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I need a sinusoidal varying capacitor to test a differential capacitance circuit. I will use the circuit to test MEMS gyroscope/accelerometer capacitance change. But right now I do not have the gyroscope/accelerometer prototype. Is there any available chip or method by which capacitance can be change by varying the input voltage by function generator.

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    \$\begingroup\$ For relatively small capacitance values, a varactor diode does the job. \$\endgroup\$ – Peter Smith Jan 2 '18 at 13:41
  • \$\begingroup\$ @PeterSmith But the capacitance is not directly proportional to applied voltage in the varactor diode. A linear relation is needed between voltage and capacitance to do this job. \$\endgroup\$ – Sumit Agrawal Jan 2 '18 at 13:47
  • \$\begingroup\$ A MOSFET perhaps. It wont be linear though. \$\endgroup\$ – Trevor_G Jan 2 '18 at 13:47
  • \$\begingroup\$ Then you'll have to find a way to precompensate the control voltage so that the capacitance varies linearly. Given that you probably only need frequencies up to the tens of kHz, I would use a microcontroller running a DDS algorithm to generate the frequency, followed by a waveform lookup, followed by a linearization lookup, followed by a DAC that drives the varactor. \$\endgroup\$ – Dave Tweed Jan 2 '18 at 14:22
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This is what a varactor diode does. The width of the depletion layer changes with applied reverse voltage. This changes the capacitance seen by small signal relative to the DC offset.

Such diodes used to be popular for fine-tuning radio receivers, the resonance of crystal oscillators, etc.

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If the bias voltage change won't bleed through too much you can use a varactor or indeed any reverse-biased diode. It's preferable to use two varactors back-to-back so that nonlinearities cancel out to a first order (for the signal, not the bias).

If you need very small or very large capacitance you may have to use a part that is not designed as a varactor.

You might want to consider a motor driving an air dielectric capacitor. Keep the separation constant and change the overlap between the plates like an old-school tuning capacitor. Keeping it sinusoidal would be an interesting field solver project.

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