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FEATURES Two 8-Bit DACs In One Package 20-Lead DIP/SOIC/TSSOP Package +2.7 V to +5.5 V Operation Internal and External Reference Capability DAC Power-Down Function Parallel Interface On-Chip Output Buffer Rail-to-Rail Operation Low Power Operation 3 mA max @ 3.3 V Power-Down to 1 A max @ 25 C APPLICATIONS Portable Battery Powered Instruments Digital Gain and Offset Adjustment Programmable Voltage and Current Sources Programmable Attenuators
2.7 V to 5.5 V, Parallel Input Dual Voltage Output 8-Bit DAC AD7302
FUNCTIONAL BLOCK DIAGRAM
AD7302
INPUT REGISTER DAC REGISTER I DAC A I/V VOUTA
D7 D0 /B
INPUT REGISTER
DAC REGISTER
I DAC B
I/V
VOUTB
CONTROL LOGIC
MUX
POWER ON RESET /2 AGND
REFIN
VDD
DGND
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7302 is a dual, 8-bit voltage out DAC that operates from a single +2.7 V to +5.5 V supply. Its on-chip precision output buffers allow the DAC outputs to swing rail to rail. The AD7302 has a parallel microprocessor and DSP-compatible interface with high speed registers and double buffered interface logic. Data is loaded to the registers on the rising edge of CS or WR and the A/B pin selects either DAC A or DAC B. Reference selection for AD7302 can be either an internal reference derived from the VDD or an external reference applied at the REFIN pin. Both DACs can be simultaneously updated using the asynchronous LDAC input and can be cleared by using the asynchronous CLR input. The low power consumption of this part makes it ideally suited to portable battery operated equipment. The power consumption is less than 10 mW at 3.3 V, reducing to 3 W in powerdown mode. The AD7302 is available in a 20-pin plastic dual-in-line package, 20-lead SOIC and a 20-lead TSSOP package.
1. Low Power, Single Supply Operation. This part operates from a single +2.7 V to +5.5 V supply and typically consumes 15 mW at 5 V, making it ideal for battery powered applications. 2. The on-chip output buffer amplifiers allow the outputs of the DACs to swing rail to rail with a settling time of typically 1.2 s. 3. Internal or external reference capability. 4. High speed parallel interface. 5. Power-Down Capability. When powered down the DAC consumes less than 1 A at 25C. 6. Packaged in 20-lead DIP, SOIC and TSSOP packages.
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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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 World Wide Web Site: http://www.analog.com Fax: 617/326-8703 (c) Analog Devices, Inc., 1997
AD7302-SPECIFICATIONS to T
Parameter STATIC PERFORMANCE Resolution Relative Accuracy Differential Nonlinearity Full-Scale Error Zero Code Error @ 25C Gain Error3 Zero Code Temperature Coefficient DAC REFERENCE INPUT REFIN Input Range REFIN Input Impedance OUTPUT CHARACTERISTICS Output Voltage Range Output Voltage Settling Time Slew Rate Digital to Analog Glitch Impulse Digital Feedthrough Digital Crosstalk Analog Crosstalk DC Output Impedance Short Circuit Current Power Supply Rejection Ratio4 LOGIC INPUTS Input Current VINL, Input Low Voltage VINL, Input Low Voltage VINH, Input High Voltage VINH, Input High Voltage Pin Capacitance POWER REQUIREMENTS VDD IDD VDD = 3.3 V @ 25C @ TMIN to TMAX VDD = 5.5 V @ 25C @ TMIN to TMAX IDD (Full Power-Down) @ 25C TMIN to TMAX 8 1 1 -0.75 3 1 100
(VDD = +2.7 V to +5.5 V, Internal Reference; CL = 100 pF, RL = 10 k MAX unless otherwise noted)
Units Bits LSB max LSB max LSB typ LSB typ % FSR typ V/C typ V min to max M typ V min to max s max V/s typ nV-s typ nV-s typ nV-s typ LSB typ typ mA typ %/% max A max V max V max V min V min pF max V min/max Conditions/Comments
to VDD and GND;
B Versions1
Note 2 Guaranteed Monotonic All Zeroes Loaded to DAC Register
1.0 to VDD/2 10 0 to VDD 2 7.5 1 0.2 0.2 0.2 40 14 0.0003 10 0.8 0.6 2.4 2.1 7 2.7/5.5
Typically 1.2 s 1 LSB Change Around Major Carry
VDD = 10%
VDD = +5 V VDD = +3 V VDD = +5 V VDD = +3 V
2.8 3 4.5 5 1 2
mA max mA max mA max mA max A max A max
Both DACs Active and Excluding Load Currents VIH = VDD and VIL = GND Typically 2.3 mA See Figures 6 and 7 VIH = VDD and VIL = GND Typically 2.8 mA See Figures 6 and 7 VIH = VDD and VIL = GND See Figure 18
NOTES 1 Temperature ranges are as follows: B Version: -40C to +105C. 2 Relative Accuracy is calculated using a reduced code range of 15 to 245. 3 Gain error is specified between Codes 15 and 245. The actual error at Code 15 is typically 3 LSB. 4 Guaranteed by characterization at product release, not production tested. Specifications subject to change without notice.
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AD7302 TIMING CHARACTERISTICS1, 2 all specifications T
Parameter t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 Limit at TMIN, T MAX (B Version) 0 0 0 0 20 15 4.5 20 20 20
(VDD = +2.7 V to +5.5 V; GND = 0 V; Reference = Internal VDD/2 Reference; MIN to T MAX unless otherwise noted)
Units ns min ns min ns min ns min ns min ns min ns min ns min ns min ns min Conditions/Comments Address to Write Setup Time Address Valid to Write Hold Time Chip Select to Write Setup Time Chip Select to Write Hold Time Write Pulse Width Data Setup Time Data Hold Time Write to LDAC Setup Time LDAC Pulse Width CLR Pulse Width
NOTES 1 Sample tested at +25C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of V DD) and timed from a voltage level of (VIL + V IH )/2. tr and tf should not exceed 1 s on any digital input. 2 See Figure 1.
t1
/B
t2
t3
t4
t5
t6
D7-D0
t7
t8
t9
t10
Figure 1. Timing Diagram for Parallel Data Write
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AD7302
ABSOLUTE MAXIMUM RATINGS*
(TA = +25C unless otherwise noted)
VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V Reference Input Voltage to AGND . . . .-0.3 V to V DD + 0.3 V Digital Input Voltage to DGND . . . . . -0.3 V to VDD + 0.3 V AGND to DGND . . . . . . . . . . . . . . . . . . . . . . . -0.3 V, 0.3 V VOUTA, VOUTB to AGND . . . . . . . . . . . . -0.3 V, VDD + 0.3 V Operating Temperature Range Commercial (B Version) . . . . . . . . . . . . . -40C to +105C Storage Temperature Range . . . . . . . . . . . . -65C to +150C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . .+150C Plastic DIP Package, Power Dissipation . . . . . . . . . . 900 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 102C/W Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . +260C
TSSOP Package, Power Dissipation . . . . . . . . . . . . . 700 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 143C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . .+220C SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 870 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 74C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . .+220C
*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.
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 the AD7302 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.
WARNING!
ESD SENSITIVE DEVICE
ORDERING GUIDE
Model AD7302BN AD7302BR AD7302BRU
Temperature Range -40C to +105C -40C to +105C -40C to +105C
Package Options* N-20 R-20 RU-20
*N = Plastic DIP; R = Small Outline; RU =Thin Shrink Small Outline.
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AD7302
PIN FUNCTION DESCRIPTIONS
Pin No. 1-8 9 10 11 12 13
Mnemonic Function D7-D0 CS WR A/B PD LDAC Parallel Data Inputs. Eight-bit data is loaded to the input register of the AD7302 under the control of CS and WR. Chip Select. Active low logic input. Write Input. WR is an active low logic input used in conjunction with CS and A/B to write data to the selected DAC register. DAC Select. Address pin used to select writing to either DAC A or DAC B. Active low input used to put the part into low power mode reducing current consumption to less than 1 A. Load DAC Logic Input. When this logic input is taken low both DAC outputs are simultaneously updated with the contents of their DAC registers. If LDAC is permanently tied low, the DACs are updated on the rising edge of WR. Asynchronous Clear Input (Active Low). When this input is taken low the DAC registers are loaded with all zeroes and the DAC outputs are cleared to zero volts. Power Supply Input. These parts can be operated from 2.7 V to 5.5 V and should be decoupled to AGND. External Reference Input. This can used as the reference for both DACs. The range on this reference input is 1 V to VDD /2. If REFIN is directly tied to VDD the internal VDD /2 reference is selected. Analog Ground reference point and return point for all analog current on the part. Analog output voltage from DAC B. The output amplifier can swing rail to rail on its output. Analog output voltage from DAC A. The output amplifier can swing rail to rail on its output. Digital Ground reference point and return point for all digital current on the part.
PIN CONFIGURATION
(MSB) DB7 1 DB6 2 DB5 3 DB4 4 DB3 5 DB2 6 20 DGND 19 VOUTA 18 VOUTB 17 AGND
14 15 16 17 18 19 20
CLR VDD REFIN AGND VOUTB VOUTA DGND
AD7302
16 REFIN
TOP VIEW 15 V DD (Not to Scale) 14 DB1 7 (LSB) DB0 8 9 10 13 12 11 /B
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AD7302
TERMINOLOGY
INTEGRAL NONLINEARITY DIGITAL FEEDTHROUGH
For the DACs, relative accuracy or endpoint nonlinearity is a measure of the maximum deviation, in LSBs, from a straight line passing through the endpoints of the DAC transfer function. A graphical representation of the transfer curve is shown in Figure 14.
DIFFERENTIAL NONLINEARITY
Digital Feedthrough is a measure of the impulse injected into the analog output of a DAC from the digital inputs of the same DAC, but is measured when the DAC is not updated. It is specified in nV-s and measured with a full-scale code change on the data bus, i.e., from all 0s to all 1s and vice versa.
DIGITAL CROSSTALK
Differential Nonlinearity is the difference between the measured change and the ideal 1 LSB change between any two adjacent codes. A specified differential nonlinearity of 1 LSB maximum ensures monotonicity.
ZERO CODE ERROR
Digital Crosstalk is the glitch impulse transferred to the output of one converter due to a digital code change to another DAC. It is specified in nV-s.
ANALOG CROSSTALK
Analog Crosstalk is a change in output of any DAC in response to a change in the output of the other DAC. It is measured in LSBs.
POWER SUPPLY REJECTION RATIO (PSRR)
Zero Code Error is the measured output voltage from VOUT of either DAC when zero code (all zeros) is loaded to the DAC latch. It is due to a combination of the offset errors in the DAC and output amplifier. Zero scale error is expressed in LSBs.
GAIN ERROR
This is a measure of the span error of the DAC. It is the deviation in slope of the DAC transfer characteristic from ideal, expressed as a percent of the full-scale value. It includes full-scale errors but not offset errors.
DIGITAL-TO-ANALOG GLITCH IMPULSE
This specification indicates how the output of the DAC is affected by changes in the power supply voltage. Power supply rejection ratio is quoted in terms of % change in output per % change in VDD for full-scale output of the DAC. VDD is varied 10%.
Digital-to-Analog Glitch Impulse is the impulse injected into the analog output when the digital inputs change state with the DAC selected and the LDAC used to update the DAC. It is normally specified as the area of the glitch in nV-s and is measured when the digital input code is changed by 1 LSB at the major carry transition.
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Typical Performance Characteristics-AD7302
800 720 640 560 VOUT - mV 480 400 320 240 160 80 0 0 2 4 6 SINK CURRENT - mA 8 VDD = 5V AND 3V INTERNAL REFERENCE TA = +25 C DAC LOADED WITH 00HEX 5 4.92 4.84 4.76 VOUT - Volts 4.68 4.6 4.52 4.44 4.36 4.28 4.2 0 VDD = 5V INTERNAL REFERENCE DAC REGISTER LOADED WITH FFHEX TA = +25C 2 4 6 SOURCE CURRENT - mA 8 VOUT - Volts 3.5 3.25 3.0 2.75 2.5 2.25 2.0 1.75 1.5 1.25 1.0 0 VDD = 3V INTERNAL REFERENCE DAC REGISTER LOADED WITH FFHex TA = +25C 1 2 3 4 5 6 SOURCE CURRENT - mA 7 8
Figure 2. Output Sink Current Capability with VDD = 3 V and VDD = 5 V
Figure 3. Output Source Current Capability with V DD = 5 V
Figure 4. Output Source Current Capability with VDD = 3 V
0.5 0.45 0.4 0.35 ERROR - LSBs 0.3 0.25 0.2 0.15 DNL ERROR 0.1 0.05 0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 REFERENCE VOLTAGE - Volts IDD - mA INL ERROR VDD = 5V TA = +25 C
5.0 4.5
7.0 6.0 BOTH DACS ACTIVE INTERNAL REFERENCE USED TA = +25C
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -50 VDD = 3.3V INTERNAL REFERENCE LOGIC INPUTS = VDD OR GND BOTH DACS ACTIVE VDD = 5.5V IDD - mA 5.0
LOGIC INPUTS = VIH OR VIL
4.0
3.0
2.0 LOGIC INPUTS = VDD OR GND 100 125 1.0 2.5 3.0 3.5 4.0 4.5 VDD - Volts 5.0 5.5
-25
0 25 50 75 TEMPERATURE - C
Figure 5. Relative Accuracy vs. External Reference
Figure 6. Typical Supply Current vs. Temperature
Figure 7. Typical Supply Current vs. Supply Voltage
10 5 0 ATTENUATION - dB -5 -10 -15 -20 -25 -30 -35 -40 1 VDD = 5V EXTERNAL SINEWAVE REFERENCE DAC REGISTER LOADED WITH FFHEX TA = +25C 10 100 1k FREQUENCY - Hz 10k
T 1 PD 2 2 VOUT VOUT VDD = 3V INTERNAL VOLTAGE REFERENCE FULL SCALE CODE CHANGE 00H-FFH TA = +25C CH1 5V, CH2 1V, CH3 20mV TIME BASE = 200 ns/Div VOUT AD7302 POWER-UP TIME VDD = 5V INTERNAL REFERENCE DAC IN POWER-DOWN INITIALLY CH1 = 2V/div, CH2 = 5V/Div, TIME BASE = 2 s/Div
1
3
Figure 8. Large Scale Signal Frequency Response
Figure 9. Full-Scale Settling Time
Figure 10. Exiting Power-Down (Full Power-Down)
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AD7302
10
T
9 VDD ZERO CODE ERROR - LSB 8 7 6 5 4
1
VDD = 2.7 TO 5.5V DAC LOADED WITH ALL ZEROES INTERNAL REFERENCE
1
2
T
VDD = 5V INTERNAL VOLTAGE REFERENCE 10 LSB STEP CHANGE TA = +25 C VOUT
VOA
T
DAC B 3 2 1 DAC A 2 -25 0 25 50 75 100 125 CH1 5.00V, CH2 50.0mV, M 250ns
3 VOB
CH1 CH3
5.00V CH2 5.00V
5.00V M20.0ms CH1
0 -50
TEMPERATURE - C
Figure 11. Power-On--RESET
Figure 12. Zero Code Error vs. Temperature
0.5 0.4 0.3
Figure 13. Small-Scale Settling Time
0.5 0.4 0.3 INL ERROR - LSB 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 0 32 64 96 128 160 192 224 256 INPUT CODE (10 to 245) DAC B VDD = 5V INTERNAL REFERENCE 5k 100pf. LOAD LIMITED CODE RANGE (10-245) TA = +25C DAC A INL ERROR - LSB
0.5 0.4 0.3 DNL ERROR - LSB VDD = 5V INTERNAL REFERENCE 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0 20 40 60 80 100 120 140 TEMPERATURE - C -0.5 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE - C VDD = 5V INTERNAL REFERENCE
0.2 0.1 0 -0.1 -0.2 -0.3 -0.4
-0.5 -60 -40 -20
Figure 14. Integral Linearity Plot
Figure 15. Typical INL vs. Temperature
Figure 16. Typical DNL vs. Temperature
1.0
POWER-DOWN CURRENT - nA
1000
%
VDD = 5V 0.8
900 800 700 600 500 400 300 200 100
VDD = 5V LOGIC INPUTS = VDD OR GND
INT REFERENCE ERROR -
0.6
0.4
0.2
0 -60 -40 -20
0 -50
-25
0 20 40 60 80 100 120 140 TEMPERATURE - C
25 50 75 0 TEMPERATURE - C
100
125
Figure 17. Typical Internal Reference Error vs. Temperature
Figure 18. Power-Down Current vs. Temperature
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AD7302
GENERAL DESCRIPTION
D/A Section The AD7302 is a dual 8-bit voltage output digital-to-analog converter. The architecture consists of a reference amplifier, a current source DAC followed by a current-to-voltage converter capable of generating rail-to-rail voltages on the output of the DAC. Figure 19 shows a block diagram of the basic DAC architecture.
VDD 30k REFIN CURRENT DAC 11.7k 30k I/V VO A/B REFERENCE AMPLIFIER
The internal reference is selected by tying the REFIN pin to VDD. If an external reference is to be used, this can be directly applied to the REFIN pin; if this is 1 V below VDD , the internal circuitry will select this externally applied reference as the reference source for the DAC.
Digital Interface
AD7302
11.7k
The AD7302 contains a fast parallel interface allowing this dual DAC to interface to industry standard microprocessors, microcontrollers and DSP machines. There are two modes in which this parallel interface can be configured to update the DAC outputs. The simultaneous update mode allows simultaneous updating of both DAC outputs. The automatic update mode allows each DAC to be individually updated following a write cycle. Figure 21 shows the internal logic associated with the digital interface. The PON STRB signal is internally generated from the power on reset circuitry and is low during the poweron reset phase of the power-up procedure.
CLR CLR PON STRB LDAC CLEAR SET SLE DAC A CONTROL LOGIC MLE A
Figure 19. DAC Architecture
Both DAC A and DAC B outputs are internally buffered and these output buffer amplifiers have rail-to-rail output characteristics. The output amplifier is capable driving a load of 10 k to both VDD and ground in parallel with a 100 pF to ground. The reference selection for the DAC can either be internally generated from VDD or externally applied through the REFIN pin. A comparator on the REFIN pin detects whether the required reference is the internally generated reference or the externally applied voltage to the REFIN pin. If REFIN is connected to VDD, the reference selected is the internally generated VDD/2 reference. When an externally applied voltage is more than one volt below VDD, the comparator selection switches to the externally applied voltage to the REFIN pin. The range on the external reference input is from 1.0 V to VDD/2. The output voltage from either DAC is given by: VO A/B = 2 x VREF x (N/256) where: VREF is the voltage applied to the external REFIN pin or VDD/2 when the internal reference is selected. N is the decimal equivalent of the code loaded to the DAC register and ranges from 0 to 255.
LDAC DAC A SEL ENABLE A/B CLEAR
SLE A
DAC B CONTROL LOGIC DAC B SEL SET SLE LDAC CS WR ENABLE
MLE B
SLE B
Figure 21. Logic Interface
The AD7302 has a double buffered interface, which allows for simultaneous updating of the DAC outputs. Figure 22 shows a block diagram of the register arrangement within the AD7302.
DB7-DB0
INPUT REGISTER 8 4 4 TO 15 DECODER 15 A/B CS MLE SLE DAC REGISTER 15 DRIVERS 30 LOWER NIBBLE 4 4 TO 15 DECODER 15 DAC REGISTER 15 DRIVERS 30 UPPER NIBBLE
Reference
The AD7302 has the facility to use either an external reference applied through the REFIN pin or an internal reference generated from VDD . Figure 20 shows the reference input arrangement where either the internal VDD /2 reference or the externally applied reference can be selected.
VDD VTH PMOS INT REF REFIN
CONTROL LOGIC
WR LDAC COMPARATOR CLR
EXT REF INT REF MUX SELECTED REFERENCE OUTPUT
Figure 22. Register Arrangement
Figure 20. Reference Selection Circuitry
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AD7302
Automatic Update Mode POWER-ON RESET
In this mode of operation the LDAC signal is permanently tied low. The state of the LDAC is sampled on the rising edge of WR. LDAC being low allows the selected DAC register to be automatically updated on the rising edge of WR. The output update occurs on the rising edge of WR. Figure 23 shows the timing associated with the automatic update mode of operation and also the status of the various registers during this frame.
A/B
The AD7302 has a power-on reset circuit designed to allow output stability during power-up. This circuit holds the DACs in a reset state until a write takes place to the DAC. In the reset state all zeros are latched into the input registers of each DAC and the DAC registers are in transparent mode, thus the output of both DACs is held at ground potential until a write takes place to the DAC. The power-on reset circuitry generates a PON STRB signal, which is a gating signal used within the logic to identify a power-on condition.
POWER-DOWN FEATURES
CS
WR D7-D0 LDAC = 0 I/P REG (MLE) HOLD TRACK HOLD
DAC REG (SLE)
TRACK
HOLD
TRACK
VOUT
Figure 23. Timing and Register Arrangement for Automatic Update Mode
Simultaneous Update Mode
The AD7302 has a power-down feature. This is implemented by exercising the external PD pin; an active low signal puts the complete DAC into power-down mode. When in power-down the current consumption of the device is reduced to 1 A max at 25C and 2 A max over temperature, making the device suitable for use in portable battery powered equipment. When power-down is activated, the reference bias servo loop and the output amplifiers with their associated linear circuitry are powered down, the reference resistors are open circuited to further reduce the power consumption. The output sees a load of approximately 23 k to GND when in power-down mode as shown in Figure 25. The contents of the data registers are unaffected when in power-down mode. The device comes out of power-down in typically 13 s (see Figure 10).
11.7k VDD IDAC 11.7k VREF
In this mode of operation the LDAC signal is used to update both DAC outputs simultaneously. The state of the LDAC is sampled on the rising edge of WR. If LDAC is high, the automatic update mode is disabled and both DAC latches are updated at any time after the write by taking LDAC low. The output update occurs on the falling edge of LDAC. LDAC must be taken back high again before the next data transfer takes place. Figure 24 shows the timing associated with the simultaneous update mode of operation and also the status of the various registers during this frame.
A/B
Figure 25. Output Stage During Power-Down
Analog Outputs
CS
WR
D7-D0
The AD7302 contains two independent voltage output DACs with 8-bit resolution and rail-to-rail operation. The output buffer provides a gain of two at the output. Figures 2 to 4 show the source and sink capabilities of the output amplifier. The slew rate of the output amplifier is typically 7.5 V/s and has a fullscale settling to 8 bits with a 100 pF capacitive load in typically 1.2 s. The input coding to the DAC is straight binary. Table I shows the binary transfer function for the AD7302. Figure 26 shows the DAC transfer function for binary coding. Any DAC output voltage can be expressed as:
TRACK HOLD
LDAC I/P REG (MLE) HOLD TRACK HOLD
DAC REG (SLE)
HOLD
VOUT = 2 x VREF (N/256) where: N is the decimal equivalent of the binary input code. N ranges from 0 to 255.
VOUT
Figure 24. Timing and Register Arrangement for Simultaneous Update Mode
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AD7302
VREF is the voltage applied to the external REFIN pin when the external reference is selected and is VDD/2 if the internal reference is used.
Table I. Output Voltage for Selected Input Codes
VDD = 3 TO 5V 0.1F 10F
Digital Input MSB . . . LSB 1111 1111 1111 1110 1000 0001 1000 0000 0111 1111 0000 0001 0000 0000
VDD
AGND DGND VOUTA
Analog Output 2 x 255/256 x VREF V 2 x 254/256 x VREF V 2 x 129/256 x VREF V VREF V 2 x 127/256 x VREF V 2 x VREF/256 V 0V
REF IN
AD7302
CLR PD VDD D7-D0 A/B CS
VOUTA
VOUTB WR LDAC
VOUTB
DATA BUS
CONTROL INPUTS
Figure 27. Typical Configuration Selecting the Internal Reference
2.VREF
DAC OUTPUT VOLTAGE
VREF
Figure 28 shows a typical setup for the AD7302 when using an external reference. The reference range for the AD7302 is from 1 V to VDD /2 V. Higher values of reference can be incorporated, but will saturate the output at both the top and bottom end of the transfer function. There is a gain of two from input to output on the AD7302. Suitable references for 5 V operation are the AD780 and REF192. For 3 V operation a suitable external reference would be the AD589 a 1.23 V bandgap reference.
VDD = 3 TO 5V
0
VIN
0.1F
10F
DAC INPUT CODE
00
01
7F
80
81
FE
FF
EXT REF
VOUT GND 0.1F CLR REF IN
VDD
AGND DGND VOUTA VOUTA
AD7302
VOUTB A/B CS WR LDAC VOUTB PD
Figure 26. DAC Transfer Function
Figure 27 shows a typical setup for the AD7302 when using its internal reference. The internal reference is selected by tying the REFIN pin to VDD . Internally in the reference section there is a reference detect circuit that will select the internal VDD /2 based on the voltage connected to the REFIN pin. If REFIN is within a threshold voltage of a PMOS device (approximately 1 V) of VDD the internal reference is selected. When the REFIN voltage is more than 1 V below VDD , the externally applied voltage at this pin is used as the reference for the DAC. The internal reference on the AD7302 is VDD/2, the output current to voltage converter within the AD7302 provides a gain of two. Thus the output range of the DAC is from 0 V to VDD, based on Table I.
AD780/REF192
WITH VDD = 5V OR VDD
D7-D0
AD589
WITH VDD = 3V DATA BUS CONTROL INPUTS
Figure 28. Typical Configuration Using An External Reference
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AD7302
MICROPROCESSOR INTERFACING AD7302-ADSP-2101/ADSP-2103 Interface
A15 ADDRESS BUS A0 A** IS EN ADDR DECODE A+1** A/B CS
Figure 29 shows an interface between the AD7302 and the ADSP-2101/ADSP-2103. The fast interface timing associated with the AD7302 allows easy interface to the ADSP-2101/ ADSP-2103.
TMS32020
DMA14 ADDRESS BUS DMA0 A** DMS EN ADDR DECODE A+1** A/B CS
AD7302*
LDAC
STRB R/W
WR DB7 DB0
ADSP-2101*/ ADSP-2103*
WR
AD7302*
LDAC
WR DB7 DB0
DMD15 DATA BUS DMD0 **ADDITIONAL CIRCUITRY OMITTED FOR CLARITY. **A DECODED ADDRESS FOR DAC A. **A+1 DECODED ADDRESS FOR DAC B.
Figure 30. AD7302-TMS32020 Interface
DMD15 DATA BUS DMD0 **ADDITIONAL CIRCUITRY OMITTED FOR CLARITY. **A DECODED ADDRESS FOR DAC A. **A+1 DECODED ADDRESS FOR DAC B.
Figure 29. AD7302-ADSP-2101/ADSP-2103 Interface
Two addresses are decoded to select loading data to either DAC A or DAC B. LDAC is permanently tied low in this circuit, so the selected DAC output is updated on the rising edge of the WR signal. Data is loaded to the AD7302 input register using the following ADSP-21xx instruction: DM (DAC) = MR0 MR0 = ADSP-21xx MR0 Register. DAC = Decoded DAC Address.
AD7302-TMS32020 Interface
In the circuit shown the LDAC is hardwired low, thus the selected DAC output is updated on the rising edge of WR. Some applications may require simultaneous updating of both DACs in the AD7302. In this case the LDAC signal can be driven from an external timer or can be controlled by the microprocessor. One option for simultaneous updating is to decode the LDAC from the address bus so that a write operation at this address will simultaneously update both DAC outputs. A simple OR gate with one input driven from the decoded address and the second input from the WR signal will implement this function.
AD7302-8051/8088 Interface
Figure 31 shows a serial interface between the AD7302 and the 8051/8088 processors. The address decoder is used to decode the addresses for DAC A and DAC B.
A15 ADDRESS BUS A8 A** PSEN OR DEN EN ADDR DECODE A/B CS A+1**
Figure 30 shows an interface between the AD7302 and the TMS32020. The address decoder is used to decode the addresses for DAC A and DAC B. Data is loaded to the AD7302 using the following instruction: OUT DAC, D DAC = Decoded DAC Address. D = Data Memory Address.
AD7302*
WR
WR
8051/8088
ALE OCTAL LATCH
LDAC
DB7 DB0
AD7 ADDRESS/DATA BUS AD0 **ADDITIONAL CIRCUITRY OMITTED FOR CLARITY. **A DECODED ADDRESS FOR DAC A. **A+1 DECODED ADDRESS FOR DAC B.
Figure 31. AD7302-8051//8088 Interface
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AD7302
APPLICATIONS Bipolar Operation Using the AD7302
DATA BUS WR VDD VCC ENABLE CODED ADDRESS 1G 1A 1B 1Y0 1Y1 1Y2
AD7302
CS WR D0 D8 LDAC VOUTA VOUTB
The AD7302 has been designed for single supply operation, but bipolar operation is achievable using the circuit shown in Figure 32. The circuit shown has been configured to achieve an output voltage range of -5 V < VO < +5 V. Rail-to-rail operation at the amplifier output is achievable using an AD820 or OP295 as the output amplifier. The output voltage for any input code can be calculated as follows:
VO = [(1+R4/R3) x (R2/(R1+R2) x (2 x VREF x D/256)] - R4 x VREF/R3
AD7302
CS WR D0 D8 LDAC VOUTA VOUTB
74HC139
DGND
1Y3
where D is the decimal equivalent of the code loaded to the DAC and VREF is the reference voltage input. With VREF = 2.5 V, R1 = R3 = 10 k and R2 = R4 = 20 k and VDD = 5 V. VOUT = (10 x D/256) - 5 V
VDD = 5V R4 20k +5V
AD7302
CS WR D0 D8 LDAC VOUTA VOUTB
AD7302
CS WR D0 D8 LDAC VOUTA VOUTB
0.1F
10F R3 10k
Figure 33. Decoding Multiple AD7302 DACs in a System
5V
VIN
AD7302 As a Digitally Programmable Window Detector
EXT REF
VOUT GND 0.1F REF IN
VDD R1 10k -5V
AD820/ OP295
AD7302
VOUTA
AD780/REF192
WITH VDD = 5V OR
AGND
DGND
R2 20k
A digitally programmable upper/lower limit detector using the two DACs in the AD7302 is shown in Figure 34. The upper and lower limits for the test are loaded to DACs A and B, which in turn set the limits on the CMP04. If a signal at the VIN input is not within the programmed window an LED will indicate the fail condition.
+5V 0.1F 10F VIN REFIN PD D7 D0 A/B CS WR DVDD CLR LDAC DGND AGND VOUTB 1/6 74HC05 VOUTA VDD 1k FAIL 1/2 1k PASS
AD589
WITH VDD = 3V
Figure 32. Bipolar Operation Using the AD7302
Decoding Multiple AD7302 in a System
The CS pin on the AD7302 can be used in applications to decode a number of DACs. In this application all DACs in the system receive the same input data, but only the CS to one of the DACs will be active at any one time allowing access to two channels in the system. The 74HC139 is used as a two-to-four line decoder to address any of the DACs in the system. To prevent timing errors from occurring, the enable input should be brought to its inactive state while the coded address inputs are changing state. Figure 33 shows a diagram of a typical setup for decoding multiple AD7302 devices in a system. The built-in power-on reset circuit on the AD7302 ensures that the outputs of all DACs in the system power up with zero volts on their outputs.
CMP04
AD7302
PASS/FAIL
Figure 34. Programmable Window Detector
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AD7302
Programmable Current Source
VDD = 5V
Figure 35 shows the AD7302 used as the control element of a programmable current source. In this circuit the full-scale current is set to 1 mA. The output voltage from the DAC is applied across the current setting resistor of 4.7 k in series with the full-scale setting resistor of 470 . Transistors suitable to place in the feedback loop of the amplifier include the BC107 or the 2N3904, which enable the current source to operate from a min VSOURCE of 6 V. The operating range is determined by the operating characteristics of the of the transistor. Suitable amplifiers include the AD820 and the OP295 both having railto-rail operation on their outputs. The current for any digital input code can be calculated as follows:
R4 51.2k
R4 390 +5V
0.1F
10F
VIN
VOUT VDD REF IN 0.1F VOUTA R2 51.2k R1 390
EXT REF
VOUT GND
AD820/ OP295
AD7302
VOUTB AGND DGND
AD780/REF192
WITH VDD = 5V OR
AD589
WITH VDD = 3V
I = 2 x VREF x D/(5E +3 x 256) mA
VDD = 5V
Figure 36. Coarse/Fine Adjust Circuit
Power Supply Bypassing and Grounding
0.1F
10F VSOURCE
VIN
EXT REF
VOUT GND 0.1F REF IN
VDD VOUTA
+5V
LOAD
AD7302
AGND DGND
AD820/ OP295
AD780/REF192
WITH VDD = 5V
4.7k 470
Figure 35. Programmable Current Source
Coarse and Fine Adjustment Using the AD7302
In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. The printed circuit board on which the AD7302 is mounted should be designed so the analog and digital sections are separated and confined to certain areas of the board. If the AD7302 is in a system where multiple devices require an AGND to DGND connection, the connection should be made at one point only, a star ground point that should be established as closely as possible to the AD7302. The AD7302 should have ample supply bypassing of 10 F in parallel with 0.1 F on the supply located as close to the package as possible, ideally right up against the device. The 10 F capacitors are the tantalum bead type. The 0.1 F capacitor should have low Effective Series Resistance (ESR) and Effective Series Inductance (ESI), such as the common ceramic types, which provide a low impedance path to ground at high frequencies to handle transient currents due to internal logic switching. The power supply lines of the AD7302 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other parts of the board and should never be run near the reference inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This reduces the effects of feedthrough through the board. A microstrip technique is by far the best, but not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground plane while signal traces are placed on the solder side.
The DACs on the AD7302 can be paired together to form a coarse and fine adjustment function as shown in Figure 36. In this circuit DAC A is used to provide the coarse function while DAC B is used to provide the fine adjustment. Varying the ratio of R1 and R2 will vary the relative effect of the coarse and fine tune elements in the circuit. For the resistor values shown DAC B has a resolution of 148 V giving a fine tune range of approximately 2 LSBs for operation with a VDD of 5 V and a reference of 2.5 V. The amplifiers shown allow a rail-to-rail output voltage to be achieved on the output. A typical application for such a circuit would be in a setpoint controller.
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AD7302
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Lead Plastic DIP (N-20)
1.060 (26.90) 0.925 (23.50)
20 1 11 10
0.280 (7.11) 0.240 (6.10)
PIN 1 0.210 (5.33) MAX 0.160 (4.06) 0.115 (2.93) 0.022 (0.558) 0.014 (0.356) 0.100 (2.54) BSC
0.060 (1.52) 0.015 (0.38) 0.130 (3.30) MIN 0.070 (1.77) SEATING 0.045 (1.15) PLANE
0.325 (8.25) 0.300 (7.62) 0.195 (4.95) 0.115 (2.93)
0.015 (0.381) 0.008 (0.204)
20-Lead SO (R-20)
0.5118 (13.00) 0.4961 (12.60)
20 11
1
10
PIN 1
0.1043 (2.65) 0.0926 (2.35)
0.4193 (10.65) 0.3937 (10.00)
0.2992 (7.60) 0.2914 (7.40)
0.0291 (0.74) x 45 0.0098 (0.25)
0.0118 (0.30) 0.0040 (0.10)
8 0.0500 0.0192 (0.49) 0 (1.27) 0.0138 (0.35) SEATING 0.0125 (0.32) PLANE BSC 0.0091 (0.23)
0.0500 (1.27) 0.0157 (0.40)
20-Lead TSSOP (RU-20)
0.260 (6.60) 0.252 (6.40)
20
11
0.177 (4.50) 0.169 (4.30)
1
10
0.006 (0.15) 0.002 (0.05)
PIN 1 0.0433 (1.10) MAX 0.0256 (0.65) BSC 0.0118 (0.30) 0.0075 (0.19) 0.0079 (0.20) 0.0035 (0.090)
0.256 (6.50) 0.246 (6.25)
SEATING PLANE
8 0
0.028 (0.70) 0.020 (0.50)
REV. 0
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C2990-12-4/97
PRINTED IN U.S.A.

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