable current sources On-chip bias voltage generator Burnout currents Low-side power switch Power supply: 2.7 V to 5.25 V -40C to +105C temperature range Independent interface power supply 24-lead TSSOP package Strain gauge transducers Gas analysis Industrial process control Instrumentation Blood analysis Smart transmitters Liquid/gas chromotography 6-digit DVM
GENERAL DESCRIPTION
The AD7794 is a low power, low noise, complete analog front end for high precision measurement applications. It contains a low noise, 24-bit - ADC with six differential inputs. The on-chip low noise instrumentation amplifier means that signals of small amplitude can be interfaced directly to the ADC. The device contains a precision low noise, low drift internal band gap reference and can also accept up to two external differential references. Other on-chip features include programmable excitation current sources, burnout currents and a bias voltage generator, this feature being used to set the common mode voltage of a channel to AVDD/2. The low-side power switch can be used to power down bridge sensors between conversions, minimizing the system's power consumption. The device can be operated with the internal clock or, alternatively, an external clock can be used. The output data rate from the part can be varied from 4.17 Hz to 500 Hz. The part operates with a power supply from 2.7 V to 5.25 V. It consumes a current of 400 A typical and is housed in a 24-lead TSSOP package.
INTERFACE
3-wire serial SPI(R)-, QSPITM-, MICROWIRETM-, and DSP-compatible Schmitt trigger on SCLK
APPLICATIONS
Temperature measurement Pressure measurement Weigh scales
GND AVDD
FUNCTIONAL BLOCK DIAGRAM
AIN4(+)/REFIN2(+) REFIN1(+) AIN4(-)/REFIN2(-) REFIN1(-)
VBIAS VDD AIN1(+) AIN1(-) AIN2(+) AIN2(-) AIN3(+) AIN3(-) AIN5(+)/IOUT2 AIN5(-)/IOUT1 AIN6(+)/P1 AIN6(+)/P2 PSW
BAND GAP REFERENCE GND
REFERENCE DETECT
BUF MUX GND
IN-AMP
- ADC
SERIAL INTERFACE AND LOGIC CONTROL
DOUT/RDY DIN SCLK CS
TEMP SENSOR VDD
INTERNAL CLOCK
DVDD
AD7794
04854-001
GND CLK
Figure 1. Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
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AD7794
TABLE OF CONTENTS Specifications..................................................................................... 3 Timing Characteristics..................................................................... 7 Absolute Maximum Ratings............................................................ 9 ESD Caution.................................................................................. 9 Pin Configuration and Function Descriptions........................... 10 Output Noise and Resolution Specifications .............................. 12 Chopping Enabled...................................................................... 12 External Reference ................................................................. 12 Internal Reference .................................................................. 13 Chopping Disabled..................................................................... 14 Typical Performance Characteristics ........................................... 15 On-Chip Registers .......................................................................... 16 Communications Register......................................................... 16 Status Register ............................................................................. 17 Mode Register ............................................................................. 17 Configuration Register .............................................................. 19 Data Register ............................................................................... 21 ID Register................................................................................... 21 IO Register................................................................................... 21 Offset Register............................................................................. 22 Full-Scale Register ...................................................................... 22 ADC Circuit Information.............................................................. 23 Overview...................................................................................... 23 Digital Interface .......................................................................... 25 Single Conversion Mode ....................................................... 26 Continuous Conversion Mode............................................. 26 Continuous Read........................................................................ 27 Circuit Description......................................................................... 28 Analog Input Channel ............................................................... 28 Instrumentation Amplifier........................................................ 28 Bipolar/Unipolar Configuration .............................................. 28 Data Output Coding .................................................................. 28 Burnout Currents ....................................................................... 29 Excitation Currents .................................................................... 29 Bias Voltage Generator .............................................................. 29 Reference ..................................................................................... 29 Reference Detect......................................................................... 30 Reset ............................................................................................. 30 AVDD Monitor ............................................................................. 30 Calibration................................................................................... 30 Grounding and Layout .............................................................. 31 Applications..................................................................................... 32 Flowmeter.................................................................................... 32 Outline Dimensions ....................................................................... 33 Ordering Guide .......................................................................... 33
REVISION HISTORY
10/04--Revision 0: Initial Version
Rev. 0 | Page 2 of 36
AD7794 SPECIFICATIONS
AVDD = 2.7 V to 5.25 V; DVDD = 2.7 V to 5.25 V; GND = 0 V; all specifications TMIN to TMAX, unless otherwise noted. Table 1.
Parameter1 AD7794 (CHOP ENABLED) Output Update Rate No Missing Codes2 Resolution Output Noise and Update Rates Integral Nonlinearity Offset Error3 Offset Error Drift vs. Temperature4 Full-Scale Error3, 5 Gain Drift vs. Temperature4 Power Supply Rejection ANALOG INPUTS Differential Input Voltage Ranges Absolute AIN Voltage Limits2 Unbuffered Mode Buffered Mode In-Amp Active Common-Mode Voltage, VCM Analog Input Current Buffered Mode or In-Amp Active Average Input Current2 AD7794B 4.17 - 500 24 See Tables in ADC Description See Tables in ADC Description 15 1 10 10 1 3 100 VREF/Gain Unit Hz nom Bits min Test Conditions/Comments Settling Time = 2/Output Update Rate fADC 250 Hz
ppm of FSR max V typ nV/C typ V typ ppm/C typ ppm/C typ dB min V nom
Gain = 1 to 16, External Reference Gain = 32 to 128, External Reference AIN = 1 V/Gain, Gain 4, External Reference VREF = REFIN(+) - REFIN(-) or Internal Reference, Gain = 1 to 128 Gain = 1 or 2 Gain = 1 or 2 Gain = 4 to 128 VCM = (AIN(+) + AIN(-))/2, Gain = 4 to 128
GND - 30 mV AVDD + 30 mV GND + 100 mV AVDD - 100 mV GND + 300 mV AVDD - 1.1 0.5
V min V max V min V max V min V max V min
Average Input Current Drift Unbuffered Mode Average Input Current Average Input Current Drift Normal Mode Rejection2 Internal Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz External Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz, 60 Hz2 @ 50 Hz, 60 Hz2
1 250 1 2 400 50
nA max pA max nA max pA/C typ nA/V typ pA/V/C typ
Gain = 1 or 2, Update Rate < 100 Hz Gain = 4 to 128, Update Rate < 100 Hz AIN6(+)/AIN6(-) Gain = 1 or 2 Input current varies with input voltage
65 80 90 80 94 90 100 100 100
dB min dB min dB min dB min dB min dB min dB min dB min dB min
80 dB typ, 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 90 dB typ, 50 1 Hz, FS[3:0] = 10016 100 dB typ, 60 1 Hz, FS[3:0] = 10006 90 dB typ, 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 100 dB typ, 50 1 Hz, FS[3:0] = 10016 100 dB typ, 60 1 Hz, FS[3:0] = 10006 AIN = 1 V/Gain, Gain 4 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 50 1 Hz (FS[3:0] = 10016), 60 1 Hz (FS[3:0] = 10006)
Rev. 0 | Page 3 of 36
AD7794
Parameter1 AD7794 (CHOP DISABLED) Output Update Rate No Missing Codes2 Resolution Output Noise and Update Rates Integral Nonlinearity Offset Error3 Offset Error Drift vs. Temperature4 Full-Scale Error3, 5 Gain Drift vs. Temperature4 Power Supply Rejection ANALOG INPUTS Differential Input Voltage Ranges Absolute AIN Voltage Limits2 Unbuffered Mode Buffered Mode In-Amp Active Common-Mode Voltage, VCM AD7794B 4.17 - 500 24 See Tables in ADC Description See Tables in ADC Description 15 100/Gain 100/Gain 10 10 1 3 100 VREF /Gain Unit Hz nom Bits min Test Conditions/Comments Settling Time = 1/Output Update Rate fADC 125 Hz
ppm of FSR max V typ nV/C typ nV/C typ V typ ppm/C typ ppm/C typ dB typ V nom
Without Calibration Gain = 1 to 16 Gain = 32 to 128 Gain = 1 to 16, External Reference Gain = 32 to 128, External Reference AIN = 1 V/Gain, Gain 4, External Reference VREF = REFIN(+)- REFIN(-) or Internal Reference, Gain = 1 to 128 Gain = 1 or 2 Gain = 1 or 2 Gain = 4 to 128 AMP-CM = 1, VCM = (AIN(+) + AIN(-))/2, Gain = 4 to 128
GND - 30 mV AVDD + 30 mV GND + 100 mV AVDD - 100 mV GND + 300 mV AVDD - 1.1 0.2 + (Gain/2 x (AIN(+) - AIN(-))) AVDD - 0.2 - (Gain/2 x (AIN(+) - AIN(-)))
V min V max V min V max V min V max V min V max
Analog Input Current Buffered Mode or In-Amp Active Average Input Current2
Average Input Current Drift Unbuffered Mode Average Input Current Average Input Current Drift Normal Mode Rejection2 Internal Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz External Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz, 60 Hz2 @ 50 Hz, 60 Hz2
1 250 1 2 400 50
nA max pA max nA max pA/C typ nA/V typ pA/V/C typ
Gain = 1 or 2 Gain = 4 to 128 AIN6(+)/AIN6(-) Gain = 1 or 2 Input current varies with input voltage.
60 78 86 60 94 90 100 100 100
dB min dB min dB min dB min dB min dB min dB min dB min dB min
70 dB typ, 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 90 dB typ, 50 1 Hz, FS[3:0] = 10016 100 dB typ, 60 1 Hz, FS[3:0] = 10006 70 dB typ, 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 100 dB typ, 50 1 Hz, FS[3:0] = 10016 100 dB typ, 60 1 Hz, FS[3:0] = 10006 AIN = 1 V/Gain with Gain = 4, AMP-CM Bit = 1 50 1 Hz, 60 1 Hz, FS[3:0] = 10106 50 1 Hz (FS[3:0] = 10016), 60 1 Hz (FS[3:0] = 10006)
Rev. 0 | Page 4 of 36
AD7794
Parameter1 AD7794 (CHOP ENABLED or DISABLED) REFERENCE INPUT Internal Reference Internal Reference Initial Accuracy Internal Reference Drift2 Power Supply Rejection External Reference External REFIN Voltage Reference Voltage Range2 AD7794B Unit Test Conditions/Comments
1.17 0.01% 4 15 85 2.5 0.1 AVDD GND - 30 mV AVDD + 30 mV 400 0.03 Same as for Analog Inputs 100 0.3 0.65
V min/max ppm/C typ ppm/C max dB typ V nom V min V max V min V max nA/V typ nA/V/C typ
AVDD = 4 V, TA = 25C
REFIN = REFIN(+) - REFIN(-) When VREF = AVDD , the differential input must be limited to 0.9xVREF/Gain if the In-Amp is active
Absolute REFIN Voltage Limits2 Average Reference Input Current Average Reference Input Current Drift Normal Mode Rejection2 Common-Mode Rejection Reference Detect Levels EXCITATION CURRENT SOURCES (IEXC1 and IEXC2) Output Current Initial Tolerance at 25C Drift Current Matching Drift Matching Line Regulation (AVDD) Load Regulation Output Compliance
dB typ
V min V max NOXREF Bit Active if VREF < 0.3 V
10/210/1000 5 200 0.5 50 2 0.2 AVDD - 0.65 AVDD - 1.1 GND - 30 mV AVDD/2 See Figure 11
A nom % typ ppm/C typ % typ ppm/C typ %/V typ %/V typ V max V max V min V nom ms/nF typ C typ mV/C typ max max mA max V min V max V min V max
Matching between IEXC1 and EXC2. VOUT = 0 V AVDD = 5 V 5% Current Sources Programmed to 10 A or 210 A Current Sources Programmed to 1 mA
BIAS VOLTAGE GENERATOR VBIAS VBIAS Generator Start-Up Time TEMPERATURE SENSOR Accuracy Sensitivity LOW SIDE POWER SWITCH RON Allowable Current2 DIGITAL OUTPUTS (P1 and P2) VOH, Output High Voltage2 VOL, Output Low Voltage2 VOH, Output High Voltage2 VOL, Output Low Voltage2 INTERNAL/EXTERNAL CLOCK Internal Clock Frequency2 Duty Cycle
Dependent on the Capacitance connected to AIN Applies if User Calibrates the Temp Sensor
2 0.81
7 9 30 AVDD - 0.6 0.4 4 0.4
AVDD = 5 V AVDD = 3 V Continuous Current AVDD = 3 V, ISOURCE = 100 A AVDD = 3 V, ISINK = 100 A AVDD = 5 V, ISOURCE = 200 A AVDD = 5 V, ISINK = 800 A
64 3% 50:50
kHz min/max % typ
Rev. 0 | Page 5 of 36
AD7794
Parameter1 External Clock Frequency Duty Cycle LOGIC INPUTS CS2 VINL, Input Low Voltage VINH, Input High Voltage SCLK, CLK and DIN (Schmitt-Triggered Input)2 VT(+) VT(-) VT(+) - VT(-) VT(+) VT(-) VT(+)- VT(-) Input Currents Input Capacitance LOGIC OUTPUTS (Including CLK) VOH, Output High Voltage2 VOL, Output Low Voltage2 VOH, Output High Voltage2 VOL, Output Low Voltage2 Floating-State Leakage Current Floating-State Output Capacitance Data Output Coding SYSTEM CALIBRATION2 Full-Scale Calibration Limit Zero-Scale Calibration Limit Input Span POWER REQUIREMENTS7 AD7794B 64 45:55 to 55:45 Unit kHz nom % typ Test Conditions/Comments A 128 kHz external clock can be used if the divide by 2 function is used (Bit CLK1 = CLK0 = 1) Applies for external 64 kHz clock. A 128 kHz clock can
have a less stringent duty cycle
0.8 0.4 2.0
V max V max V min
DVDD = 5 V DVDD = 3 V DVDD = 3 V or 5 V
1.4/2 0.8/1.7 0.1/0.17 0.9/2 0.4/1.35 0.06/0.13 10 10 DVDD - 0.6 0.4 4 0.4 10 10 Offset Binary 1.05 x FS -1.05 x FS 0.8 x FS 2.1 x FS
V min/V max V min/V max V min/V max V min/V max V min/V max V min/V max A max pF typ V min V max V min V max A max pF typ
DVDD = 5 V DVDD = 5 V DVDD = 5 V DVDD = 3 V DVDD = 3 V DVDD = 3 V VIN = DVDD or GND All Digital Inputs DVDD = 3 V, ISOURCE = 100 A DVDD = 3 V, ISINK = 100 A DVDD = 5 V, ISOURCE = 200 A DVDD = 5 V, ISINK = 1.6 mA (DOUT/RDY)/800 A (CLK)
V max V min V min V max
Power Supply Voltage AVDD - GND DVDD - GND Power Supply Currents IDD Current
2.7/5.25 2.7/5.25 140 185 400 500
V min/max V min/max A max A max A max A max A max 110 A typ @ AVDD = 3 V, 125 A typ @ AVDD = 5 V, Unbuffered Mode, Ext. Reference 130 A typ @ AVDD = 3 V, 165 A typ @ AVDD = 5 V, Buffered Mode, Gain = 1 or 2, Ext Ref 300 A typ @ AVDD = 3 V, 350 A typ @ AVDD = 5 V, Gain = 4 to 128, Ext. Ref 400 A typ @ AVDD = 3 V, 450 A typ @ AVDD = 5 V, Gain = 4 to 128, Int Ref
IDD (Power-Down Mode)
1
1 2
Temperature Range: -40C to +105C. Specification is not production tested but is supported by characterization data at initial product release. 3 Following a calibration, this error will be in the order of the noise for the programmed gain and update rate selected. 4 Recalibration at any temperature will remove these errors. 5 Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (AVDD = 4 V, gain = 1, TA = 25C ). 6 FS[3:0] are the four bits used in the mode register to select the output word rate. 7 Digital inputs equal to DVDD or GND with excitation currents and bias voltage generator disabled.
Rev. 0 | Page 6 of 36
AD7794 TIMING CHARACTERISTICS
AVDD = 2.7 V to 5.25 V; DVDD = 2.7 V to 5.25 V; GND = 0 V, Input Logic 0 = 0 V, Input Logic 1 = DVDD, unless otherwise noted. Table 2.
Parameter1, 2 t3 t4 Read Operation t1 Limit at TMIN, TMAX (B Version) 100 100 0 60 80 0 60 80 10 80 0 10 0 30 25 0 Unit ns min ns min ns min ns max ns max ns min ns max ns max ns min ns max ns min ns min ns min ns min ns min ns min Conditions/Comments SCLK High Pulse Width SCLK Low Pulse Width CS Falling Edge to DOUT/RDY Active Time DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V SCLK Active Edge to Data Valid Delay4 DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V Bus Relinquish Time after CS Inactive Edge SCLK Inactive Edge to CS Inactive Edge SCLK Inactive Edge to DOUT/RDY High CS Falling Edge to SCLK Active Edge Setup Time4 Data Valid to SCLK Edge Setup Time Data Valid to SCLK Edge Hold Time CS Rising Edge to SCLK Edge Hold Time
t23
t55, 6 t6 t7 Write Operation t8 t9 t10 t11
1 2 3
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.6 V. See Figure 3 and Figure 4. These numbers are measured with the load circuit of Figure 2 and defined as the time required for the output to cross the VOL or VOH limits. 4 SCLK active edge is falling edge of SCLK. 5 These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus relinquish times of the part and, as such, are independent of external bus loading capacitances. 6 RDY returns high after a read of the ADC. In single conversion mode and continuous conversion mode, the same data can be read again, if required, while RDY is high, although care should be taken to ensure that subsequent reads do not occur close to the next output update. In continuous read mode, the digital word can be read only once.
ISINK (1.6mA WITH DVDD = 5V, 100A WITH DVDD = 3V)
TO OUTPUT PIN 50pF
1.6V
ISOURCE (200A WITH DVDD = 5V, 100A WITH DVDD = 3V)
Figure 2. Load Circuit for Timing Characterization
Rev. 0 | Page 7 of 36
04854-002
AD7794
CS (I)
t1
DOUT/RDY (O) MSB LSB
t6 t5
t2 t3
SCLK (I)
t7
t4
I = INPUT, O = OUTPUT
Figure 3. Read Cycle Timing Diagram
CS (I)
t8
SCLK (I)
t11
t9 t10
DIN (I) MSB LSB
04854-004
I = INPUT, O = OUTPUT
Figure 4. Write Cycle Timing Diagram
Rev. 0 | Page 8 of 36
04854-003
AD7794 ABSOLUTE MAXIMUM RATINGS
TA= 25C, unless otherwise noted. Table 3.
Parameter AVDD to GND DVDD to GND Analog Input Voltage to GND Reference Input Voltage to GND Digital Input Voltage to GND Digital Output Voltage to GND AIN/Digital Input Current Operating Temperature Range Storage Temperature Range Maximum Junction Temperature TSSOP JA Thermal Impedance JC Thermal Impedance Lead Temperature, Soldering Vapor Phase (60 sec) Infrared (15 sec) Rating -0.3 V to +7 V -0.3 V to +7 V -0.3 V to AVDD + 0.3 V -0.3 V to AVDD + 0.3 V -0.3 V to DVDD + 0.3 V -0.3 V to DVDD + 0.3 V 10 mA -40C to +85C -65C to +85C 150C
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
97.9C/W 14C/W 215C 220C
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. 0 | Page 9 of 36
AD7794 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
SCLK 1 CLK 2 CS 3 NC 4 AIN6(+)/P1 5 AIN6(-)/P2 6
24 23 22 21
DIN DOUT/RDY DVDD AVDD GND
19 PSW TOP VIEW AIN1(+) 7 (Not to Scale) 18 AIN4(-)/REFIN2(-) 17 AIN4(+)/REFIN2(+) AIN1(-) 8
AD7794
20
AIN2(+) 9 AIN2(-) 10 AIN3(+) 11 AIN3(-) 12
16 15 14 13
AIN5(-)/IOUT1 AIN5(+)/IOUT2 REFIN1(-)
04854-005
REFIN1(+)
NC = NO CONNECT
Figure 5. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. 1 Mnemonic SCLK Description Serial Clock Input for Data Transfers to and from the ADC. The SCLK has a Schmitt-triggered input, making the interface suitable for opto-isolated applications. The serial clock can be continuous with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to or from the ADC in smaller batches of data. Clock In/Clock Out. The internal clock can be made available at this pin. Alternatively, the internal clock can be disabled and the ADC can be driven by an external clock. This allows several ADCs to be driven from a common clock, allowing simultaneous conversions to be performed. Chip Select Input. This is an active low logic input used to select the ADC. CS can be used to select the ADC in systems with more than one device on the serial bus or as a frame synchronization signal in communicating with the device. CS can be hardwired low, allowing the ADC to operate in 3-wire mode with SCLK, DIN, and DOUT used to interface with the device. No Connect. Analog Input/Digital Output Pin. AIN6(+) is the positive terminal of the differential analog input pair AIN6(+)/AIN6(-). Alternatively, this pin can function as a general purpose output bit referenced between AVDD and GND. Analog Input/ Digital Output Pin. AIN6(-) is the negative terminal of the differential analog input pair AIN6(+)/AIN6(-). Alternatively, this pin can function as a general purpose output bit referenced between AVDD and GND. Analog Input. AIN1(+) is the positive terminal of the differential analog input pair AIN1(+)/AIN1(-). Analog Input. AIN1(-) is the negative terminal of the differential analog input pair AIN1(+)/AIN1(-). Analog Input. AIN2(+) is the positive terminal of the differential analog input pair AIN2(+)/AIN2(-). Analog Input. AIN2(-) is the negative terminal of the differential analog input pair AIN2(+)/AIN2(-). Analog Input. AIN3(+) is the positive terminal of the differential analog input pair AIN3(+)/AIN3(-). Analog Input. AIN3(-) is the negative terminal of the differential analog input pair AIN3(+)/AIN3(-). Positive Reference Input. An external reference can be applied between REFIN1(+) and REFIN1(-). REFIN1(+) can lie anywhere between AVDD and GND + 0.1 V. The nominal reference voltage (REFIN1(+)- REFIN1(-)) is 2.5 V, but the part functions with a reference from 0.1 V to AVDD. Negative Reference Input. This reference input can lie anywhere between GND and AVDD - 0.1 V. Analog Input/Output of Internal Excitation Current Source. AIN5(+) is the positive terminal of the differential analog input pair AIN5(+)/AIN5(-). Alternatively, the internal excitation current source can be made available at this pin. The excitation current source is programmable so that the current can be 10 A, 210 A or 1 mA. Either IEXC1 or IEXC2 can be switched to this output Analog Input/Output of Internal Excitation Current Source. AIN5(-) is the negative terminal of the differential analog input pair AIN5(+)/AIN5(-). Alternatively, the internal excitation current source can be made available at this pin. The excitation current source is programmable so that the current can be 10 A, 210 A or 1 mA. Either IEXC1 or IEXC2 can be switched to this output. Analog Input/Positive Reference Input. AIN4(+) is the positive terminal of the differential analog input pair AIN4(+)/AIN4(-). This pin can also function as a reference input. REFIN2(+) can lie anywhere between AVDD and GND + 0.1 V. The nominal reference voltage (REFIN2(+)- REFIN2(-)) is 2.5 V, but the part functions with a reference from 0.1 V to AVDD.
Rev. 0 | Page 10 of 36
2
CLK
3
CS
4 5 6 7 8 9 10 11 12 13
NC AIN6(+)/P1 AIN6(-)/P2 AIN1(+) AIN1(-) AIN2(+) AIN2(-) AIN3(+) AIN3(-) REFIN1(+)
14 15
REFIN1(-) AIN5(+)/IOUT2
16
AIN5(-)/IOUT1
17
AIN4(+)/REFIN2(+)
AD7794
Pin No. 18 Mnemonic AIN4(-)/REFIN2(-) Description Analog Input/Negative Reference Input. AIN4(-) is the negative terminal of the differential analog input pair AIN4(+)/AIN4(-). This pin also functions as the negative reference input for REFIN2. This reference input can lie anywhere between GND and AVDD - 0.1 V. Low-Side Power Switch to GND. Ground Reference Point. Supply Voltage, 2.7 V to 5.25 V. Serial Interface Supply Voltage, 2.7 V to 5.25 V. DVDD is independent of AVDD. Therefore, the serial interface can be operated at 3 V with AVDD at 5 V or vice versa. Serial Data Output/Data Ready Output. DOUT/RDYserves a dual purpose. It functions as a serial data output pin to access the output shift register of the ADC. The output shift register can contain data from any of the on-chip data or control registers. In addition, DOUT/RDYoperates as a data ready pin, going low to indicate the completion of a conversion. If the data is not read after the conversion, the pin will go high before the next update occurs. The DOUT/RDY falling edge can be used as an interrupt to a processor, indicating that valid data is available. With an external serial clock, the data can be read using the DOUT/RDY pin. With CS low, the data/control word information is placed on the DOUT/RDY pin on the SCLK falling edge and is valid on the SCLK rising edge. Serial Data Input to the Input Shift Register on the ADC. Data in this shift register is transferred to the control registers within the ADC, the register selection bits of the communications register identifying the appropriate register.
19 20 21 22 23
PSW GND AVDD DVDD DOUT/RDY
24
DIN
Rev. 0 | Page 11 of 36
AD7794 OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
The AD7794 can be operated with chopping enabled or chopping disabled, allowing the ADC to be optimized for switching time or optimized for drift performance. With chopping enabled, the settling time is two times the conversion time. However, the offset is continuously removed by the ADC leading to low offset and low offset drift. With chopping disabled, the allowable update rates are the same as in chop enable mode. However, the settling time now equals the conversion time. With chopping disabled, the offset is not removed by the ADC so periodic offset calibrations may be required to remove offset due to drift.
CHOPPING ENABLED
External Reference
Table 5 shows the AD7794's output rms noise for some of the update rates and gain settings. The numbers given are for the bipolar input range with an external 2.5 V reference. These numbers are typical and are generated with a differential input voltage of 0 V. Table 6 shows the effective resolution while the output peak-to-peak (p-p) resolution is listed in brackets. It is important to note that the effective resolution is calculated using the rms noise while the p-p resolution is calculated based on peak-to-peak noise. The p-p resolution represents the resolution for which there will be no code flicker. These numbers are typical and are rounded to the nearest LSB.
Table 5. Output RMS Noise (V) vs. Gain and Output Update Rate Using an External 2.5 V Reference with Chop Enabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 0.64 1.04 1.55 2.3 2.95 4.89 11.76 11.33 Gain of 2 0.6 0.96 1.45 2.13 2.85 4.74 9.5 9.44 Gain of 4 0.29 0.38 0.54 0.74 0.92 1.49 4.02 3.07 Gain of 8 0.22 0.26 0.36 0.5 0.58 1 1.96 1.79 Gain of 16 0.1 0.13 0.18 0.23 0.29 0.48 0.88 0.99 Gain of 32 0.065 0.078 0.11 0.17 0.2 0.32 0.45 0.63 Gain of 64 0.039 0.057 0.087 0.124 0.153 0.265 0.379 0.568 Gain of 128 0.041 0.055 0.086 0.118 0.144 0.283 0.397 0.593
Table 6. Typical Resolution (Bits) vs. Gain and Output Update Rate Using an External 2.5 V Reference with Chop Enabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 22.5 (20) 21.5 (19) 21 (18.5) 20.5 (18) 20 (17.5) 19.5 (17) 18 (15.5) 18 (15.5) Gain of 2 21.5 (19) 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 17.5 (15) 17.5 (15) Gain of 4 21.5 (19) 21 (18.5) 20.5 (18) 20 (17.5) 20 (17.5) 19 (16.5) 17.5 (15) 18 (15.5) Gain of 8 21 (18.5) 20.5 (18) 20 (17.5) 19.5 (17) 19.5 (17) 18.5 (16) 17.5 (15) 18 (15.5) Gain of 16 21 (18.5)) 20.5 (18) 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18 (15.5) 17.5 (15) Gain of 32 20.5 (18) 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) Gain of 64 20.5 (18) 20 (17.5) 19.5 (17) 18.5 (16) 18.5 (16) 17.5 (15) 17 (14.5) 16.5 (14) Gain of 128 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) 16.5 (14) 16 (13.5) 15.5 (13)
Rev. 0 | Page 12 of 36
AD7794
Internal Reference
Table 7 shows the AD7794's output rms noise for some of the update rates and gain settings. The numbers given are for the bipolar input range with the internal 1.17 V reference. These numbers are typical and are generated with a differential input voltage of 0V. Table 8 shows the effective resolution while the output peak-to-peak (p-p) resolution is listed in brackets. It is important to note that the effective resolution is calculated using the rms noise while the p-p resolution is calculated based on peak-to-peak noise. The p-p resolution represents the resolution for which there will be no code flicker. These numbers are typical and are rounded to the nearest LSB.
Table 7. Output RMS Noise (V) vs. Gain and Output Update Rate (Internal Reference) with Chop Enabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 0.81 1.18 1.96 2.99 3.6 5.83 11.22 12.46 Gain of 2 0.67 1.11 1.72 2.48 3.25 5.01 8.64 10.58 Gain of 4 0.32 0.41 0.55 0.83 1.03 1.69 2.69 4.58 Gain of 8 0.2 0.25 0.36 0.48 0.65 0.96 1.9 2 Gain of 16 0.13 0.16 0.25 0.33 0.46 0.67 1.04 1.27 Gain of 32 0.065 0.078 0.11 0.17 0.2 0.32 0.45 0.63 Gain of 64 0.04 0.058 0.088 0.13 0.15 0.25 0.35 0.50 Gain of 128 0.039 0.059 0.088 0.12 0.15 0.26 0.34 0.49
Table 8. Typical Resolution (Bits) vs. Gain and Output Update Rate (Internal Reference) with Chop Enabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 21 (18.5) 20.5 (18) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17 (14.5) 17 (14.5) Gain of 2 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) 16.5 (14) 16.5 (14) Gain of 4 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17 (14.5) 16.5 (14) Gain of 8 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18.5 (16) 17.5 (15) 16.5 (14) 16.5 (14) Gain of 16 19.5 (17) 19.5 (17) 18.5 (16) 18 (15.5) 18 (15.5) 17 (14.5) 16.5 (14) 16.5 (14) Gain of 32 19.5 (17) 19.5 (17) 19 (16.5) 18 (15.5) 18 (15.5) 17.5 (15) 17 (14.5) 16.5 (14) Gain of 64 19.5 (17) 18.5 (16) 18 (15.5) 17.5 (15) 17.5 (15) 16.5 (14) 16 (13.5) 15.5 (13) Gain of 128 18.5 (16) 17.5 (15) 17 (14.5) 16.5 (14) 16.5 (14) 15.5 (13) 15 (12.5) 14.5 (12)
Rev. 0 | Page 13 of 36
AD7794
CHOPPING DISABLED
With chopping disabled, the switching time or settling time is reduced by a factor of 2. However, periodic offset calibrations may now be required to remove offset and offset drift. When chopping is disabled, the AMP-CM bit in the mode register should be set to 1. This limits the allowable common-mode voltage that can be used. However, the common-mode rejection will degrade if the bit is not set. Table 9 shows the AD7794's output rms noise for some of the update rates and gain settings with chopping disabled. The numbers given are for the bipolar input range with the internal 1.17 V reference. These numbers are typical and are generated with a differential input voltage of 0 V. Table 10 shows the effective resolution while the output peak-to-peak (p-p) resolution is listed in brackets. It is important to note that the effective resolution is calculated using the rms noise, while the p-p resolution is calculated based on peak-to-peak noise. The p-p resolution represents the resolution for which there will be no code flicker. These numbers are typical and are rounded to the nearest LSB.
Table 9. Output RMS Noise (V) vs. Gain and Output Update Rate Using the Internal Reference with Chop Disabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 1.22 1.74 2.64 4.55 5.03 8.13 15.12 17.18 Gain of 2 0.98 1.53 2.44 3.52 4.45 7.24 13.18 14.63 Gain of 4 0.33 0.49 0.79 1.11 1.47 2.27 3.77 8.86 Gain of 8 0.18 0.29 0.48 0.66 0.81 1.33 2.09 2.96 Gain of 16 0.13 0.21 0.33 0.46 0.58 0.96 1.45 1.92 Gain of 32 0.062 0.1 0.16 0.21 0.27 0.48 0.64 0.89 Gain of 64 0.053 0.079 0.13 0.17 0.2 0.36 0.5 0.69 Gain of 128 0.051 0.07 0.12 0.16 0.22 0.37 0.47 0.7
Table 10. Typical Resolution (Bits) vs. Gain and Output Update Rate Using the Internal Reference with Chop Disabled
Update Rate 4.17 Hz 8.33 Hz 16.7 Hz 33.3 Hz 62.5 Hz 125 Hz 250 Hz 500 Hz Gain of 1 20.5 (18) 20 (17.5) 19 (16.5) 18.5 (16) 18.5 (16) 17.5 (15) 16.5 (14) 16.5 (14) Gain of 2 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) 17 (14.5) 16 (13.5) 16 (13.5) Gain of 4 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) 16.5 (14) 15.5 (13) Gain of 8 20 (17.5) 19.5 (17) 18.5 (16) 18 (15.5) 18 (15.5) 17 (14.5) 16.5 (14) 16 (13.5) Gain of 16 19.5 (17) 19 (16.5) 18 (15.5) 18 (15.5) 17.5 (15) 16.5 (14) 16 (13.5) 15.5 (13) Gain of 32 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17.5 (15) 16.5 (14) 16.5 (14) 15.5 (13) Gain of 64 19 (16.5) 18.5 (16) 17.5 (15) 17.5 (15) 17 (14.5) 16 (13.5) 15.5 (13) 15 (12.5) Gain of 128 18 (15.5) 17.5 (15) 16.5 (14) 16.5 (14) 16 (13.5) 15 (12.5) 14.5 (12) 14 (11.5)
Rev. 0 | Page 14 of 36
AD7794 TYPICAL PERFORMANCE CHARACTERISTICS
8388800 8388750
14 12
8388700
OCCURANCE
CODE READ
10 8 6 4
8388650 8388600 8388550 8388500
8388450 0 200 400 600 800 READING NUMBER
1000
8388068 8388100
0
8388150
8388200
8388250
8388300
8388350
8388396
CODE
Figure 6. Typical Noise Plot (Internal Reference, Gain = 64, Update Rate = 16.7 Hz, Chop Enabled)
16 14
Figure 9. Noise Distribution Histogram (Internal Reference, Gain = 64, Update Rate = 16.7 Hz, Chop Disabled, AMP-CM = 1)
20
12
OCCURANCE
10 8 6 4
04854-007
(%)
10
2 0
0 -2.0 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2.0 MATCHING (%)
8388482
8388520
8388560
8388600
8388640
8388680
8388720
8388750
Figure 7. Noise Distribution Histogram (Internal Reference, Gain = 64, Update Rate = 16.7 Hz, Chop Enabled)
8388450 8388400 8388350
Figure 10. Excitation Current Matching (210 A) at Ambient Temperature
90 80 70
POWER-UP TIME (ms)
CODE READ
8388300 8388250 8388200 8388150
04854-008
60 50 40 30 20 10 0 0 200 400 600 800 LOAD CAPACITANCE (nF)
04854-011
8388100 8388050 0 200 400 600 800 READING NUMBER
1000
1000
Figure 8. Typical Noise Plot when Gain = 64 and Internal Reference Selected (Chop Disabled, AMP-CM = 1)
Figure 11. Bias Voltage Generator Power Up Time vs. Load Capacitance
Rev. 0 | Page 15 of 36
04854-010
04854-009
04854-006
2
AD7794 ON-CHIP REGISTERS
The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise stated.
COMMUNICATIONS REGISTER
(RS2, RS1, RS0 = 0, 0, 0)
The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the communications register. The data written to the communications register determines whether the next operation is a read or write operation, and to which register this operation takes place. For read or write operations, once the subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write operation to the communications register. This is the default state of the interface and, on power-up or after a reset, the ADC is in this default state waiting for a write operation to the communications register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns the ADC to this default state by resetting the entire part. Table 11 outlines the bit designations for the communications register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
CR7 WEN(0)
CR6 R/W(0)
CR5 RS2(0)
CR4 RS1(0)
CR3 RS0(0)
CR2 CREAD(0)
CR1 0(0)
CR0 0(0)
Table 11. Communications Register Bit Designations
Bit Location CR7 Bit Name WEN Description Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually occurs. If a 1 is the first bit written, the part will not clock on to subsequent bits in the register. It will stay at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits will be loaded to the communications register. A 0 in this bit location indicates that the next operation will be a write to a specified register. A 1 in this position indicates that the next operation will be a read from the designated register. Register Address Bits. These address bits are used to select which of the ADC's registers are being selected during this serial interface communication. See Table 12. Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the serial interface is configured so that the data register can be continuously read, i.e., the contents of the data register are placed on the DOUT pin automatically when the SCLK pulses are applied after the RDY pin goes low to indicate that a conversion is complete. The communications register does not have to be written to for data reads. To enable continuous read mode, the instruction 01011100 must be written to the communications register. To exit the continuous read mode, the instruction 01011000 must be written to the communications register while the RDY pin is low. While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit continuous read mode. Additionally, a reset will occur if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to the device. These bits must be programmed to logic 0 for correct operation.
CR6 CR5-CR3 CR2
R/W RS2-RS0 CREAD
CR1-CR0
0
Table 12. Register Selection
RS2 0 0 0 0 0 1 1 1 1 RS1 0 0 0 1 1 0 0 1 1 RS0 0 0 1 0 1 0 1 0 1 Register Communications Register during a Write Operation Status Register during a Read Operation Mode Register Configuration Register Data Register ID Register IO Register Offset Register Full-Scale Register
Rev. 0 | Page 16 of 36
Register Size 8-Bit 8-Bit 16-Bit 16-Bit 24-Bit 8-Bit 8-Bit 24-Bit 24-Bit
AD7794
STATUS REGISTER
(RS2, RS1, RS0 = 0, 0, 0; Power-On/Reset = 0x88)
The status register is an 8-bit read-only register. To access the ADC status register, the user must write to the communications register, select the next operation to be a read, and load bits RS2, RS1, and RS0 with 0. Table 13 outlines the bit designations for the status register. SR0 through SR7 indicate the bit locations, SR denoting the bits are in the status register. SR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
SR7 RDY(1) SR6 ERR(0) SR5 NOREF(0) SR4 0(0) SR3 1(1) SR2 CH2(0) SR1 CH1(0) SR0 CH0(0)
Table 13. Status Register Bit Designations
Bit Location SR7 Bit Name RDY Description Ready Bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically after the ADC data register has been read or a period of time before the data register is updated with a new conversion result to indicate to the user not to read the conversion data. It is also set when the part is placed in power-down mode. The end of a conversion is also indicated by the DOUT/RDY pin. This pin can be used as an alternative to the status register for monitoring the ADC for conversion data. ADC Error Bit. This bit is written to at the same time as the RDY bit. Set to indicate that the result written to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange, underrange, or the absence of a reference voltage. Cleared by a write operation to start a conversion. No External Reference Bit. Set to indicate that the selected reference (REFIN1 or REFIN2) is at a voltage that is below a specified threshold. When set, conversion results are clamped to all ones. Cleared to indicate that a valid reference is applied to the selected reference pins. The NOXREF bit is enabled by setting the REF_DET bit in the configuration register to 1. The ERR bit is also set if the voltage applied to the selected reference input is invalid. This bit is automatically cleared. This bit is automatically set. These bits indicate which channel is being converted by the ADC.
SR6
ERR
SR5
NOREF
SR4 SR3 SR2-SR0
0 1 CH2-CH0
MODE REGISTER
(RS2, RS1, RS0 = 0, 0, 1; Power-On/Reset = 0x000A)
The mode register is a 16-bit register from which data can be read or to which data can be written. This register is used to select the operating mode, the update rate and the clock source. Table 14 outlines the bit designations for the mode register. MR0 through MR15 indicate the bit locations, MR denoting the bits are in the mode register. MR15 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. Any write to the setup register resets the modulator and filter and sets the RDY bit.
MR15 MD2(0) MR7 CLK1(0) MR14 MD1(0) MR6 CLK0(0) MR13 MD0(0) MR5 0(0) MR12 PSW(0) MR4 CHOP-DIS(0) MR11 0(0) MR3 FS3(1) MR10 0(0) MR2 FS2(0) MR9 AMP-CM(0) MR1 FS1(1) MR8 0(0) MR0 FS0(0)
Table 14. Mode Register Bit Designations
Bit Location MR15-MR13 MR12 Bit Name MD2-MD0 PSW Description Mode Select Bits. These bits select the operational mode of the AD7794 (see Table 15). Power Switch Control Bit. Set by user to close the power switch PSW to GND. The power switch can sink up to 30 mA. Cleared by user to open the power switch. When the ADC is placed in power-down mode, the power switch is opened. These bits must be programmed with a Logic 0 for correct operation. Instrumentation Amplifier Common-Mode Bit. It is used in conjunction with the CHOP-DIS bit. When chopping is disabled, the user can operate with a wider range of common mode voltages when AMP-CM is cleared. However, the dc common-mode rejection will degrade.
Rev. 0 | Page 17 of 36
MR11-MR10 MR9
0 AMP-CM
AD7794
Bit Location Bit Name Description With AMP-CM set, the span for the common-mode voltage is reduced (see Specifications section). However, the dc common-mode rejection is significantly better. This bit must be programmed with a Logic 0 for correct operation. These bits are used to select the clock source for the AD7794. Either the on-chip 64 kHz clock can be used or an external clock can be used. The ability to use an external clock allows several AD7794 devices to be synchronized. Also, 50 Hz/60 Hz rejection is improved when an accurate external clock drives the AD7794. CLK1 CLK0 ADC Clock Source 0 0 Internal 64 kHz clock. Internal clock is not available at the CLK pin 0 1 Internal 64 kHz clock. This clock is made available at the CLK pin 1 0 External 64 kHz clock used. The external clock can have a 45:55 duty cycle. See specifications for external clock. 1 1 External clock used. The external clock is divided by 2 within the AD7794. This bit must be programmed with a Logic 0 for correct operation. This bit is used to enable or disable chopping. On power-up or following a reset, CHOP-DIS is cleared so chopping is enabled. When CHOP-DIS is set, chopping is disabled. This bit is used in conjunction with the AMP-CM bit. When chopping is disabled, the AMP-CM bit should be set. This will limit the common mode voltage which can be used by the ADC but the dc common-mode rejection will not degrade. Filter Update Rate Select Bits (see Table 16).
MR8 MR7-MR6
0 CLK1-CLK0
MR5 MR4
0 CHOP-DIS
MR3-MR0
FS3-FS0
Table 15. Operating Modes
MD2 0 MD1 0 MD0 0 Mode Continuous Conversion Mode (Default). In continuous conversion mode, the ADC continuously performs conversions and places the result in the data register. RDY goes low when a conversion is complete. The user can read these conversions by placing the device in continuous read mode whereby the conversions are automatically placed on the DOUT line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to output the conversion by writing to the communications register. After power-on, the first conversion is available after a period 2/fADC when chopping is enabled or 1/fADC when chopping is disabled. Subsequent conversions are available at a frequency of fADC with chopping either enabled or disabled. Single Conversion Mode. When single conversion mode is selected, the ADC powers up and performs a single conversion. The oscillator requires 1 ms to power up and settle. The ADC then performs the conversion which takes a time of 2/fADC when chopping is enabled or 1/fADC when chopping is disabled. The conversion result in placed in the data register, RDY goes low, and the ADC returns to power-down mode. The conversion remains in the data register and RDY remains active (low) until the data is read or another conversion is performed. Idle Mode. In idle mode, the ADC filter and modulator are held in a reset state although the modulator clocks are still provided. Power-Down Mode. In power-down mode, all the AD7794 circuitry is powered down including the current sources, power switch, burnout currents, bias voltage generator, and CLKOUT circuitry. Internal Zero-Scale Calibration. An internal short is automatically connected to the enabled channel. A calibration takes 2 conversion cycles to complete when chopping is enabled and 1 conversion cycle when chopping is disabled. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured offset coefficient is placed in the offset register of the selected channel. Internal Full-Scale Calibration. A full-scale input voltage is automatically connected to the selected analog input for this calibration. When the gain equals 1, a calibration takes 2 conversion cycles to complete when chopping is enabled and 1 conversion cycle when chopping is disabled. For higher gains, 4 conversion cycles are required to perform the full-scale calibration when chopping is enabled and 2 conversion cycles when chopping is disabled. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the full-scale register of the selected channel.
Rev. 0 | Page 18 of 36
0
0
1
0 0
1 1
0 1
1
0
0
1
0
1
AD7794
MD2 MD1 MD0 Mode Internal full-scale calibrations cannot be performed when the gain equals 128. With this gain setting, a system fullscale calibration can be performed. A full-scale calibration is required each time the gain of a channel is changed to minimize the Full-Scale error. System Zero-Scale Calibration. User should connect the system zero-scale input to the channel input pins as selected by the CH2-CH0 bits. A system offset calibration takes 2 conversion cycles to complete when chopping is enabled and one conversion cycle when chopping is disabled. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured offset coefficient is placed in the offset register of the selected channel. System Full-Scale Calibration. User should connect the system full-scale input to the channel input pins as selected by the CH2-CH0 bits. A calibration takes 2 conversion cycles to complete when chopping is enabled and one conversion cycle when chopping is disabled. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the full-scale register of the selected channel. A full-scale calibration is required each time the gain of a channel is changed.
1
1
0
1
1
1
Table 16. Update Rates Available (Chopping Enabled)
FS3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 FS2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 FS1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 FS0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 fADC(Hz) x 500 250 125 62.5 50 39.2 33.3 19.6 16.7 16.7 12.5 10 8.33 6.25 4.17 TSETTLE (ms) x 4 8 16 32 40 48 60 101 120 120 160 200 240 320 480 Rejection@ 50 Hz/60 Hz (Internal Clock)
90 dB (60 Hz only) 80 dB (50 Hz only) 65 dB (50 Hz and 60 Hz) 66 dB (50 Hz and 60 Hz) 69 dB (50 Hz and 60 Hz) 70 dB (50 Hz and 60 Hz) 72 dB (50 Hz and 60 Hz) 74 dB (50 Hz and 60 Hz)
With chopping disabled, the update rates remain unchanged but the settling time for each update rate is reduced by a factor of 2. The rejection at 50 Hz/60 Hz for a 16.6 Hz update rate degrades to 60 dB.
CONFIGURATION REGISTER
(RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710)
The configuration register is a 16-bit register from which data can be read or to which data can be written. This register is used to configure the ADC for unipolar or bipolar mode, enable or disable the buffer, enable or disable the burnout currents, select the gain, and select the analog input channel. Table 17 outlines the bit designations for the filter register. CON0 through CON15 indicate the bit locations, CON denoting the bits are in the configuration register. CON15 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
CON15 VBIAS1(0) CON7 REFSEL1(0)
CON14 VBIAS0(0) CON6 REFSEL0(0)
CON13 BO(0) CON5 REF_DET(0)
CON12 U/B (0) CON4 BUF(1)
Rev. 0 | Page 19 of 36
CON11 BOOST0) CON3 CH3(0)
CON10 G2(1) CON2 CH2(0)
CON9 G1(1) CON1 CH1(0)
CON8 G0(1) CON0 CH0(0)
AD7794
Table 17. Configuration Register Bit Designations
Bit Location CON15- CON14 Bit Name VBIAS1 - VBIAS0 Description Bias Voltage Generator Enable. The negative terminal of the analog inputs can be biased up to AVDD/2. These bits are used in conjunction with the BOOST bit. VBIAS1 VBIAS0 Bias Voltage 0 0 Bias Voltage Generator Disabled 0 1 Bias Voltage Generator connected to AIN1(-) 1 0 Bias Voltage Generator connected to AIN2(-) 1 1 Bias Voltage Generator connected to AIN3(-) This bit must be programmed with a Logic 0 for correct operation. Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal path are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled only when the buffer or in-amp is active. Unipolar/Bipolar Bit. Set by user to enable unipolar coding, i.e., zero differential input will result in 0x000000 output and a full-scale differential input will result in 0xFFFFFF output. Cleared by the user to enable bipolar coding. Negative full-scale differential input will result in an output code of 0x000000, zero differential input will result in an output code of 0x800000, and a positive full-scale differential input will result in an output code of 0xFFFFFF. This bit is used in conjunction with the VBIAS1 and VBIAS0 bits. When set, the current consumed by the bias voltage generator is increased which reduces its power-up time. Gain Select Bits. Written by the user to select the ADC input range as follows: G2 G1 G0 Gain ADC Input Range (2.5 V Reference) 0 0 0 1 (In-Amp not used) 2.5 V 0 0 1 2 (In-Amp not used) 1.25 V 0 1 0 4 625 mV 0 1 1 8 312.5 mV 1 0 0 16 156.2 mV 1 0 1 32 78.125 mV 1 1 0 64 39.06 mV 1 1 1 128 19.53 mV Reference Select Bits. The reference source for the ADC is selected using these bits. REFSEL1 REFSEL0 Reference Source 0 0 External Reference applied between REFIN1(+) and REFIN1(-) 0 1 External Reference applied between REFIN2(+) and REFIN2(-) 1 0 Internal 1.17 V Reference 1 1 Reserved Enables the Reference Detect Function. When set, the NOXREF bit in the status register indicates when the external reference being used by the ADC is open circuit or less than 0.5 V. When cleared, the reference detect function is disabled. Configures the ADC for buffered or unbuffered mode of operation. If cleared, the ADC operates in unbuffered mode, lowering the power consumption of the device. If set, the ADC operates in buffered mode, allowing the user to place source impedances on the front end without contributing gain errors to the system. For gains of 1 and 2, the buffer can be enabled or disabled. For higher gains, the buffer is automatically enabled. With the buffer disabled, the voltage on the analog input pins can be from 30 mV below GND to 30 mV above AVDD. When the buffer is enabled, it requires some headroom so the voltage on any input pin must be limited to 100 mV within the power supply rails. Channel Select Bits. Written by the user to select the active analog input channel to the ADC.
CON13
BO
CON12
U/B
CON11 CON10- CON8
BOOST G2-G0
CON7- CON6
REFSEL1/REFSEL0
CON5
REF_DET
CON4
BUF
CON3- CON0
CH3-CH0
Rev. 0 | Page 20 of 36
AD7794
Bit Location Bit Name Description CH3 CH2 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 1 1 1 1 1 1 1 0 0 0 1 1 1 1
CH1 0 0 1 1 0 0 1 1 0 0 1 0 0 1 1
CH0 0 1 0 1 0 1 0 1 0 1 1 0 1 0 1
Channel AIN1(+) - AIN1(-) AIN2(+) - AIN2(-) AIN3(+)- AIN3(-) AIN4(+)- AIN4(-) AIN5(+)- AIN5(-) AIN6(+)- AIN6(-) Temp Sensor AVDD Monitor AIN1(-)- AIN1(-) Reserved Reserved Reserved Reserved Reserved Reserved
Calibration Pair 0 1 2 3 3 3 Automatically selects the internal reference and sets the gain to 1 Automatically selects the internal 1.17 V reference and sets the gain to 1/6 0
DATA REGISTER
(RS2, RS1, RS0 = 0, 1, 1; Power-On/Reset = 0x000000)
The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from this register, the RDY bit/pin is set.
ID REGISTER
(RS2, RS1, RS0 = 1, 0, 0; Power-On/Reset = 0xXF)
The identification number for the AD7794 is stored in the ID register. This is a read-only register.
IO REGISTER
(RS2, RS1, RS0 = 1, 0, 1; Power-On/Reset = 0x00)
The IO register is an 8-bit register from which data can be read or to which data can be written. This register is used to enable the excitation currents and select the value of the excitation currents. Table 18 outlines the bit designations for the IO register. IO0 through IO7 indicate the bit locations, IO denoting the bits are in the IO register. IO7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
IO7 0(0)
IO6 IOEN(0)
IO5 IO2DAT(0)
IO4 IO1DAT(0)
IO3 IEXCDIR1(0)
IO2 IEXCDIR0(0)
IO1 IEXCEN1(0)
IO0 IEXCEN0(0)
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AD7794
Table 18. IO Register Bit Designations
Bit Location IO7 IO6 Bit Name 0 IOEN Description This bit must be programmed with a Logic 0 for correct operation. Configures the pins AIN6(+)/P2 and AIN6(-)/P2 as analog input pins or digital output pins. When this bit is set, the pins are configured as Digital Output Pins P1 and P2. When this bit is cleared, these pins are configured as Analog Input Pins AIN6(+) and AIN6(-). P2/P1 Data. When IOEN is set, the data for the Digital Output Pins P1 and P2 is written to Bits IO2DAT and IO1DAT. Direction of Current Sources Select bits. IEXCDIR0 Current Source IEXC1 connected to Pin IOUT1. Current Source IEXC2 connected to Pin IOUT2. 0 1 Current Source IEXC1 connected to Pin IOUT2. Current Source IEXC2 connected to Pin IOUT1. 1 0 Both current sources connected to Pin IOUT1. Permitted only when the current sources are set to 10 A or 210 A. 1 1 Both current sources connected to Pin IOUT2. Permitted only when the current sources are set to 10 A or 210 A. These bits are used to enable and disable the current sources along with selecting the value of the excitation currents. IEXCEN1 IEXCEN0 Current Source Value 0 0 Excitation Currents Disabled 0 1 10 A 1 0 210 A 1 1 1 mA EXCDIR1 0 IEXCDIR0 0
IO5-IO4
IO2DAT/IO1DAT
IO3-IO2
IEXCDIR1- IEXCDIR0
IO3-IO2
IEXCEN1- IEXCEN0
OFFSET REGISTER
(RS2, RS1, RS0 = 1, 1, 0; Power-On/Reset = 0x800000)
The offset register holds the offset calibration coefficient for the ADC. The power-on reset value of the offset register is 0x800000. The AD7794 has four offset registers. Channels AIN1 to AIN3 have dedicated offset registers while channels AIN4, AIN5 and AIN6 share an offset register. Each of these registers is a 24-bit read/write register. This register is used in conjunction with its associated full-scale register to form a register pair. The power-on reset value is automatically overwritten if an internal or system zero-scale calibration is initiated by the user. The AD7794 must be placed in power-down mode or idle mode when writing to the offset register.
FULL-SCALE REGISTER
(RS2, RS1, RS0 = 1, 1, 1; Power-On/Reset = 0x5XXX00)
The full-scale register is a 24-bit register that holds the full-scale calibration coefficient for the ADC. The AD7794 has 4 full-scale registers. Channels AIN1, AIN2 and AIN3 have dedicated full-scale registers while channels AIN4, AIN5, and AIN6 share a register. The full-scale registers are read/write registers. However, when writing to the full-scale registers, the ADC must be placed in power-down mode or idle mode. These registers are configured on power-on with factory-calibrated full-scale calibration coefficients, the calibration being performed at gain = 1. Therefore, every device will have different default coefficients. The coefficients are different depending on whether the internal reference or an external reference is selected. The default value will be automatically overwritten if an internal or system full-scale calibration is initiated by the user, or the full-scale register is written to.
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AD7794 ADC CIRCUIT INFORMATION
OVERVIEW
The AD7794 is a low power ADC that incorporates a - modulator, a buffer, reference, In-amp, and on-chip digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those in pressure transducers, weigh scales, and temperature measurement applications. The part has six differential inputs that can be buffered or unbuffered. The device can be operated with the internal 1.17 V reference or an external reference can be used. Figure 12 shows the basic connections required to operate the part.
VDD
0
-20
-40
(dB)
-60
-80
04854-017
-100 0
IN+ OUT- IN+ OUT- OUT+ OUT+ REFIN1(+) GND AVDD
20
40
60 FREQUENCY (Hz)
80
100
120
AIN1(+) AIN1(-) VDD AIN2(+) AIN2(-) MUX BUF IN-AMP
AD7794
Figure 13. Filter Profile with Update Rate = 4.17 Hz (Chop Enabled)
DOUT/RDY
IN-
0
IN-
AIN3(+) AIN3(-) REFIN2(+) RCM REFIN2(-) IOUT1 REFIN1(-) PSW
- ADC
SERIAL INTERFACE AND LOGIC CONTROL
DIN SCLK CS
GND VDD INTERNAL CLOCK DVDD
-20
-40
CLK
(dB)
GND
04854-012
Figure 12. Basic Connection Diagram
-60
The output rate of the AD7794 (fADC) is user programmable. The allowable update rates along with the corresponding settling times are listed in Table 16 for chop enabled. With chop disabled, the allowable update rates remain unchanged but the settling time equals 1/fADC. Normal mode rejection is the major function of the digital filter. Simultaneous 50 Hz and 60 Hz rejection is optimized when the update rate equals 16.7 Hz or less as notches are placed at both 50 Hz and 60 Hz with these update rates (see Figure 14). The AD7794 uses slightly different filter types depending on the output update rate so that the rejection of quantization noise and device noise is optimized. When the update rate is from 4.17 Hz to 12.5 Hz, a Sinc3 filter along with an averaging filter is used. When the update rate is from 16.7 Hz to 39.2 Hz, a modified Sinc3 filter is used. This filter gives simultaneous 50 Hz/60 Hz rejection when the update rate equals 16.7 Hz. A Sinc4 filter is used when the update rate is from 50 Hz to 250 Hz. Finally, an integrate-only filter is used when the update rate equals 500 Hz. Figure 13 to Figure 16 show the frequency response of the different filters types for some of the update rates when chopping is enabled. In this mode, the settling time equals twice the update rate. Figure 17 to Figure 20 show the filter response with chopping disabled.
-80
04854-018
-100 0 20 40 60 80 100 120 140 160 180 FREQUENCY (Hz)
200
Figure 14. Filter Profile with Update Rate = 16.7 Hz (Chop Enabled)
0
-20
-40
(dB)
-60
-80
04854-019
-100 0 500 1000 1500 2000 2500 FREQUENCY (Hz)
3000
Figure 15. Filter Profile with Update Rate = 250 Hz (Chop Enabled)
Rev. 0 | Page 23 of 36
AD7794
0
0
-10
-20
-20
-40
-30
(dB)
-60 -80
04854-020
04854-023
(dB)
-40
-50
-60 0 FREQUENCY (Hz)
-100 0 500 1000 1500 2000 2500 FREQUENCY (Hz)
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
3000
Figure 16. Filter Response at 500 Hz Update Rate (Chop Enabled)
Figure 19. Filter Response at 250 Hz Update Rate (Chop Disabled)
0
0
-20
-10
-20
-40
(dB) (dB)
-30
-60
-40
-80
04854-021
-50
04854-024
-100 0 20 40 60 FREQUENCY (Hz) 80 100
-60 0 FREQUENCY (Hz)
120
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Figure 17. Filter Response at 4.17 Hz Update Rate (Chop Disabled)
0
Figure 20. Filter Response at 500 Hz Update Rate (Chop Disabled)
-20
-40
(dB)
-60 -80
04854-022
-100 0 20 40 60 80 100 120 140 160 180 FREQUENCY (Hz)
200
Figure 18. Filter Response at 16.7 Hz Update Rate (Chop Disabled)
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AD7794
DIGITAL INTERFACE
As previously outlined, the AD7794's programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the part's serial interface and read access to the on-chip registers is also provided by this interface. All communications with the part must start with a write to the communications register. After power-on or reset, the device expects a write to its communications register. The data written to this register determines whether the next operation is a read operation or a write operation and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part begins with a write operation to the communications register followed by a write to the selected register. A read operation from any other register (except when continuous read mode is selected) starts with a write to the communications register followed by a read operation from the selected register. The AD7794's serial interface consists of four signals: CS, DIN, SCLK, and DOUT/RDY. The DIN line is used to transfer data into the on-chip registers while DOUT/RDY is used for accessing from the on-chip registers. SCLK is the serial clock input for the device and all data transfers (either on DIN or DOUT/RDY) occur with respect to the SCLK signal. The DOUT/ RDY pin operates as a data ready signal also, the line going low when a new data-word is available in the output register. It is reset high when a read operation from the data register is complete. It also goes high prior to the updating of the data register to indicate when not to read from the device, to ensure that a data read is not attempted while the register is being updated. CS is used to select a device. It can be used to decode the AD7794 in systems where several components are connected to the serial bus. Figure 3 and Figure 4 show timing diagrams for interfacing to the AD7794 with CS being used to decode the part. Figure 3 shows the timing for a read operation from the AD7794's output shift register while Figure 4 shows the timing for a write operation to the input shift register. It is possible to read the same word from the data register several times even though the DOUT/RDY line returns high after the first read operation. However, care must be taken to ensure that the read operations have been completed before the next output update occurs. In continuous read mode, the data register can be read only once. The serial interface can operate in 3-wire mode by tying CS low. In this case, the SCLK, DIN, and DOUT/RDY lines are used to communicate with the AD7794. The end of the conversion can be monitored using the RDY bit in the status register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port pin. For microcontroller interfaces, it is recommended that SCLK idles high between data transfers. The AD7794 can be operated with CS being used as a frame synchronization signal. This scheme is useful for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS since CS would normally occur after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers, provided the timing numbers are obeyed. The serial interface can be reset by writing a series of 1s on the DIN input. If a Logic 1 is written to the AD7794 line for at least 32 serial clock cycles, the serial interface is reset. This ensures that the interface can be reset to a known state if the interface gets lost due to a software error or some glitch in the system. Reset returns the interface to the state in which it is expecting a write to the communications register. This operation resets the contents of all registers to their power-on values. Following a reset, the user should allow a period of 500 s before addressing the serial interface. The AD7794 can be configured to continuously convert or to perform a single conversion. See Figure 21 through Figure 23.
Rev. 0 | Page 25 of 36
AD7794
CS
DIN
0x08
0x200A
0x58
DATA DOUT/RDY
SCLK
Figure 21. Single Conversion
Single Conversion Mode
In single conversion mode, the AD7794 is placed in shutdown mode between conversions. When a single conversion is initiated by setting MD2, MD1, MD0 to 0, 0, 1 in the mode register, the AD7794 powers up, performs a single conversion, and then returns to shutdown mode. The on-chip oscillator requires 1 ms to power up. A conversion will require a time period of 2 x tADC. DOUT/RDY goes low to indicate the completion of a conversion. When the data-word has been read from the data register, DOUT/RDY will go high. If CS is low, DOUT/RDY will remain high until another conversion is initiated and completed. The data register can be read several times, if required, even when DOUT/ RDY has gone high.
Continuous Conversion Mode
This is the default power-up mode. The AD7794 continuously converts, the RDY pin in the status register going low each time a conversion is complete. If CS is low, the DOUT/RDY line also goes low when a conversion is complete. To read a conversion, the user can write to the communications register, indicating that the next operation is a read of the data register. The digital conversion is placed on the DOUT/RDY pin as soon as SCLK pulses are applied to the ADC. DOUT/RDY returns high when the conversion is read. The user can read this register additional times, if required. However, the user must ensure that the data register is not being accessed at the completion of the next conversion or else the new conversion word will be lost.
CS
0x58 DIN
0x58
DOUT/RDY
DATA
DATA
SCLK
Figure 22. Continuous Conversion
Rev. 0 | Page 26 of 36
04854-015
04854-014
AD7794
CONTINUOUS READ
Rather than write to the communications register each time a conversion is complete to access the data, the AD7794 can be configured so that the conversions are placed on the DOUT/ RDY line automatically. By writing 01011100 to the communications register, the user needs only to apply the appropriate number of SCLK cycles to the ADC and the 24-bit word will automatically be placed on the DOUT/RDY line when a conversion is complete. The ADC should be configured for continuous conversion mode. When DOUT/RDY goes low to indicate the end of a conversion, sufficient SCLK cycles must be applied to the ADC and the data conversion will be placed on the DOUT/RDY line. When the conversion is read, DOUT/RDY will return high until the next conversion is available. In this mode, the data can be read only once. Also, the user must ensure that the data-word is read before the next conversion is complete. If the user has not read the conversion before the completion of the next conversion or if insufficient serial clocks are applied to the AD7794 to read the word, the serial output register is reset when the next conversion is complete and the new conversion is placed in the output serial register. To exit the continuous read mode, the instruction 01011000 must be written to the communications register while the RDY pin is low. While in the continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit the continuous read mode. Additionally, a reset will occur if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to the device.
CS
0x5C DIN
DOUT/RDY
DATA
DATA
DATA
SCLK
Figure 23. Continuous Read
Rev. 0 | Page 27 of 36
04854-016
AD7794 CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7794 has six differential analog input channels. These are connected to the on-chip buffer amplifier when the device is operated in buffered mode and directly to the modulator when the device is operated in unbuffered mode. In buffered mode (the BUF bit in the mode register is set to 1), the input channel feeds into a high impedance input stage of the buffer amplifier. Therefore, the input can tolerate significant source impedances and is tailored for direct connection to external resistive-type sensors such as strain gauges or resistance temperature detectors (RTDs). When BUF = 0, the part is operated in unbuffered mode. This results in a higher analog input current. Note that this unbuffered input path provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on the input pins can cause gain errors, depending on the output impedance of the source that is driving the ADC input. Table 19 shows the allowable external resistance/capacitance values for unbuffered mode such that no gain error at the 20-bit level is introduced.
Table 19. External R-C Combination for No 20-Bit Gain Error C (pF) R () 50 9K 100 6K 500 1.5 K 1000 900 5000 200
within the AD7794 while still maintaining excellent noise performance. For example, when the gain is set to 64, the rms noise is 40 nV typically which is equivalent to 20.5 bits effective resolution or 18 bits peak-to-peak resolution. The AD7794 can be programmed to have a gain of 1, 2, 4, 8, 16, 32, 64, and 128 using the Bits G2 to G0 in the configuration register. Therefore, with an external 2.5V reference, the unipolar ranges are from 0 mV to 20 mV to 0 V to 2.5 V while the bipolar ranges are from 20 mV to 2.5 V. When the in-amp is active (Gain > 4), the common-mode voltage ((AIN(+) + AIN(-))/2) must be greater than or equal to 0.5 V when chopping is enabled. With chopping disabled, and with the AMP-CM bit set to 1 to prevent degradation in the common-mode rejection, the allowable common-mode voltage is limited to between 0.2 + (Gain/2 x (AIN(+) - AIN(-))) and
AVDD - 0.2 - (Gain/2 x (AIN(+) - AIN(-)))
If the AD7794 is operated with an external reference that has a value equal to AVDD, for correct operation the analog input signal must be limited to 90% of VREF/gain when the in-amp is active.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7794 can accept either unipolar or bipolar input voltage ranges. A bipolar input range does not imply that the part can tolerate negative voltages with respect to system GND. Unipolar and bipolar signals on the AIN(+) input are referenced to the voltage on the AIN(-) input. For example, if AIN(-) is 2.5 V and the ADC is configured for unipolar mode with a gain of 1, the input voltage range on the AIN(+) pin is 2.5 V to 5 V. If the ADC is configured for bipolar mode, the analog input range on the AIN(+) input is 0 V to 5 V. The bipolar/unipolar option is chosen by programming the B/U bit in the configuration register.
The AD7794 can be operated in unbuffered mode only when the gain equals 1 or 2. At higher gains, the buffer is automatically enabled. The absolute input voltage range in buffered mode is restricted to a range between GND + 100 mV and AVDD - 100 mV. When the gain is set to 4 or higher, the in-amp is enabled. The absolute input voltage range when the in-amp is active is restricted to a range between GND + 300 mV and AVDD - 1.1 V. Care must be taken in setting up the commonmode voltage so that these limits are not exceeded. Otherwise, there will be degradation in linearity and noise performance. The absolute input voltage in unbuffered mode includes the range between GND - 30 mV and AVDD + 30 mV as a result of being unbuffered. The negative absolute input voltage limit does allow the possibility of monitoring small true bipolar signals with respect to GND.
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output code is natural (straight) binary with a zero differential input voltage resulting in a code of 00...00, a mid-scale voltage resulting in a code of 100...000, and a full-scale input voltage resulting in a code of 111...111. The output code for any analog input voltage can be represented as Code = 2N x (AIN/VREF)
INSTRUMENTATION AMPLIFIER
Amplifying the analog input signal by a gain of 1 or 2 is performed digitally within the AD7794. However, when the gain equals 4 or higher, the output from the buffer is applied to the input of the on-chip instrumentation amplifier. This low noise in-amp means that signals of small amplitude can be gained
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AD7794
When the ADC is configured for bipolar operation, the output code is offset binary with a negative full-scale voltage resulting in a code of 000...000, a zero differential input voltage resulting in a code of 100...000, and a positive full-scale input voltage resulting in a code of 111...111. The output code for any analog input voltage can be represented as Code = 2N - 1 x [(AIN/VREF) + 1] where AIN is the analog input voltage and N = 24. amplifier requires headroom so signals close to GND or AVDD will not be converted accurately. The bias voltage generator is controlled using the VBIAS1 and VBIAS0 bits in conjunction with the boost bit in the configuration register. The power up time of the bias voltage generator is dependent on the load capacitance. To accommodate higher load capacitances, the AD7794 has a boost bit. When this bit is set to 1, the current consumed by the bias voltage generator is increased so that the power up time is considerably reduced. Figure 11 shows the power up times when boost equals 0 and 1 for different load capacitances. The current consumption of the AD7794 increases by 40 A when the bias voltage generator is enabled and boost equals 0. With the boost function enabled, the current consumption increases by 250 A.
BURNOUT CURRENTS
The AD7794 contains two 100 nA constant current generators, one sourcing current from AVDD to AIN(+) and one sinking current from AIN(-) to GND. The currents are switched to the selected analog input pair. Both currents are either on or off, depending on the burnout current enable (BO) bit in the configuration register. These current s can be used to verify that an external transducer is still operational before attempting to take measurements on that channel. Once the burnout currents are turned on, they will flow in the external transducer circuit, and a measurement of the input voltage on the analog input channel can be taken. If the resultant voltage measured is full scale, the user needs to verify why this is the case. A full-scale reading could mean that the front end sensor is open circuit, it could also mean that the front end sensor is overloaded and is justified in outputting full scale or, the reference may be absent and the NOXREF bit is set, thus clamping the data to all ones. When reading all ones from the output, the user needs to check these three cases before making a judgment. If the voltage measured is 0 V, it may indicate that the transducer has short circuited. For normal operation, these burnout currents are turned off by writing a 0 to the BO bit in the configuration register. The current sources work over the normal absolute input voltage range specifications with buffers on.
REFERENCE
The AD7794 has an embedded 1.17 V reference. This reference can be used to supply the ADC or an external reference can be applied. The embedded reference is a low noise, low drift reference, the drift being 4 ppm/OC typically. For external references, the ADC has a fully differential input capability for the channel. In addition, the user has the option of selecting one of two external reference options (REFIN1 or REFIN2). The reference source for the AD7794 is selected using the REFSEL1 and REFSEL0 bits in the configuration register. When the internal reference is selected, it is internally connected to the modulator (it is not available on the REFIN pins). The common-mode range for these differential inputs is from GND to AVDD. The reference input is unbuffered and, therefore, excessive R-C source impedances will introduce gain errors. The reference voltage REFIN (REFIN(+) - REFIN(-)) is 2.5 V nominal, but the AD7794 is functional with reference voltages from 0.1 V to AVDD. In applications where the excitation (voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source will be removed because the application is ratiometric. If the AD7794 is used in a nonratiometric application, a low noise reference should be used. Recommended 2.5 V reference voltage sources for the AD7794 include the ADR381 and ADR391, which are low noise, low power references. Also note that the reference inputs provide a high impedance, dynamic load. Because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source driving the reference inputs. Reference voltage sources like those recommended above (e.g., ADR391) will typically have low output impedances and are, therefore, tolerant to having decoupling capacitors on REFIN(+) without introducing gain errors in the system. Deriving the reference input voltage across an external resistor will mean that
EXCITATION CURRENTS
The AD7794 also contains two matched, software configurable constant current sources which can be programmed to equal 10 A, 210 A or 1 mA. Both source currents from AVDD are directed to either IOUT1 or IOUT2 pins of the device. These current sources are controlled via bits in the IO register. The configuration bits enable the current sources, direct the current sources to IOUT1 or IOUT2 along with selecting the value of the current. These current sources can be used to excite external resistive bridge or RTD sensors.
BIAS VOLTAGE GENERATOR
A bias voltage generator is included on the AD7794. This will bias the negative terminal of the selected input channel to AVDD/2. This function is available on inputs AIN1 to AIN3. It is useful in thermocouple applications as the voltage generated by the thermocouple must be biased about some dc voltage if the gain is greater than 2. This is required since the instrumentation
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AD7794
the reference input sees a significant external source impedance. External decoupling on the REFIN pins would not be recommended in this type of circuit configuration. conversion result is scaled using the ADC calibration registers before being written to the data register. The offset calibration coefficient is subtracted from the result prior to multiplication by the full-scale coefficient. To start a calibration, write the relevant value to the MD2 to MD0 bits in the mode register. After the calibration is complete, the contents of the corresponding calibration registers are updated, the RDY bit in the status register is set, the DOUT/ RDY pin goes low (if CS is low) and the AD7794 reverts to idle mode. During an internal zero-scale or full-scale calibration, the respective zero input and full-scale input are automatically connected internally to the ADC input pins. A system calibration, however, expects the system zero-scale and system full-scale voltages to be applied to the ADC pins before initiating the calibration mode. In this way, external ADC errors are removed. From an operational point of view, a calibration should be treated like another ADC conversion. A zero-scale calibration (if required) should always be performed before a full scale calibration. System software should monitor the RDY bit in the status register or the DOUT/RDY pin to determine the end of calibration via a polling sequence or an interrupt-driven routine. With chopping enabled, both an internal offset calibration and a system offset calibration take two conversion cycles. With chopping enabled, an internal offset calibration is not needed as the ADC itself removes the offset continuously. With chopping disabled, an internal offset calibration or system offset calibration takes one conversion cycle to complete. Internal offset calibrations are required with chopping disabled and should occur before the full-scale calibration. To perform an internal full-scale calibration, a full-scale input voltage is automatically connected to the selected analog input for this calibration. When the gain equals 1 a calibration takes 2 conversion cycles to complete when chopping is enabled and 1 conversion cycle when chopping is disabled. For higher gains, 4 conversion cycles are required to perform the full-scale calibration when chopping is enabled and 2 conversion cycles when chopping is disabled. DOUT/RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the fullscale register of the selected channel. Internal full-scale calibrations cannot be performed when the gain equals 128. With this gain setting, a system full-scale calibration can be performed. A full-scale calibration is required each time the gain of a channel is changed to minimize the full-scale error. An internal full-scale calibration can be performed at specified update rates only. For gains of 1, 2, and 4, an internal full-scale
REFERENCE DETECT
The AD7794 includes on-chip circuitry to detect if the part has a valid reference for conversions or calibrations if the user selects an external reference as the reference source. This feature is enabled when the REF-DET bit in the configuration register is set to 1. If the voltage between the selected REFIN(+) and REFIN(-) pins goes below 0.3 V or either the REFIN(+) or REFIN(-) inputs are open circuit, the AD7794 detects that it no longer has a valid reference. In this case, the NOXREF bit of the status register is set to 1. If the AD7794 is performing normal conversions and the NOXREF bit becomes active, the conversion results revert to all 1s. Therefore it is not necessary to continuously monitor the status of the NOXREF bit when performing conversions. It is only necessary to verify its status if the conversion result read from the ADC's data register is all 1s. If the AD7794 is performing either an offset of full-scale calibration and the NOXREF bit becomes active, the updating of the respective calibration registers is inhibited to avoid loading incorrect coefficients to these registers and the ERR bit in the status register is set. If the user is concerned about verifying that a valid reference is in place every time a calibration is performed, the status of the ERR bit should be checked at the end of the calibration cycle.
RESET
The circuitry and serial interface of the AD7794 can be reset by writing 32 consecutive 1s to the device. This will reset the logic, the digital filter and the analog modulator while all on-chip registers are reset to their default values. A reset is automatically performed on power up. When a reset is initiated, the user must allow a period of 500 s before accessing any of the on-chip registers. A reset is useful if the serial interface becomes asynchronous due to noise on the SCLK line.
AVDD MONITOR
Along with converting external voltages, the ADC can be used to monitor the voltage on the AVDD pin. When bit CH2 to CH0 equals 1, the voltage on the AVDD pin is internally attenuated by 6 and the resultant voltage is applied to the - modulator using an internal 1.17 V reference for analog to digital conversion. This is useful because variations in the power supply voltage can be monitored.
CALIBRATION
The AD7794 provides four calibration modes that can be programmed via the mode bits in the mode register. These are internal zero-scale calibration, internal full-scale calibration, system zero-scale calibration and system full-scale calibration which will effectively reduce the offset error and full-scale error to the order of the noise. After each conversion, the ADC
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AD7794
calibration can be performed at any update rate. However, for higher gains, internal full-scale calibrations can only be performed when the update rate is less than or equal to 16.7 Hz, 33.3Hz, and 50 Hz only. However, the full-scale error does not vary with update rate so a calibration at one update is valid for all update rates (assuming the gain or reference source is not changed). A system full-scale calibration takes 2 conversion cycles to complete irrespective of the gain setting when chopping is enabled and 1 conversion cycle when chopping is disabled. A system full-scale calibration can be performed at all gains and all update rates. With chopping disabled, the offset calibration (internal or system offset) should be performed before the system full-scale calibration is initiated. It is recommended that the AD7794's GND pin be tied to the AGND plane of the system. In any layout, it is important that the user keep in mind the flow of currents in the system, ensuring that the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND sections of the layout. The AD7794's ground plane should be allowed to run under the AD7794 to prevent noise coupling. The power supply lines to the AD7794 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals such as clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough through the board. A microstrip technique is by far the best, but it is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes, while signals are placed on the solder side. Good decoupling is important when using high resolution ADCs. AVDD should be decoupled with 10 F tantalum in parallel with 0.1 F capacitors to GND. DVDD should be decoupled with 10 F tantalum in parallel with 0.1 F capacitors to the system's DGND plane with the system's AGND to DGND connection being close to the AD7794. To achieve the best from these decoupling components, they should be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 F ceramic capacitors to DGND.
GROUNDING AND LAYOUT
Since the analog inputs and reference inputs of the ADC are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part will remove common-mode noise on these inputs. The digital filter will provide rejection of broadband noise on the power supply, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs, provided that these noise sources do not saturate the analog modulator. As a result, the AD7794 is more immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7794 is so high, and the noise levels from the AD7794 are so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7794 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. A minimum etch technique is generally best for ground planes because it gives the best shielding.
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AD7794 APPLICATIONS
The AD7794 provides a low-cost, high resolution analog-todigital function. Because the analog-to-digital function is provided by a - architecture, it makes the part more immune to noisy environments, making it ideal for use in sensor measurement and industrial and process control applications. A second advantage of using the AD7794 in transducer-based applications is that the low-side power switch can be fully utilized in low power applications. The low-side power switch is connected in series with the cold side of the bridges. In normal operation, the switch is closed and measurements can be taken. In applications where power is of concern, the AD7794 can be placed in standby mode, thus significantly reducing the power consumed in the application. In addition, the low-side power switch can be opened while in standby mode, thus avoiding unnecessary power consumption by the front-end transducers. When the part is taken out of standby mode and the low-side power switch is closed, the user should ensure that the frontend circuitry is fully settled before attempting a read from the AD7794. In the diagram, temperature compensation is performed using a thermistor. The on-chip excitation current supplies the thermistor. In addition, the reference voltage for the temperature measurement is derived from a precision resistor in series with the thermistor. This allows a ratiometric measurement so that variation of the excitation current has no affect on the measurement (it is the ratio of the precision reference resistance to the thermistor resistance which is measured).
FLOWMETER
Figure 24 shows the AD7794 being used in a flowmeter application that consists of two pressure transducers, the rate of flow being equal to the pressure difference. The pressure transducers shown are the BP01 from Sensym. The pressure transducers are arranged in a bridge network and give a differential output voltage between its OUT+ and OUT- terminals. With rated full-scale pressure (in this case 300 mmHg) on the transducer, the differential output voltage is 3 mV/V of the input voltage (i.e. the voltage between its IN(+) and IN(-) terminals). Assuming a 5 V excitation voltage, the full-scale output range from the transducer is 15 mV. The excitation voltage for the bridge can be used to directly provide the reference for the ADC as the reference input range includes the supply voltage.
VDD
IN+ OUT- IN+ OUT- OUT+ OUT+
REFIN1(+) GND
AVDD
AIN1(+) AIN1(-) VDD AIN2(+) AIN2(-) MUX BUF IN-AMP
AD7794
IN-
DOUT/RDY - ADC SERIAL INTERFACE AND LOGIC CONTROL DIN SCLK CS
IN-
AIN3(+) AIN3(-) REFIN2(+) RCM REFIN2(-) IOUT1 REFIN1(-) PSW
GND VDD INTERNAL CLOCK DVDD
GND CLK
Figure 24. Typical Application (Flowmeter)
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04854-012
AD7794 OUTLINE DIMENSIONS
7.90 7.80 7.70
24
13
4.50 4.40 4.30 6.40 BSC
1 12
PIN 1 0.65 BSC 0.15 0.05 0.30 0.19 0.10 COPLANARITY COMPLIANT TO JEDEC STANDARDS MO-153AD 1.20 MAX
SEATING PLANE
0.20 0.09
8 0
0.75 0.60 0.45
Figure 25. 24-Lead Thin Shrink Small Outline Package [TSSOP] (RU-24) Dimensions shown in millimeters
ORDERING GUIDE
Models AD7794BRU AD7794BRU-REEL Temperature Range -40C to +105C -40C to +105C Package Description 24-Lead TSSOP 24-Lead TSSOP Package Option RU-24 RU-24
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AD7794 NOTES
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AD7794 NOTES
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AD7794 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04854-0-10/04(0)
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