The IVC102 precision integrator features internal capacitors. The circuit in the figure allows you to measure very-high-resistance values of RX. A precision difference amplifier, 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. The delta time between two such events relates to the system parameters by the expression: ΔT=CINT[(VA– VB)/VREF]RX, 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: ΔT=CINTRX. Also note that the precision of resistors R1, R2, and R3 is not critical. The difference amplifier guarantees the precision of the ohmmeter.

EDNAOL 2016JUN03 AN 03FigFigure: Using an IVC102 precision integrator, this circuit can measure resistances in the gigohm range.

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.