Enabling a simple ohmmeter with gigohm range

Article By : Stefano Salvatori

Through the use of an external counter and control circuitry, this system can measure resistances in the tens of gigohms.

The IVC102 precision integrator features internal capacitors. The circuit in the figure allows you to measure very-high-resistance values of R_X. A precision difference amplifier, 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.

Figure: 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.

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