TC7135CPI

1 TC7135 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER FEATURES s s s s s s s s s s s s s s Low Roll-Over Error ......................... ±1 Count Max Guaranteed Nonlinearity Error ........ ±1 Count Max Guaranteed Zero Reading for 0V Input True Polarity Indication at Zero for Null Detection Multiplexed BCD Data Output TTL-Compatible Outputs Differential Input Control Signals Permit Interface to UARTs and µProcessors Auto-Ranging Supported With Overrange and Underrange Signals Blinking Display Visually Indicates Overrange Condition Low Input Current ............................................. 1 pA Low Zero Reading Drift ............................... 2 µV/°C Interfaces to TC7211A (LCD) and TC7212A (LED) Display Drivers Available in DIP and Surface-Mount Packages GENERAL DESCRIPTION The TC7135 4-1/2 digit analog-to-digital converter (ADC) offers 50 ppm (1 part in 20,000) resolution with a maximum nonlinearity error of 1 count. An auto-zero cycle reduces zero error to below 10 µV and zero drift to 0.5 µV/°C. Source impedance errors are minimized by a 10 pA maximum input current. Roll-over error is limited to ±1 count. By combining the TC7135 with a TC7211A (LCD) or TC7212A (LED) driver, a 4-1/2 digit display DVM or DPM can be constructed. Overrange and underrange signals support automatic range switching and special display blanking/flashing applications. Microprocessor-based measurement systems are supported by BUSY, STROBE, and RUN/HOLD control signals. Remote data acquisition systems with data transfer via UARTs are also possible. The additional control pins and multiplexed BCD outputs make the TC7135 the ideal converter for display or microprocessor-based measurement systems. 2 3 4 5 6 ORDERING INFORMATION Part No. TC7135CBU TC7135CLI TC7135CPI Package 64-Pin Plastic Flat Package 28-Pin PLCC 28-Pin Plastic DIP Temperature Range 0°C to +70°C 0°C to +70°C 0°C to +70°C TYPICAL 4-1/2 DIGIT DVM WITH LCD 4-1/2 DIGIT LCD 6.8 k Ω 0.1 µF +5V +5V –5V 100 k Ω 1 2 V UR 28 27 26 25 24 23 22 21 20 19 18 17 16 15 +5V 120 Hz = 3 READING/SEC CLOCK IN 5 BP 31 D1 32 33 34 30 29 28 D2 D3 D4 B3 B2 B1 +5V 1 V+ 1 16 15 14 12 5 3 4 SEGMENT TC04 ANALOG GROUND 1 µF 0.1 µF INPUT 100 k Ω REF IN OR 3 ANALOG STROBE 0.47 µF 4 COMMON RUN/HOLD INT OUT 5 1 µF DGND AZ IN 6 POL BUFF OUT 100 k Ω 7 – CLOCK CREF 8+ BUSY CREF 9 D1 –INPUT 10 D2 +INPUT 11 +5V D3 V+ 12 D5 D4 13 TC7135 B1 B8 14 B2 B4 CD4054A 7 8 13 11 10 9 2 6 BACKPLANE D R I V E 1/4 CD4030 CD4081 7 SEG OUT 2,3,4 6–26 37–40 36 +5V OSC OPTIONAL CAP 35 GND TC7211A 27 B 0 8 TC7135-10 11/6/96 TELCOM SEMICONDUCTOR, INC. 3-113 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 ABSOLUTE MAXIMUM RATINGS* (Note 1) Positive Supply Voltage ............................................. +6V Negative Supply Voltage .............................................–9V Analog Input Voltage (Pin 9 or 10) ......... V+ to V– (Note 2) Reference Input Voltage (Pin 2) ........................... V+ to V– Clock Input Voltage .............................................. 0V to V+ Operating Temperature Range .................... 0°C to +70°C Storage Temperature Range ................. –65°C to +160°C Lead Temperature (Soldering, 10 sec) ................. +300°C Package Power Dissipation (TA ≤ 70°C) Plastic DIP ........................................................ 1.14W PLCC ................................................................ 1.00W Plastic Flat Package .........................................1.14W *Static-sensitive device. Unused devices must be stored in conductive material to protect them 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 operational sections of the specifications is not implied. ELECTRICAL CHARACTERISTICS: TA = +25°C, fCLOCK = 120 kHz, V+ = +5V, V– = –5V (Figure 1) Symbol Analog Display Reading With Zero Volt Input Zero Reading Temperature Coefficient Full-Scale Temperature Coefficient Nonlinearity Error Differential Linearity Error Display Reading in Ratiometric Operation ± Full-Scale Symmetry Error (Roll-Over Error) Input Leakage Current Noise Notes 2 and 3 VIN = 0V Note 4 VIN = 2V Notes 4 and 5 Note 6 Note 6 VIN = VREF Note 2 –VIN = +VIN Note 7 Note 3 Peak-to-Peak Value Not Exceeded 95% of Time VIN = 0V VIN = +5V IOL = 1.6 mA IOH = 1 mA IOH = 10 µA Note 8 –0.0000 — — — — +0.9996 — — — ±0.0000 0.5 — 0.5 0.01 +0.9999 0.5 1 15 +0.0000 2 5 1 — +1.0000 1 10 — Display Reading µV/°C ppm/°C Count LSB Display Reading Count pA µVP-P Parameter Test Conditions Min Typ Max Unit TCZ TCFS NL DNL ±FSE IIN VN Digital IIL IIH VOL VOH fCLK Power Supply V+ V– I+ I– PD NOTES: Input Low Current Input High Current Output Low Voltage Output High Voltage B1, B2, B4, B8, D1–D5 Busy, Polarity, Overrange, Underrange, Strobe Clock Frequency Positive Supply Voltage Negative Supply Voltage Positive Supply Current Negative Supply Current Power Dissipation — — — 2.4 4.9 0 4 –3 — — — 10 0.08 0.2 4.4 4.99 120 5 –5 1 0.7 8.5 100 10 0.4 5 5 1200 6 –8 3 3 30 µA µA V V V kHz V V mA mA mW fCLK = 0 Hz fCLK = 0 Hz fCLK = 0 Hz 5. 6. 7. 8. 1. Limit input current to under 100µA if input voltages exceed supply voltage. 2. Full-scale voltage = 2V. 3. VIN = 0V. 4. 0°C ≤ TA ≤ +70°C. External reference temperature coefficient less than 0.01 ppm/°C. –2V ≤ VIN ≤ +2V. Error of reading from best fit straight line. |VIN| = 1.9959. Specification related to clock frequency range over which the TC7135 correctly performs its various functions. Increased errors result at higher operating frequencies. 3-114 TELCOM SEMICONDUCTOR, INC. 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 PIN CONFIGURATIONS RUN/HOLD CLOCK IN STROBE 1 DGND BUSY POL V– NC NC D1 D2 NC 1 2 3 4 5 6 7 8 9 28 UNDERRANGE 27 OVERRANGE 2 3 4 5 6 7 NC NC NC NC NC 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 NC 1 NC 2 NC 3 NC 4 NC 5 NC 6 OVERRANGE 7 UNDERRANGE 8 NC 9 V – 10 REF IN 11 ANALOG COM 12 NC 13 NC 14 NC 15 NC 16 q 48 NC 47 NC 46 NC 45 D3 44 D4 43 B8 42 B4 41 B2 40 NC REF IN ANALOG COM INT OUT AZ IN BUFF OUT – C REF + CREF – INPUT 26 STROBE 25 RUN/HOLD 24 DIGTAL GND 23 POLARITY TC7135CPI (PDIP) 22 CLOCK IN 21 BUSY 20 D1 (LSD) 19 D2 18 D3 17 D4 16 B8 (MSB) 15 B4 +INPUT 10 V + 11 (MSD) D5 12 (LSB) B1 13 B2 14 TC7135CBU (PFP) (NOTES 1) 39 B1 38 D5 37 NC 36 NC 33 NC 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 4 3 2 1 28 27 26 25 RUN/HOLD 24 DIGTAL GND 23 POLARITY BUFF OUT – CREF INT OUT AZ IN +INPUT NC NC NC NC NC + C REF NC –INPUT NC NC V+ AZ IN 5 BUFF OUT 6 REF CAP– 7 REF CAP+ 8 –INPUT 9 OR UR 34 NC STROBE 22 CLOCK IN 21 BUSY 20 D1 (LSD) 19 D2 35 NC NOTES: 1. NC = No internal connection. +INPUT 10 V + 11 12 13 14 15 16 17 18 B4 (MSB) B8 D4 INT OUT ANALOG COM REF IN V– TC7135CLI (PLCC) (MSD) D5 (LSB) B1 B2 D3 8 TELCOM SEMICONDUCTOR, INC. 3-115 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 SET VREF = 1V VREF IN 100 k Ω –5V SW I ANALOG INPUT BUFFER + – SW RI SW + RI – CSZ SWIZ REF IN SWR CREF SWZ COMPARATOR + – RINT CINT 1 2 V– REF IN + IN UNDERRANGE 28 27 26 25 24 23 22 21 20 19 18 17 16 15 – IN SWZ SW + RI – SW RI SWZ INTEGRATOR CLOCK INPUT 120 kHz ANALOG COM SW I SW1 SWITCH OPEN SWITCH CLOSED Figure 3B. System Zero Phase Figure 1. Test Circuit + IN SW I ANALOG INPUT BUFFER + – SW RI SW + RI – CSZ SWIZ SWZ COMPARATOR + – RINT CINT V+ SWR CREF SWZ BUFFER LOGIC INPUT SW + RI – SW RI SWZ INTEGRATOR ANALOG COM SW I – IN SW1 SWITCH OPEN SWITCH CLOSED Figure 3C. Input Signal Integration Phase Figure 2. Digital Logic Input SW I + IN – SW RI SW + RI ANALOG INPUT BUFFER + – CSZ SWIZ SWZ COMPARATOR + – RINT CINT SW I + IN – SW RI SW + RI ANALOG INPUT BUFFER + – CSZ SWIZ SWZ COMPARATOR + – RINT CINT SWZ ANALOG COM SW I – IN SW + RI – SW RI SWZ INTEGRATOR SWZ ANALOG COM SW + RI – SW RI SWZ INTEGRATOR SW1 SW I – IN SW1 SWITCH OPEN SWITCH CLOSED Figure 3A. Internal Analog Switches 3-116 Figure 3D. Reference Voltage Integration Phase TELCOM SEMICONDUCTOR, INC. – TO DIGITAL SECTION + – + REF IN SWR CREF REF IN SWR CREF – + REF IN – + TO DIGITAL SECTION OVERRANGE 3 ANALOG STROBE COMMON ANALOG GND 4 RUN/HOLD INT OUT 0.47 1 µF 5 µF DIGTAL GND AZ IN 6 POLARITY BUFF OUT 100 k Ω 7 – CLOCK IN CREF 100 SIGNAL 1 µF 8 + BUSY kΩ INPUT CREF 9 –INPUT (LSD) D1 0.1 µF 10 D2 +INPUT TC7135 11 + D3 +5V V 12 D5 (MSD) D4 13 B1 (LSB) (MSB) B8 14 B2 B4 TO DIGITAL SECTION TO DIGITAL SECTION 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 For a constant VIN: SW I + IN – SW RI SW + RI ANALOG INPUT BUFFER + – CSZ SWIZ REF IN SWR CREF SWZ COMPARATOR + – RINT CINT 1 VIN = VR [] tRI . tSI 2 3 4 5 6 CLOCK SWZ ANALOG COM SW I – IN SW + RI – SW RI SWZ INTEGRATOR TO DIGITAL SECTION SW1 SWITCH OPEN SWITCH CLOSED The dual-slope converter accuracy is unrelated to the integrating resistor and capacitor values, as long as they are stable during a measurement cycle. Noise immunity is an inherent benefit. Noise spikes are integrated, or averaged, to zero during integration periods. Integrating ADCs are immune to the large conversion errors that plague successive approximation converters in high-noise environments. (See Figure 4.) Figure 3E. Integrator Output Zero Phase GENERAL THEORY OF OPERATION (All Pin Designations Refer to 28-Pin DIP) Dual-Slope Conversion Principles The TC7135 is a dual-slope, integrating analog-todigital converter. An understanding of the dual-slope conversion technique will aid in following detailed TC7135 operational theory. The conventional dual-slope converter measurement cycle has two distinct phases: (1) Input signal integration (2) Reference voltage integration (deintegration) The input signal being converted is integrated for a fixed time period, measured by counting clock pulses. An opposite polarity constant reference voltage is then integrated until the integrator output voltage returns to zero. The reference integration time is directly proportional to the input signal. In a simple dual-slope converter, a complete conversion requires the integrator output to "ramp-up" and "rampdown." A simple mathematical equation relates the input signal, reference voltage, and integration time: 1 RC SWITCH DRIVER REF VOLTAGE PHASE CONTROL CONTROL LOGIC POLARITY CONTROL ∫0 tSI VIN(t) dt = VR tRI , RC INTEGRATOR OUTPUT DISPLAY VIN VIN VARIABLE REFERENCE INTEGRATE TIME VFULL SCALE 1/2 VFULL SCALE where: VR = Reference voltage tSI = Signal integration time (fixed) tRI = Reference voltage integration time (variable). TELCOM SEMICONDUCTOR, INC. FIXED SIGNAL INTEGRATE TIME Figure 4. Basic Dual-Slope Converter 3-117 + – + – – + TC7135 Operational Theory The TC7135 incorporates a system zero phase and integrator output voltage zero phase to the normal twophase dual-slope measurement cycle. Reduced system errors, fewer calibration steps, and a shorter overrange recovery time result. The TC7135 measurement cycle contains four phases: (1) (2) (3) (4) System zero Analog input signal integration Reference voltage integration Integrator output zero Internal analog gate status for each phase is shown in Table 1. ANALOG INPUT SIGNAL INTEGRATOR COMPARATOR COUNTER 7 8 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 Table 1. Internal Analog Gate Status Conversion Cycle Phase System Zero Input Signal Integration Reference Voltage Integration Integrator Output Zero Closed Closed* Closed Closed – SWI SWR + Internal Analog Gate Status – SWR SWZ SWR Closed Closed SW1 Closed SWIZ Reference Schematic 3B 3C 3D Closed 3E *NOTE: Assumes a positive polarity input signal. SWR would be closed for a negative input signal. System Zero Phase During this phase, errors due to buffer, integrator, and comparator offset voltages are compensated for by charging CAZ (auto-zero capacitor) with a compensating error voltage. With zero input voltage, the integrator output remains at zero. The external input signal is disconnected from the internal circuitry by opening the two SWI switches. The internal input points connect to analog common. The reference capacitor charges to the reference voltage potential through SWR. A feedback loop, closed around the integrator and comparator, charges the CAZ with a voltage to compensate for buffer amplifier, integrator, and comparator offset voltages. (See Figure 3B.) Analog Input Signal Integration Phase The TC7135 integrates the differential voltage between the +INPUT and –INPUT. The differential voltage must be within the device's common-mode range; –1V from either supply rail, typically. The input signal polarity is determined at the end of this phase. (See Figure 3C.) Reference Voltage Integration Phase The previously-charged reference capacitor is connected with the proper polarity to ramp the integrator output back to zero. (See Figure 3D.) The digital reading displayed is: Reading = 10,000 Analog Section Functional Description Differential Inputs The TC7135 operates with differential voltages (+INPUT, pin 10 and –INPUT, pin 9) within the input amplifier common-mode range which extends from 1V below the positive supply to 1V above the negative supply. Within this common-mode voltage range, an 86 dB common-mode rejection ratio is typical. The integrator output also follows the common-mode voltage and must not be allowed to saturate. A worst-case condition exists, for example, when a large positive common-mode voltage with a near full-scale negative differential input voltage is applied. The negative input signal drives the integrator positive when most of its swing has been used up by the positive common-mode voltage. For these critical applications, the integrator swing can be reduced to less than the recommended 4V full-scale swing, with some loss of accuracy. The integrator output can swing within 0.3V of either supply without loss of linearity. Analog Common ANALOG COMMON (pin 3) is used as the –INPUT return during the auto-zero and deintegrate phases. If – INPUT is different from analog common, a common-mode voltage exists in the system. This signal is rejected by the excellent CMRR of the converter. In most applications, – INPUT will be set at a fixed known voltage (power supply common, for instance). In this application, analog common should be tied to the same point, thus removing the commonmode voltage from the converter. The reference voltage is referenced to analog common. Reference Voltage The reference voltage input (REF IN, pin 2) must be a positive voltage with respect to analog common. Two reference voltage circuits are shown in Figure 5. [ Differential Input . VREF ] Integrator Output Zero Phase This phase guarantees the integrator output is at 0V when the system zero phase is entered and that the true system offset voltages are compensated for. This phase normally lasts 100 to 200 clock cycles. If an overrange condition exists, the phase is extended to 6200 clock cycles. (See Figure 3E.) 3-118 TELCOM SEMICONDUCTOR, INC. 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 Digital Section Functional Description V+ V+ REF IN 1 TC05 2.5VREF TC7135 ANALOG COMMON I REF The major digital subsystems within the TC7135 are illustrated in Figure 6, with timing relationships shown in Figure 7. The multiplexed BCD output data can be displayed on an LCD with the TC7211A. The digital section is best described through a discussion of the control signals and data outputs. RUN/HOLD Input When left open, the RUN/HOLD (R/H) input (pin 25) assumes a logic "1" level. With R/H = 1, the TC7135 performs conversions continuously, with a new measurement cycle beginning every 40,002 clock pulses. When R/H changes to logic "0," the measurement cycle in progress will be completed, and data held and displayed, as long as the logic "0" condition exists. A positive pulse (>300nsec) at R/H initiates a new measurement cycle. The measurement cycle in progress when R/H initially assumed logic "0" must be completed before the positive pulse can be recognized as a single conversion run command. The new measurement cycle begins with a 10,001count auto-zero phase. At the end of this phase, the busy signal goes high. 2 3 4 5 V– V+ V+ 6.8 kΩ REF IN TC04 20 kΩ 1.25V REF TC7135 ANALOG COMMON ANALOG GROUND Figure 5. Using an External Reference Voltage POLARITY D5 MSB D4 DIGIT D3 DRIVE MULTIPLEXER D2 SIGNAL D1 LSB DATA OUTPUT 13 B1 14 B2 15 B4 16 B8 FROM ANALOG SECTION LATCH LATCH LATCH LATCH LATCH 6 7 POLARITY FF ZERO CROSS DETECT COUNTERS CONTROL LOGIC 24 DIGITAL GND 22 CLOCK IN 25 RUN/ HOLD 27 OVER– RANGE 28 UNDER– RANGE 26 STROBE 21 BUSY Figure 6. Digital Section Functional Diagram 8 3-119 TELCOM SEMICONDUCTOR, INC. 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 TC7135 OUTPUTS INTEGRATOR OUTPUT SIGNAL INTE SYSTEM 10,000 ZERO 10,001 COUNTS COUNTS (FIXED) REFERENCE INTEGRATE 20,001 COUNTS (MAX) BUSY END OF CONVERSION * B1–B8 D5 (MSD) DATA D4 DATA D3 DATA D2 DATA D1 (LSD) DATA D5 DATA FULL MEASUREMENT CYCLE 40,002 COUNTS BUSY STROBE 200 COUNTS 201 COUNTS NOTE ABSENCE OF STROBE 
,
,   OVERRANGE WHEN APPLICABLE UNDERRANGE WHEN APPLICABLE EXPANDED SCALE BELOW DIGIT SCAN D5 D4 D3 D2 D1 100 COUNTS STROBE AUTO ZERO DIGIT SCAN FOR OVERRANGE D5 200 COUNTS D4 200 COUNTS D3 200 COUNTS D2 200 COUNTS * FIRST D5 OF SYSTEM ZERO D1 200 COUNTS AND REFERENCE INTEGRATE ONE COUNT LONGER. *DELAY BETWEEN BUSY GOING LOW AND FIRST STROBE PULSE IS DEPENDENT ON ANALOG INPUT. * D5 D4 D3 D2 D1 SIGNAL INTEGRATE REFERENCE INTEGRATE Figure 8. Strobe Signal Pulses Low Five Times per Conversion * The active-low STROBE pulses aid BCD data transfer to UARTs, microprocessors, and external latches. (See Application Note AN-16.) BUSY Output At the beginning of the signal-integration phase, BUSY (pin 21) goes high and remains high until the first clock pulse after the integrator zero crossing. BUSY returns to logic "0" after the measurement cycle ends in an overrange condition. The internal display latches are loaded during the first clock pulse after BUSY and are latched at the clock pulse end. The BUSY signal does not go high at the beginning of the measurement cycle, which starts with the auto-zero phase. OVERRANGE Output If the input signal causes the reference voltage integration time to exceed 20,000 clock pulses, the OVERRANGE output (pin 27) is set to logic "1." The OVERRANGE output register is set when BUSY goes low and reset at the beginning of the next reference-integration phase. UNDERRANGE Output If the output count is 9% of full scale or less (≤1800 counts), the UNDERRANGE output (pin 28) register bit is set at the end of BUSY. The bit is set low at the next signalintegration phase. TELCOM SEMICONDUCTOR, INC. Figure 7. Timing Diagrams for Outputs STROBE Output During the measurement cycle, the STROBE output (pin 26) control line is pulsed low five times. The five low pulses occur in the center of the digit drive signals (D1, D2, D3, D4 and D5; see Figure 8). D5 goes high for 201 counts when the measurement cycles end. In the center of D5 pulse, 101 clock pulses after the end of the measurement cycle, the first STROBE occurs for one-half clock pulse. After D5 strobe, D4 goes high for 200 clock pulses. STROBE goes low 100 clock pulses after D4 goes high. This continues through the D1 drive pulse. The digit drive signals will continue to permit display scanning. STROBE pulses are not repeated until a new measurement is completed. The digit drive signals will not continue if the previous signal resulted in an overrange condition. 3-120 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 POLARITY Output A positive input is registered by a logic "1" polarity signal. The POLARITY output (pin 23) is valid at the beginning of reference integrate and remains valid until determined during the next conversion. The POLARITY bit is valid even for a zero reading. Signals less than the converter's LSB will have the signal polarity determined correctly. This is useful in null applications. Digit Drive Outputs Digit drive outputs are positive-going signals. Their scan sequence is D5, D4, D3, D2 and D1 (pins 12, 17, 18, 19 and 20, respectively). All positive signals are 200 clock pulses wide, except D5, which is 201 clock pulses. All five digits are continuously scanned, unless an overrange condition occurs. In an overrange condition, all digit drives are held low from the final STROBE pulse until the beginning of the next reference-integrate phase. The scanning sequence is then repeated, providing a blinking visual display. BCD Data Outputs The binary coded decimal (BCD) outputs, B8, B4, B2 and B1 (pins 16, 15, 14 and 13, respectively) are positive truelogic signals. They become active simultaneously with digit drive signals. In an overrange condition, all data bits are logic "0". adequate. A 0.10 µF to 0.47 µF is recommended. In general, the value of CINT is given by: CINT = [10,000 x clock period] x IINT Integrator output voltage swing (10,000) (clock period) (20 µA) . Integrator output voltage swing 1 2 3 4 5 6 7 = A very important characteristic of the CINT is that it has low dielectric absorption to prevent roll-over or ratiometric errors. A good test for dielectric absorption is to use the capacitor with the input tied to the reference. This ratiometric condition should read half-scale 0.9999. Any deviation is probably due to dielectric absorption. Polypropylene capacitors give undetectable errors at reasonable cost. Polystyrene and polycarbonate capacitors may also be used in less critical applications. Auto-Zero and Reference Capacitors The size of the auto-zero capacitor (CAZ) has some influence on system noise. A large capacitor reduces noise. The reference capacitor (CREF) should be large enough such that stray capacitance from its nodes to ground is negligible. The dielectric absorption of CREF and CAZ is only important at power-on, or when the circuit is recovering from an overload. Smaller or cheaper capacitors can be used if accurate readings are not required during the first few seconds of recovery. Reference Voltage The analog input required to generate a full-scale output is VIN = 2 VREF. The stability of the reference voltage is a major factor in overall absolute accuracy of the converter. Therefore, it is recommended that high-quality references be used where high-accuracy, absolute measurements are being made. Suitable references are: Part Type TC04 TC05 APPLICATIONS INFORMATION Component Value Selection Integrating Resistor The integrating resistor (RINT) is determined by the fullscale input voltage and output current of the buffer used to charge the integrator capacitor (CINT). Both the buffer amplifier and the integrator have a Class A output stage, with 100 µA of quiescent current. A 20 µA drive current gives negligible linearity errors. Values of 5 µA to 40 µA give good results. The exact value of RINT for a 20 µA current is easily calculated: Full-scale voltage RINT = . 20 µA Integrating Capacitor The product of RINT and CINT should be selected to give the maximum voltage swing to ensure tolerance build-up will not saturate integrator swing (approximately 0.3V from either supply). For ±5V supplies, and analog common tied to supply ground, a ±3.5V to ±4V full-scale integrator swing is Manufacturer TelCom Semiconductor TelCom Semiconductor Conversion Timing Line Frequency Rejection A signal-integration period at a multiple of the 60 Hz line frequency will maximize 60 Hz "line noise" rejection. A 100 kHz clock frequency will reject 50 Hz, 60 Hz and 400 Hz noise, corresponding to 2.5 readings per second. 8 TELCOM SEMICONDUCTOR, INC. 3-121 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 Table 2. Line Frequency Rejection Oscillator Frequency (kHz) 300, 200, 150, 120, 100, 40, 33-1/3 250, 166-2/3, 125, 100 100 High-Speed Operation The maximum conversion rate of most dual-slope ADCs is limited by frequency response of the comparator. The comparator in this circuit follows the integrator ramp with a 3 µs delay, and at a clock frequency of 160 kHz (6 µs period), half of the first reference integrate clock period is lost in delay. This means the meter reading will change from 0 to 1 with a 50 µV input, 1 to 2 with 150 µV, 2 to 3 with 250 µV, etc. This transition at midpoint is considered desirable by most users; however, if clock frequency is increased appreciably above 160 kHz, the instrument will flash "1" on noise peaks even when the input is shorted. For many dedicated applications, where the input signal is always of one polarity, comparator delay need not be a limitation. Since nonlinearity and noise do not increase substantially with frequency, clock rates up to ~1 MHz may be used. For a fixed clock frequency, the extra count (or counts) caused by comparator delay will be constant and can be digitally subtracted. The clock frequency may be extended above 160 kHz without this error, however, by using a low value resistor in series with the integrating capacitor. The effect of the resistor is to introduce a small pedestal voltage onto the integrator output at the beginning of reference-integrate phase. By careful selection of the ratio between this resistor and the integrating resistor (a few tens of ohms in the recommended circuit), the comparator delay can be compensated for and maximum clock frequency extended by approximately a factor of 3. At higher frequencies, ringing and second-order breaks will cause significant nonlinearities during the first few counts of the instrument. The minimum clock frequency is established by leakage on the auto-zero and reference capacitors. With most devices, measurement cycles as long as 10 seconds give no measurable leakage error. The clock used should be free from significant phase or frequency jitter. Several suitable low-cost oscillators are shown in the applications section. The multiplexed output means if the display takes significant current from the logic supply, the clock should have good PSRR. Frequency Rejected (Hz) 60 50 50, 60, 400 Table 3. Conversion Rate vs Clock Frequency Conversion Rate (Conv/Sec) 2.5 3.0 5.0 7.5 10.0 20.0 30.0 Clock Frequency (kHz) 100 120 200 300 400 800 1200 Displays and Driver Circuits TelCom Semiconductor manufactures three display decoder/driver circuits to interface the TC7135 to LCDs or LED displays. Each driver has 28 outputs for driving four 7segment digit displays. Device TC7211AIPL Package 40-Pin Epoxy Description 4-Digit LCD Driver/Encoder Several sources exist for LCDs and LED displays. Manufacturer Hewlett Packard Components AND Epson America, Inc. Address 640 Page Mill Road Palo Alto, CA 94304 720 Palomar Ave. Sunnyvale, CA 94086 3415 Kanhi Kawa St. Torrance, CA 90505 Display Type LED LCD and LED LCD Zero-Crossing Flip-Flop The flip-flop interrogates data once every clock pulse after transients of the previous clock pulse and half-clock pulse have died down. False zero-crossings caused by clock pulses are not recognized. Of course, the flip-flop delays the true zero-crossing by up to one count in every instance, and if a correction were not made, the display would always be one count too high. Therefore, the counter 3-122 TELCOM SEMICONDUCTOR, INC. 4-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTER TC7135 is disabled for one clock pulse at the beginning of the reference integrate (deintegrate) phase. This one-count delay compensates for the delay of the zero-crossing flipflop, and allows the correct number to be latched into the display. Similarly, a one-count delay at the beginning of auto-zero gives an overload display of 0000 instead of 0001. No delay occurs during signal integrate, so true ratiometric readings result. 1 +5V V+ V– 11 8 1 (–5V) 10 µF 5 2 TC7660 + 10 µF 2 3 Generating a Negative Supply A negative voltage can be generated from the positive supply by using a TC7660. (See Figure 9.) TC7135 24 + 4 3 4 Figure 9. Negative Supply Voltage Generator TYPICAL APPLICATIONS RC Oscillator Circuit R2 C fO R1 Comparator Clock Circuit +5V 16 kΩ 56 kΩ 2 8 7 1 4 1 kΩ 0.22 µF 3 16 kΩ 1. fO ≈ R1 R2 1 , RP = 2 C[0.41 RP + 0.70 R1] R1 + R2 R2 100 kΩ C1 0.1 µF TELCOM SEMICONDUCTOR, INC. – 3 + a. If R = R1 = R2, f ≅ 0.55/RC b. If R2 >> R1, f ≅ 0.45/R1C c. If R2