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Build your own capacitance meter

-December 01, 2009

If you need to measure capacitance but don’t have the necessary meter, you may be able to build a circuit that can handle the task. For example, if you have a data-acquisition board or module that measures frequency or signal period, you can build a circuit that converts capacitance to frequency and then measure the period with a counter.

The figure shows the block diagram that engineer Jim McLucas developed for a capacitance meter that can measure from 10 pF to 10 µF with ±1-pF accuracy. You can get a detailed schematic from “Capacitance meter uses PLL for high accuracy” on the EDN Website (Ref. 1).

TMW09_12Dig_capmtr_300px

This block diagram illustrates a capacitance meter that can measure from 10 pF to 10 µF with ±1-pF accuracy.


The circuit generates two signals of the same frequency that are 60° apart in phase. A PLL (phase-locked loop) adjusts the frequency of its VCO (voltage-controlled output) to get the two signals in phase. With the signals in phase, the VCO’s output frequency is 6 F0, which is six times the frequency produced by the RC circuit containing the capacitor under test.

The meter has two ranges: 1 pF to 0.01 µF and 0.01 µF to 10 µF. When using the higher capacitance range, the meter’s divide-by-1000 counter is in the circuit to reduce F0 so that the VCO can attain a frequency lock.

The meter’s circuit includes a divide-by-six counter in the VCO feedback loop that reduces the VCO’s input signal to its original frequency. A divide-by-three counter reduces the VCO output frequency to 2 F0. That signal can go to a frequency counter, which can display the unknown capacitance.


Reference
  1. McLucas, Jim, “Capacitance meter uses PLL for high accuracy,” EDN, October 8, 2009. www.edn.com/designideas.

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