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I've been trying to design precise distance measurements similar to the digital caliper or dial micrometer for my CNC project by capacitance sensing between prob and base metal. I can connect base metal to ground and prob to 5-12 DC volt for capacitance measurement OR connect base metal to AC for impedance measurement THEN feed to 1 or multi-stage OPAMP for amplitude signals My QUESTION is which one is more ACCURATE and more immune to NOISE

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I am afraid no one will be able to give you an ultimate answer, since your question has uncertainties regarding the accuracy you need and the time and cost budget for the project, but I can give some direction.

Note, that impedance and capacitance measurement is not the same, and that you can't measure the capacitance with a DC voltage. For impedance measurement you have to use either an AC voltage and measure the amplitude and phase of the current, or vice versa. This is way too complicated for what you need.

There are readout solutions for a capacitive sensor. Google them! The two basic ideas I am aware of are to

  1. form an oscillator with the capacitance and measure the frequency - even digitally with a microcontroller - or
  2. measure the dV/dt due to a known current reference.

For 1) a precise solution could be to implement a Wien-bridge oscilaltor (+1opamp +1 cap +1 res +1 FET). Bridge structures in general are always good ideas to improve the measurement accuracy.

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