I have a sensor that can be modeled as a capacitor and resistor in parallel. The problem with this sensor is the resistance is on the order of 10's MOhm and the capacitance (we believe) is single pFs.

I have tried using a resonant circuit to find the capacitance, but the results end up super noisy and with an extremely low q factor. Does anybody have any ideas of how to determine the capacitance?

  • \$\begingroup\$ Could you charge it with a moderately high voltage and effectively use something like a coulomb counter to monitor how much charge flows into it? \$\endgroup\$ – MadHatter Apr 28 '20 at 23:42
  • \$\begingroup\$ Could you add more detail about how much of an impedance change you need to measure and the resonant circuit you tried to use? \$\endgroup\$ – Matt Apr 29 '20 at 0:14

basically you want to apply a square wave of 10,000 Hertz (100 microsecond period) to charge a 10uS RC load.

You will initially show a step change in current, and then a slewing as the 10MegOHm continues to charge the 1pF capacitor.

The slewrate will tell you the RC.


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