Capacitance meter (Part 2)

Part 1234

Before getting further on with maths and code, let’s have a look to the electronics. The proposed circuit is as usual as simple as possible. However, care shall be taken with the choice of components. The resistor has a high ohmic value. If you cannot get a precision resistor, you will have to measure its resistance with a precision meter and record the measured value for future use. The operational amplifier shall satisfy the the following requirements:

  • Single supply (5 V compatible)
  • Low input biasing current (pA range)
  • Rail to Rail input and output
  • Low noise, low drift

I personnaly used a MAX492 from my shelves, any equivalent will work. Do not use this component for massive future productions as it sounds like it is outdated 😉

The analog circuit paths shall be constrained to their minimal length in order to avoid parasitic phenomenom and prevent radio electric waves pickup. See the synchronisation line which shall be very usefull for debugging and displaying signals on a scope. Again, nothing really serious about the electronics. Here is an illustration of the prototype that I used: it is based on a Duemilanove, an Adafruit Industries proto shield and a couple of my special quick prototyping components.

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