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 R_X. A precision difference amplifier, a TI INA105, applies a reference voltage to R_X. During integration, a negative voltage ramp, V_O, is generated at the output of the IVC102. The two LM311s compare the amplitude of V_O with two fixed thresholds and generate the two digital signals: start and stop. The delta time between two such events relates to the system parameters by the expression: ΔT=C_INT[(V_A– V_B)/V_REF]R_X, where ΔT is the delta time and C_INT is the internal integrating capacitance of the IVC102, which external connections on pins 4, 5, and 6 select. (Note: when S₁ is open, C_INT=10 pF, whereas, when S₁ is closed, C_INT=100 pF.) The V_A threshold allows the circuit to see the output ramp without any offset on the V_O signal. Because of the INA105 difference amplifier, V_REF=V_A–V_B, so the previous equation reduces to: ΔT=C_INTR_X. 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.