Integrator enables simple ohmmeter with gigohm range
With the aid of an external counter and control circuitry, this system can measure resistances in the tens of gigohms.
Stefano Salvatori and Gennaro Conte, Università degli Studi di Roma Tre, Rome, Italy; Edited by Charles H Small and Fran Granville -- EDN, May 24, 2007
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. 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.
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.
