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ICL7136CM44ZT Datasheet(PDF) 7 Page - Intersil Corporation |
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ICL7136CM44ZT Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 15 page 7 FN3086.6 July 21, 2005 are connected to the external pins. The converter then integrates the differential voltage between IN HI and IN LO for a fixed time. This differential voltage can be within a wide common mode range: up to 1V from either supply. If, on the other hand, the input signal has no return with respect to the converter power supply, IN LO can be tied to analog COMMON to establish the correct common mode voltage. At the end of this phase, the polarity of the integrated signal is determined. De-Integrate Phase The final phase is de-integrate, or reference integrate. Input low is internally connected to analog COMMON and input high is connected across the previously charged reference capacitor. Circuitry within the chip ensures that the capacitor will be connected with the correct polarity to cause the integrator output to return to zero. The time required for the output to return to zero is proportional to the input signal. Specifically the digital reading displayed is: . Zero Integrator Phase The final phase is zero integrator. First, input low is shorted to analog COMMON. Second, the reference capacitor is charged to the reference voltage. Finally, a feedback loop is closed around the system to IN HI to cause the integrator output to return to zero. Under normal conditions, this phase lasts for between 11 to 140 clock pulses, but after a “heavy” overrange conversion, it is extended to 740 clock pulses. Differential Input The input can accept differential voltages anywhere within the common mode range of the input amplifier, or specifically from 0.5V below the positive supply to 1V above the negative supply. In this range, the system has a CMRR of 86dB typical. However, care must be exercised to assure the integrator output does not saturate. A worst case condition would be a large positive common mode voltage with a near full scale negative differential input voltage. 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 output swing can be reduced to less than the recommended 2V full scale swing with little loss of accuracy. The integrator output can swing to within 0.3V of either supply without loss of linearity. Differential Reference The reference voltage can be generated anywhere within the power supply voltage of the converter. The main source of common mode error is a roll-over voltage caused by the reference capacitor losing or gaining charge to stray capacity on its nodes. If there is a large common mode voltage, the reference capacitor can gain charge (increase voltage) when called up to de-integrate a positive signal but lose charge (decrease voltage) when called up to de- integrate a negative input signal. This difference in reference for positive or negative input voltage will give a roll-over error. However, by selecting the reference capacitor such that it is large enough in comparison to the stray capacitance, this error can be held to less than 0.5 count worst case. (See Component Value Selection.) DISPLAY READING = 1000 VIN VREF --------------- DE- DE+ CINT CAZ RINT BUFFER A-Z INT - + A-Z COMPARATOR IN HI COMMON IN LO 31 32 30 DE- DE+ INT A-Z 34 CREF + 36 REF HI CREF REF LO 35 A-Z, A-Z, 33 CREF- 28 29 27 TO DIGITAL SECTION A-Z AND DE (±) INTEGRATOR INT STRAY STRAY V+ 10 µA V- N INPUT HIGH 2.8V 6.2V V+ 1 INPUT LOW - + - + - + ZI ZI AND ZI ZI 26 FIGURE 2. ANALOG SECTION OF ICL7136 ICL7136 |
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