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How to measure differential Capacitance at 10kSPS

Question asked by ThomasG on Apr 16, 2015
Latest reply on Apr 30, 2015 by jdobler

Hello

My setup shall measure tiny differences between two capacitors.

 

The setup has two Capacitors in the order of a few pF, build of discrete metal plates at some millimeters distance.

One is CREF and does not change (temperature and humidity now neglected).

The other is CVAR and changes when objects appear between the metal plates.

The capacitance change might be in the order of 20-100 fF.

 

Is there a way to use some modulation/demodulation scheme to detect the capacitance change?

I assume that simple AM/FM radio circuits do more or less the same.

A capacitive voltage divider, a generator for stimulation and something like the AD8302 Gain/Phase detector might work, but this device is too costly.

Connecting both Caps on one side to GND, with the other terminal to one resistor each which are connected to a stimulating generator would make a bridge. The two midpoints lead again to some Gain/Phase information. But how to setup an inexpensive detection circuit?

 

An analog output is perfectly acceptable.

The environment is industrial, hence noisy.

 

Best regards and thanks in advance.

Thomas

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