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1
TC811 3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS
FEATURES
s s s s s s s s s s s s s s Differential Reference Input Display Hold Function Fast Over-Range Recovery, Guaranteed Next Reading Accuracy Low Temperature Drift Internal Reference ....................................... 35ppm/C (Typ) Guaranteed Zero Reading With Zero Input Low Noise ..................................................... 15Vp-p High Resolution (0.05%) and Wide Dynamic Range (72 dB) High Impedance Differential Input Low Input Leakage Current ....................... 1pA Typ 10pA Max Direct LCD Drive - No External Components Precision Null Detection with True Polarity at Zero Crystal Clock Oscillator Available in DIP, Compact Flat Package or PLCC Convenient 9V Battery Operation with Low Power Dissipation (600A Typical, 1mW Maximum)
2 3 4
Max VREF Tempco
TYPICAL APPLICATIONS
s s Thermometry Digital Meters -- Voltage/Current/Power -- pH Measurement -- Capacitance/Inductance -- Fluid Flow Rate/Viscosity -- Humidity -- Position Panel Meters LVDT Indicators Portable Instrumentation Digital Scales Process Monitors Gaussometers Photometers
s s s s s s s
ORDERING INFORMATION
Part No. Package Temp. Range
FUNCTIONAL BLOCK DIAGRAM
TC811CKW 44-PQFP 0C to +70C 75 ppm/C TC811CPL 40-Pin Plastic DIP 0C to +70C 75 ppm/C
TYPICAL SEGMENT OUTPUT + V 0.5mA SEGMENT OUTPUT 2mA
5 6 7
LCD DISPLAY
TC811CPL
INTERNAL DIGITAL GROUND BACKPLANE 21
+ C REF
+ VREF 35 36
CREF
- VREF 33
RINT - C REF V BUFF 34 28 V + 38
CAZ
CINT VINT LCD SEGMENT DRIVERS
29 INTEGRATOR
27 TO DIGITAL SECTION 7 SEGMENT DECODE 7 SEGMENT DECODE 7 SEGMENT DECODE 200
10 A 31 INT A/Z 32
ZI & A/Z
ZI & A/Z
- + ZI
- +
+ -
DATA LATCH
A/Z COMPARATOR
THOUSANDS HUNDREDS TENS UNITS
+ V IN
DE (-)
DE (+) - LOW TEMPCO VREF
ANALOG COMMON
DE (+)
DE (-)
+ V+- 3.0V
TO SWITCH DRIVERS FROM COMPARATOR OUTPUT CLOCK fOSC 70k 4 CONTROL LOGIC
38 6.2V
V+
- 30 V IN INT
AZ & DE () 26 -
INTERNAL DIGITAL GOUND
TEST VTH = 1V 1 500 26 V-
V
40 OSC 1
39 22M OSC 2
HLDR
470k V + 10pF kH 20pF V +
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TC811-7 11/5/96
TELCOM SEMICONDUCTOR, INC.
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
GENERAL DESCRIPTION
The TC811 is a low power, 3-1/2 digit, LCD display analog-to-digital converter. This device incorporates both a display hold feature and differential reference inputs. A crystal oscillator, which only requires two pins, permits added features while retaining a 40-pin package. An additional feature is an "Integrator Output Zero" phase which guarantees rapid input overrange recovery. The TC811 display hold (HLDR) function can be used to "freeze" the LCD display. The displayed reading will remain indefinitely as long as HLDR is held high. Conversions continue but the output data display latches are not updated. The TC811 also includes a differential reference for easy ratiometric measurements. Circuits which use the 7106/26/36 can easily be upgraded to include the hold function with the TC811. The TC811 has an improved internal zener reference voltage circuit which maintains the Analog Common temperature drift to 35ppm/C (typical) and 75ppm/C (maximum). This represents an improvement of two to four times over similar 3-1/2 digit converters, eliminating the need for a costly, space consuming external reference source. The TC811 limits linearity error to less than one count on both the 200mV and the 2.00V full-scale ranges. Rollover error--the difference in readings for equal magnitude but opposite polarity input signals--is below 1 count. High impedance differential inputs offer 1pA leakage currents and a 1012 input impedance. The 15Vp-p noise performance guarantees a "rock solid" reading. The Auto Zero cycle guarantees a zero display readout for a zero volt input. The single chip CMOS TC811 incorporates all the active devices for a 3-1/2 digit analog to digital converter to directly drive an LCD display. Onboard oscillator, precision voltage reference and display segment and backplane drivers simplify system integration, reduce board space requirements and lower total cost. A low cost, high resolution (0.05%) indicating meter requires only a TC811, an LCD display, five resistors, six capacitors, a crystal, and a 9V battery. Compact, hand held multimeter designs benefit from the TelCom Semiconductor small footprint package option. The TC811 uses a dual slope conversion technique which will reject interference signals if the converters integration time is set to a multiple of the interference signal period. This is especially useful in industrial measurement environments where 50, 60 and 400Hz line frequency signals are present.
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage (V+ to V -)............................................ 15V Analog Input voltage (Either Input)1 ..................... V+ to V - Reference Input Voltage ...................................... V+ to V - Clock Input ...................................................... TEST to V+ Power Dissipation2 (TA 70C) 44-Pin Flat Package .........................................1.00W 40-Pin Plastic DIP .............................................1.23W Operating Temperature Range Commercial Package (C) ...................... 0C to +70C Industrial Package (I) ........................ - 25C to +85C Storage Temperature Range ................ - 65C to +150C Lead Temperature (Soldering, 10 sec) ................. +300C
*Static-sensitive device. Unused devices must be stored in conductive material. Protect devices from static discharge and static fields. Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operation sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS: VSupply = 9V, fCLOCK = 32.768kHz, and TA = 25C, unless otherwise noted.
Symbol
Input -- -- -- NL ER eN IL CMRR
Parameter
Zero Input Reading Zero Reading Drift Ratiometric Reading Linearity Error Roll Over Error Noise Input Leakage Current Common-Mode Rejection
Test Conditions
VIN = 0V VFS = 200mV VIN = 0V, 0C TA 70C VIN = VREF, VREF = 100mV VFS = 200mV or 2.000V VIN- = VIN+ 200mV VIN = 0V, VFS = 200mV VIN = 0V VCM = 1V, VIN = 0V, VFS = 200mV
Min
- 000.0 -- 999 -1 -1 -- -- --
Typ
000.0 0.2 999/1000 0.2 0.2 15 1 50
Max
+000.0 1 1000 +1 +1 -- 10 --
Unit
Digital Reading V/C Digital Reading Counts Counts VP-P pA V/V
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TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
ELECTRICAL CHARACTERISTICS: VSupply = 9V, fCLOCK = 32.768kHz, and TA = 25C, unless otherwise noted.
Symbol
TCSF
1
Parameter
Scale Factor Temperature Coefficient
Test Conditions
VIN = 199mV, 0C TA 70C (ext. VREF tc = 0ppm) 250K from V+ to Analog Common 0C TA 70C "C" Commercial "I" Industrial 250k from V+ to Analog Common Pin 1 to Pin 37 Pin 1 Pin 1 V + to V - = 9V V+ to V- = 9V VIN = 0V, V + to V- = 9V fOSC = 16kHz fOSC = 48kHz
Min
--
Typ
1
Max
5
Unit
ppm/C
2 3 4 5 6 7
Analog Common Section VCTC Analog Common Temperature Coefficient
VC
Analog Common Voltage
-- -- -- -- 2.7 -- -- V+ - 1.5 4 4 -- -- --
-- -- 35 35 3.05 70 -- -- 5 5 -- 70 90
-- -- 75 100 3.35 -- Test +1.5 -- 6 6 -- 100 125
-- -- ppm/C ppm/C Volts k V V VP-P VP-P -- A A
Hold Pin Input Section Input Resistance VIL Input Low Voltage VIH Input High Voltage LCD Drive Section3 VSD LCD Segment Drive Voltage VSD LCD Backplane Drive Voltage Power Supply ISUP Power Supply Current
NOTES: 1. Input voltages may exceed supply voltages when input current is limited to 100A. 2. Dissipation rating assumes device is mounted with all leads soldered to a printed circuit board. 3. Backplane drive is in phase with the segment drive for "segment off" 180 out of phase for "segment on." Frequency is 20 times the conversion rate. Average DC component is less than 50mV.
PIN CONFIGURATIONS
V INT V-
35 34
D1 C1 B1 1's A1 F1 G1 E1 D2 C2 10's B2 A2 F2 E2 D3 100's B3 F3 E3 1000's AB4
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
44 43
42
41 40 39
VIN + VIN -
38
COM
HLDR
1
40 OSC 1 NORMAL PIN CONFIGURATION 39 OSC 2 38 V + 37 36 35 34 33 32 31 TC811CPL (40-PIN PDIP) 30 29 28 27 26 25 24 23 22 21 TEST V REF+ CREF+ CREF- VREF- ANALOG COMMON + V IN V IN- CAZ VBUFF V INT V- G2 C3 A3 G3 BP (BACKPLANE) 100's 10's
NC 1 NC 2 TEST 3
V+ 4
VREF+ + CREF CREF - VREF-
37
C AZ V BUFF
36
33 NC 32 G
2
31 C 3 30 A 3 29 G 3
NC 5
OSC 2 6 OSC 1 7
TC811CKW (PQFP)
28 BP 27 POL 26 AB 4 25 E3 24 F3 23 B3
HLDR 8 D1 9 C 1 10 B 1 11
12 13 14 15 16 17 18 19 20 21 22
D2
C2
B2
A2
A1
G1
POL 20 (MINUS SIGN)
E1
E2
D3
F1
F2
NC = NO INTERNAL CONNECTION
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
PIN DESCRIPTION
Pin No. 40-Pin Plastic DIP
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Symbol
HLDR D1 C1 B1 A1 F1 G1 E1 D2 C2 B2 A2 F2 E2 D3 B3 F3 E3 AB4 POL BP G3 A3 C3 G2 V- VINT VBUFF CAZ VIN- VIN+ COM VREF- CREF- CREF+ VREF+ TEST V+ OSC2 OSC1
Description
Hold pin, logic 1 holds present display reading. Activates the D section of the units display. Activates the C section of the units display. Activates the B section of the units display. Activates the A section of the units display. Activates the F section of the units display. Activates the G section of the units display. Activates the E section of the units display. Activates the D section of the tens display. Activates the C section of the tens display. Activates the B section of the tens display. Activates the A section of the tens display. Activates the F section of the tens display. Activates the E section of the tens display. Activates the D section of the hundreds display. Activates the B section of the hundreds display. Activates the F section of the hundreds display. Activates the E section of the hundreds display. Activates both halves of the 1 in the thousands display. Activates the negative polarity display. Backplane drive output. Activates the G section of the hundreds display. Activates the A section of the hundreds display. Activates the C section of the hundreds display. Activates the G section of the tens display. Negative power supply voltage. Integrator output, connection for CINT. Buffer output, connection for RINT. Integrator input, connection for CAZ. Analog input low. Analog input high. Analog Common: Internal zero reference. Reference input low. Negative connection for reference capacitor. Positive connection for reference capacitor. Reference input high. All LCD segment test when pulled high (V+). Positive power supply voltage. Crystal oscillator output. Crystal oscillator input.
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
0.1F 35 C+ REF 31 + V IN - 30 V IN 32 ANALOG COMMON 28 180k 0.47 F 29 27 VBUFF TC811 33 C 34 REF
1
C INTEGRATOR - COMPARATOR - VIN VIN VARIABLE REFERENCE INTEGRATE TIME
LCD 9-19 SEGMENT 22-25 DRIVE POL BP V + 20 21 38 240k 1 HLDR 36 + VREF - V REF 33 - 26 V OSC1 40 + 9V 10k MINUS SIGN BACKPLANE
ANALOG INPUT SIGNAL
1M + ANALOG INPUT - 0.01F
2
CLOCK
+/- SWITCH DRIVER REF VOLTAGE PHASE CONTROL CONTROL LOGIC POLARITY CONTROL
CAZ VINT OSC2 39 22M
0.068F
2 CONVERSION/SEC
INTEGRATOR OUTPUT
DISPLAY
TO ANALOG COMMON (PIN 32) 10pF V+
VFULL SCALE 1.2 VFULL SCALE
20pF V+
470k
FIXED SIGNAL INTEGRATE TIME
Figure 1. Typical Operating Circuit
Figure 2. Basic Dual Slope Converter
(All Pin Designations Refer to 40-Pin DIP Package) The TC811 is a dual slope, integrating analog-to-digital converter. An understanding of the dual slope conversion technique will aid the user in following the detailed TC811 theory of operation following this section. A conventional dual slope converter measurement cycle has two distinct phases: 1) Input Signal Integration 2) Reference Voltage Integration (Deintegration) Referring to Figure 2, the unknown input signal to be converted is integrated from zero for a fixed time period (TINT), measured by counting clock pulses. A constant reference voltage of the opposite polarity is then integrated until the integrator output voltage returns to zero. The reference integration (deintegration) time (TDEINT) is then directly proportional to the unknown input voltage (VIN). In a simple dual slope converter, a complete conversion requires the integrator output to "ramp-up" from zero and "ramp-down" back to zero. A simple mathematical equation relates the input signal, reference voltage and integration time: tINT 1 VREF tDEINT VIN(t) dt = RINT CINT 0 RINT CINT
NORMAL MODE REJECTION (dB)
GENERAL THEORY OF OPERATION Dual-Slope Conversion Principles
30
20
10
T = MEASUREMENT PERIOD 0 0.1/T 1/T INPUT FREQUENCY 10/T
Figure 3. Normal-Mode Rejection of Dual Slope Converter
For a constant VINT: VIN = VREF
[]
tDEINT tINT
where: VREF = Reference voltage tINT = Integration Time tDEINT = Deintegration Time TELCOM SEMICONDUCTOR, INC.
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+
+
3 4 5 6 7
COUNTER
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
Accuracy in a dual slope converter is unrelated to the integrating resistor and capacitor values as long as they are stable during a measurement cycle. An inherent benefit of the dual slope technique is noise immunity. Noise spikes are integrated or averaged to zero during the integration periods, making integration ADCs immune to the large conversion errors that plague successive approximation converters in high noise environments. Interfering signals, with frequency components at multiples of the averaging (integrating) period, will be attenuated. (see Figure 3). Integrating ADCs commonly operate with the signal integration period set to a multiple of the 50/60Hz power line period.
1000 INT 1-2000 11-140 ZI AZ 910-2900 4000
DE-INT
THEORY OF OPERATION Analog Section
In addition to the basic integrate and deintegrate dualslope cycles discussed above, the TC811 design incorporates an "Integrator Output Zero" cycle and an "Auto Zero" cycle. These additional cycles ensure the integrator starts at 0V (even after a severe overrange conversion) and that all offset voltage errors (buffer amplifier, integrator and comparator) are removed from the conversion. A true digital zero reading is assured without any external adjustments. A complete conversion consists of four distinct phases: (1) (2) (3) (4) Integrator Output Zero Cycle Auto Zero Cycle Signal Integrate Cycle Reference Deintegrate Cycle
Figure 4a. Conversion Timing During Normal Operation
INT
1000
DE-INT
2001-2090 31-640
ZI AZ 300-910 4000
Figure 4b. Conversion Timing During Overrange Operation
Integrator Output Zero Cycle
This phase guarantees that the integrator output is at zero volts before the system zero phase is entered, ensuring that the true system offset voltages will be compensated for even after an overrange conversion. The duration of this phase is variable, being a function of the number of counts (clock cycles) required for deintegration. The Integrator Output Zero cycle will last from 11 to 140 counts for non-over-range conversions and from 31 to 640 counts for overrange conversions.
during the measurement cycle. The Auto Zero cycle residual is typically 10 to 15V. The Auto Zero duration is from 910 to 2,900 counts for non-over-range conversions and from 300 to 910 counts for overrange conversions.
Signal Integration Cycle
Upon completion of the Auto Zero cycle, the Auto Zero loop is opened and the internal differential inputs connect to VIN+ and VIN-. The differential input signal is then integrated for a fixed time period which, in the TC811 is 1000 counts (4000 clock periods). The externally set clock frequency is divided by four before clocking the internal counters. The integration time period is: TINT = 4000 fOSC
Auto Zero Cycle
During the Auto Zero cycle, the differential input signal is disconnected from the measurement circuit by opening internal analog switches and the internal nodes are shorted to Analog Common (0V ref.) to establish a zero input condition. Additional analog switches close a feedback loop around the integrator and comparator to permit comparator offset voltage error compensation. A voltage established on CAZ then compensates for internal device offset voltages
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The differential input voltage must be within the device common-mode range when the converter and measured system share the same power supply common (ground).
TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
If the converter and measured system do not share the same power supply common, as in battery powered applications, VIN- should be tied to Analog Common. Polarity is determined at the end of signal integration phase. The sign bit is a "true polarity" indication in that signals less than 1 LSB are correctly determined. This allows precision null detection which is limited only by device noise and Auto Zero residual offsets. the need for an external reference. Some minor component changes are required to upgrade existing designs, reduce power dissipation, and improve the overall performance. (see Oscillator Components)
1
2 3 4 5 6
Digital Section
The TC811 contains all the segment drivers necessary to directly drive a 3-1/2 digit liquid crystal display (LCD). An LCD backplane driver is included. The backplane frequency is the external clock frequency divided by 800. For three conversions/second the backplane frequency is 60Hz with a 5V nominal amplitude. When a segment driver is in phase with the backplane signal the segment of "OFF". An out of phase segment drive signal causes the segment to be "ON" or visible. This AC drive configuration results in negligible DC voltage across each LCD segment. This insures long LCD display life. The polarity segment driver is "ON" for negative analog inputs. If VIN+ and VIN- are reversed then this indicator would reverse.
Reference Integrate (Deintegrate) Cycle
The reference capacitor, which was charged during the Auto Zero cycle, is connected to the input of the integrating amplifier. The internal sign logic insures that the polarity of the reference voltage is always connected in the phase which is opposite to that of the input voltage. This causes the integrator to ramp back to zero at a constant rate which is determined by the reference potential. The amount of time required (TDEINT) for the integrating amplifier to reach zero is directly proportional to the amplitude of the voltage that was put on the integrating capacitor (VINT) during the integration cycle: TDEINT = RINT CINT VINT VREF
+ VIN - V IN VREF -
TEST Function (TEST)
On the TC811, when TEST is pulled to a logical "HIGH", all segments are turned "ON". The display will read "-1888". During this mode the LCD segments have a constant DC voltage impressed. Do not leave the display in this mode for more than several minutes. LCD displays may be destroyed if operated with DC levels for extended periods. The display FONT and segment drive assignment are shown in Figure 5.
DISPLAY FONT
The digital reading displayed Is: Digital Count = 1000
The oscillator frequency is divided by 4 prior to clocking the internal decade counters. The four phase measurement cycle takes a total of 4000 counts or 16000 clock pulses. The 4000 count cycle is independent of input signal magnitude or polarity. Each phase of the measurement cycle has the following length: 1) Auto Zero: 300 to 2900 Counts 2) Signal Integrate: 1000 Counts This time period is fixed. The integration period is: TINT = 4000 fOSC = 1000 Counts
1000's
100's
10's
1's
Figure 5. Display FONT and Segment Assignment
Where fOSC is the crystal oscillator frequency. 3) Reference Integrate: 0 to 2000 Counts 4) Integrator Output Zero: 11 to 640 Counts The TC811 can replace the ICL7106/26/36 in circuits which require both the hold function and a differential reference. The TC811 offers a greatly improved internal reference temperature coefficient, which can often eliminate TELCOM SEMICONDUCTOR, INC.
7
HOLD Reading Input (HLDR)
When HLDR is at a logic "HI" the latch will not be updated. Conversions will continue but will not be updated until HLDR is returned to "LOW". To continuously update the display, connect HLDR to ground or leave it open. This input is CMOS compatible and has an internal resistance of 70k (typical) tied to TEST.
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
COMPONENT VALUE SELECTION Auto Zero Capacitor - CAZ
The value of the Auto Zero capacitor (CAZ) has some influence on system noise. A 0.47F capacitor is recommended for 200mV full-scale applications where 1LSB is 100V. A 0.10F capacitor should be used for 2.0V fullscale applications. A capacitor with low dielectric absorption (Mylar) is required.
Oscillator Components
The internal oscillator has been designed to operate with a quartz crystal, such as the Statek CX-1V series. Such crystals are very small and are available in a variety of standard frequencies. Note that fOSC is divided by four to generate the TC811 internal control clock. The backplane drive signal is derived by dividing fOSC by 800. To achieve maximum rejection of ac-line noise pickup, a 40kHz crystal should be used. This frequency will yield an integration period of 100msec and will reject both 50Hz and 60Hz noise. For prototyping or cost-sensitive applications a 32.768kHz watch crystal can be used, and will produce about 25dB of line-noise rejection. Other crystal frequencies, from 16kHz to 48kHz, can also be used. Pins 39 and 40 make up the oscillator section of the TC811. Figures 6a and 6b show some typical conversion rate component values. The LCD backplane frequency is derived by dividing the oscillator frequency by 800. Capacitive loading of the LCD may compromise display performance if the oscillator is run much over 48kHz.
Reference Voltage Capacitor -CREF
The reference voltage used to ramp the integrator output voltage back to zero during the reference integrate cycle is stored on CREF. A 0.1F capacitor is typical. If the application requires a sensitivity of 200mV full-scale, increase CREF to 1.0F. Rollover error will be held to less than 1/2 count. A good quality, low leakage capacitor, such as Mylar, should be used.
Integrating Capacitor - CINT
CINT should be selected to maximize integrator output voltage swing without causing output saturation. Analog common will normally supply the differential voltage reference. For this case a 2V integrator output swing is optimum when the analog input is near full-scale. For 2 or 2.5 reading/ second (fOSC = 32kHz or 40kHz) and VFS = 200mV, a .068F value is suggested. If a different oscillator frequency is used, CINT must be changed in inverse proportion to maintain the nominal 2V integrator swing. An exact expression for CINT is : CINT = 4000 VFS VINT RINT fOSC
Reference Voltage (VREF)
A full-scale reading (2000 counts) requires the input signal be twice the reference voltage. In some applications a scale factor other than unity may exist, such as between a transducer output voltage and the required digital reading. Assume, for example, a pressure transducer output is 400mV for 2000lb/in2. Rather than dividing the input voltage by two, the reference voltage should be set to 200mV. This permits the transducer input to be used directly.
where: fOSC = Clock frequency at Pin 39 VFS = Full-scale input voltage RINT = Integrating resistor VINT = Desired full-scale integrator output swing CINT must have low dielectric absorption to minimize roll-over error. A polypropylene capacitor is recommended.
TC811
Integrating Resistor -RINT
The input buffer amplifier and integrator are designed with class A output stages which have idling currents of 6A. The integrator and buffer can supply 1A drive currents with negligible linearity errors. RINT is chosen to remain in the output stage linear drive region but not so large that printed circuit board leakage currents induce errors. For a 200mV full-scale, RINT should be about 180k. A 2.0V full-scale requires abut 1.8M.
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OSC1 40 22M
OSC2 39
V+ 38
40.0 kHz
470k
10pF
20pF +
9V
Figure 6a. TC811 Oscillator
TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
Oscillator Freq. (kHz)
32.768 40
1
Full-Scale Voltage (VFS) 200mV 2.0V RINT CINT RINT CINT
180k 150k 0.068F 0.068F
Figure 6b.
1.8M 1.5M
0.068F 0.068F
DEVICE PIN FUNCTIONAL DESCRIPTION Differential Signal Inputs (VIN+ (Pin 31), VIN- (Pin 30))
The TC811 is designed with true differential inputs and accepts input signals within the input stage common mode voltage range (VCM). The typical range is V+ - 1.0 to V- + 1.5V. Common-mode voltages are removed from the system when the TC811 operates from a battery or floating power source (isolated from measured system) and VIN- is connected to Analog Common. (see Figure 8) In systems where common-mode voltages exist, the 86dB common-mode rejection ratio minimizes error. Common-mode voltages do, however, affect the integrator output level. A worse case condition exists if a large positive VCM exists in conjunction with a full-scale negative differential signal. The negative signal drives the integrator output positive along with VCM (Figure 8). For such applications the integrator output swing can be reduced below the recommended 2.0V full-scale swing. The integrator output will swing within 0.3V of V+ or V- without increased linearity error.
inputs permit ratiometric measurements and simplify interfacing with sensors such as load cells and temperature sensors. The TC811 is ideally suited to applications in handheld multimeters, panel meters, and portable instrumentation. The reference voltage can be generated anywhere within the V+ to V- power supply range. To prevent rollover type errors from being induced by large common-mode voltages, CREF should be large compared to stray node capacitance. A 0.1F capacitor is a typical value. The TC811 offers a significantly improved Analog Common temperature coefficient. This provides a very stable voltage suitable for use as a voltage reference. The temperature coefficient of Analog Common is typically 35ppm/C.
2 3 4
INPUT BUFFER + VIN - VCM + -
CI RI - + VI INTEGRATOR
TI VI = V - VIN RI CI CM Where: 4000 T I = INTEGRATION TIME = f OSC
[
[
5 6 7
C I = INTEGRATION CAPACITOR R I = INTEGRATION RESISTOR
Reference (V REF (Pin 36), V REF (Pin 33)
Unlike the ICL7116, the TC811 has a differential reference as well as the "hold" function. The differential reference
Figure 8. Common-Mode Voltage Reduces Available Integrator Swing. (VCOM VIN)
+
-
SEGMENT DRIVE
LCD DISPLAY
MEASURED SYSTEM V+ V- V+ V - GND
10pF VBUF V+ V- CAZ VINT POL BP OSC1 20M TC811 OSC2 V- 470k 40kHZ 20 pF
V+
ANALOG - + + COMMON VREF VREF V
V+
GND POWER SOURCE
+ 9V
-
Figure 7. Common-Mode Voltage Removed in Battery Operation With V IN = Analog Common
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3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
Analog Common (Pin 32)
The Analog Common pin is set at a voltage potential approximately 3.0V below V+. This potential is guaranteed to be between 2.70V and 3.35V below V+. Analog common is tied internally to an N channel FET capable of sinking 100A. This FET will hold the common line at 3.0V below V+ should an external load attempt to pull the common line toward V+. Analog common source current is limited to 1A. Analog common is therefore easily pulled to a more negative voltage (i.e. below V+- 3.0V). + - The TC811 connects the internal VIN and VIN inputs to Analog Common during the Auto Zero cycle. During the - reference integrate phase VIN is connected to Analog - Common. If VIN is not externally connected to Analog Common, a common-mode voltage exists. This is rejected by the converter's 86dB common-mode rejection ratio. In - battery powered applications, Analog Common and VIN are usually connected, removing common-mode voltage concerns. In systems where VIN- is connected to the power supply ground or to a given voltage, Analog Common should - be connected to VIN . The Analog Common pin serves to set the analog section reference or common point. The TC811 is specifically designed to operate from a battery or in any measurement system where input signals are not referenced (float) with respect to the TC811 power source. The Analog Common potential of V+ - 3.0V gives a 7V end of battery life voltage. The analog common potential has a voltage coefficient of 0.001%/%. With a sufficiently high total supply voltage (V+ - V- > 7.0V), Analog Common is a very stable potential with excellent temperature stability (typically 35ppm/C). This potential can be used to generate the TC811 reference voltage. An external voltage reference will be unnecessary in most cases because of the 35ppm/C temperature coefficient. See TC811 Internal Voltage Reference discussion.
APPLICATIONS INFORMATION Decimal Point and Annunciator Drive
The TEST pin is connected to the internally generated digital logic supply ground through a 500 resistor. The TEST pin may be used as the negative supply for external CMOS gate segment drivers. LCD display annunciators for decimal points, low battery indication, or function indication may be added without adding an additional supply. No more than 1mA should be supplied by the TEST pin. The TEST pin potential is approximately 5V below V+.
Internal Voltage Reference
The TC811 Analog Common voltage temperature stability has been significantly improved. This improved device can be used to upgrade old systems and design new systems without external voltage references. External R and C values do not need to be changed, however, noise performance will be improved by increasing CAZ (See Auto Zero Capacitor section). Figure 10 shows Analog Common supplying the necessary voltage reference for the TC811.
V+ V+ 4049
TC811
BP GND TO LCD BACKPLANE TO LCD DECIMAL POINT
TEST
V+ V+ BP TO LCD DECIMAL POINTS
TEST (Pin 37)
The TEST pin potential is 5V less the V+. TEST may be used as the negative power supply connection when interfacing the TC811 to external CMOS logic. The TEST pin is tied to the internally generated negative logic supply through a 500 resistor. The TEST pin may be used to sink up to 1mA. See the applications section for additional information on using TEST as a negative digital logic supply. If TEST is pulled "HIGH" (V+), all segments plus the minus sign will be activated. Do not operate in this mode for more than several minutes, because when TEST is pulled to V+, the LCD Segments are impressed with a DC voltage which may cause damage to the LCD.
TC811
HDLR
DECIMAL POINT SELECT
TO "HOLD" ANNUNCIATOR 4030 GND
TEST
Figure 9. Display Annunciator Drivers
3-146
TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
Liquid Crystal Display Sources
Several LCD manufactures supply standard LCD displays to interface with the TC811 3-1/2 digit analog-to-digital converter.
26 9V +
1
2
38 V+ 240k 36 VREF 10k
Manufacturer
Crystaloid Electronics AND
Address/Phone
5282 Hudson Dr., Hudson, OH 44236 216-655-2429 770 Airport Blvd., Burlingame, CA 94010 415-347-9916 3415 Kashikawa St., Torrence, CA 90505 212-534-0360 612 E. Lake St., Lake Mills, WI 53551 414-648-2361
Representative Part Numbers*
C5335, H5535, T5135, SX440 FE 0801, FE 0203 LD-B709BZ LD-H7992AZ 3902, 3933, 3903
V- TC811
+ VREF - VREF
3 4 5 6
33
EPSON
ANALOG 32 COMMON SET VREF = 1/2 VFULL SCALE Figure 10. TC811 Internal Voltage Reference Connection
Hamlin, Inc.
*NOTE:
Contact LCD manufacturer for full product listing/specifications.
Oscillator Crystal Source
Manufacturer
STATEK
Address/Phone
512 N-Main Orange, CA 92668 714-639-7810
Representative Part Numbers
CX-1V 40.0
R STANDARD
+ + VREF V HLDR - V
REF
+ V IN R UNKNOWN - V IN ANALOG COMMON TC811
LCD
Ratiometric Resistance Measurements
The TC811 true differential input and differential reference make ratiometric readings possible. In ratiometric operation, an unknown resistance is measured with respect to a known standard resistance. No accurately defined reference voltage is needed. The unknown resistance is put in series with a known standard and a current is passed through the pair (Figure 11). The voltage developed across the unknown is applied to the input and the voltage across the known resistor applied to the reference input. If the unknown equals the standard, the input voltage will equal the reference voltage and the display will read 1000. The displayed reading can be determined from the following expression: Displayed reading = RUNKNOWN RSTANDARD x 1000
Figure 11. Low Parts Count Ratio Metric Resistance Measurement
7
The display will overrange for RUNKNOWN 2 X RSTANDARD.
8
3-147
TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS TC811
+
9V
+ 5.6 k 160 k + V - VIN + VIN 0.7%/C PTC R3 R2 20 k + VREF - VREF
9V
160 k
300 k
300 k
V+ - VIN + VIN
V-
V
-
1N914
1N4148 SENSOR R2 50 k
R1 50 k
R1 20 k
TC811
TC811
VFS = 200 MV + VREF - VREF COMMON HLDR V+
COMMON
HLDR
V+
Figure 12. Temperature Sensor
Figure 13. Positive Temperature Coefficient Resistor Temperature Sensor
3-148
TELCOM SEMICONDUCTOR, INC.

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