The Texas Instruments IVC102 precision integrator has high-quality internal capacitors. The circuit in Figure 1 allows you to measure very-high-resistance values of RX. A precision difference amplifier, a TI INA105, applies a reference voltage to RX. During integration, a negative voltage ramp, VO, is generated at the output of the IVC102. The two LM311s compare the amplitude of VO with two fixed thresholds and generate the two digital signals: start and stop.
|Figure 1.||Using an IVC102 precision integrator, this circuit can measure resistances in the gigohm range.|
The delta time between two such events relates to the system parameters by the expression:
where ΔT is the delta time and CINT is the internal integrating capacitance of the IVC102, which external connections on pins 4, 5, and 6 select. (Note: when S1 is open, CINT = 10 pF, whereas, when S1 is closed, CINT = 100 pF.) The VA threshold allows the circuit to see the output ramp without any offset on the VO signal. Because of the INA105 difference amplifier, VREF = VA – VB, so the previous equation reduces to:
Also note that the precision of resistors R1, R2, and R3 is not critical. The difference amplifier guarantees the precision of the ohmmeter.
External digital-control circuitry can measure delta time by counting the clock periods between the start and the stop events. At the end, the control circuit can generate a reset signal for the IVC102 to perform a new measurement.