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FDC2214EVM: input range issue

Gerald
Genius 9570 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Hi Experts,

Our customer is working with a FDC2214EVM board but have issues with measuring higher capacitance range. 
The range should go to 250nF according to the data sheet, but they can't measure a 100nF connected to channel 3.

Is there some parameter that they need to change? I hope you can help.

Regards,
Gerald

  • 0
    TI__Mastermind 40295 points

    Gerald,
    As the data sheet shows, a typical max capacitance of 250nF is for a 1mH inductor and a 10kHz sensor resonant frequency. 
    Is this what the customer is using, or have they picked different values?
    What are their goals?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 9570 points

    Hi John,

    Thank you for your repsonse. Customer said that it is indeed the data-sheet shows it can go up to 250nF, but in reality it doesn't go over ~33nF. He is using default EVM module with FDC2214 chip. The frequencies to choose from in GUI are: 1-10Mhz. 10kHz is not an option in the default settings.
    So the question is, how to measure with the EVM standard hardware (1mH inductor if I'm right) the max capacitance of 250nF? Thank you.

    Kind regards,
    Gerald

  • +1 in reply to Gerald
    TI__Mastermind 40295 points

    Gerald,

    An important point to remember is these devices are not like conventional data converters; the sensor is a tuned circuit that provides the FDC device input.

    The varying sensor capacitance causes a shift in the resonant frequency and this is what the device measures (by design).
    These devices are really intended to measure a change in capacitance, and not a precise value of a fixed capacitance.

    The customer will most likely need to experiment with different values of sensor capacitance, and the EVM's lumped-element L & C to get the capacitance shift they are looking for.
    These tuned circuits have bandwidths that must be considered, and they can do that with the well-known, simple equations relating R,L,C and Q.
    Otherwise they may looking for a sensor capacitance range that exceeds their input circuit's bandwidth.
    That may be what is happening now.

    There is some information in one of the later sections (10.2.4) of the  FDC2214 data sheet that may help too.

    Regards,
    John