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# FDC2114EVM: Question about LC capacitive sensing

Part Number: FDC2114EVM

I am using the FDC2114 chip for a university senior project, and I want to make sure that I understand the technical side of what the sensor is actually doing, because this is not clear from the datasheet.

As I understand it, the capacitor is fully charged (in the attached diagram it would be done by closing S1 and reopening after charging), then the charged capacitor is connected in series to an inductor (in this diagram, closing S2, though there are other ways to achieve this), and then an ADC measures the frequency across the inductor as Vout oscillates between +V and -V. This should be the resonant frequency, from which it backs out the capacitance from sqrt(LC).

Is this the general idea behind what the FDC2114 does, even if there is some extra circuitry to clean up the signal?

• Evan,

That method would work, but it isn't what the FDC2114 does.

Instead of charging up the capacitor one time, the device uses a current source to drive the circuit at its resonant frequency. It can detect the resonant frequency by monitoring the zero crossings of INxA and INxB. You can think of it as a pendulum. You push in one direction, then when it crosses the center, you start pushing in the other direction. Later, there is a counter that measures the frequency which can be used to calculate the capacitance.
• In reply to Clancy Soehren:

That makes sense. Thanks!
• In reply to Clancy Soehren:

So just to make sure I'm understanding you correctly, does this diagram (and calculations) represent the mechanism that is going on in the FDC2114? (technically speaking, the frequency output would be the reciprocal of the resonant frequency, but the parameters can still be detected)

• In reply to Evan Archibald Meikleham:

Evan,

You re-posted this follow-up question in a new thread, so we will continue the discussion there:

e2e.ti.com/.../730378