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FDC2214: Some question about the capacitance calculation formula when using FDC2214

user6519346
Prodigy 151 points
Part Number: FDC2214

As we know, FDC2214 uses parallel resonance to achieve capacitance measurement. And the  formula of resonance frequency is f=1/(2pi LC).

I now use single-ended mode,and the sensor is only attached to one end.

In this case, can the sensor and the external capacitance/inductance be considered in parallel?

Or can we use the resonance formula above to calculate the capacitance of the sensor?

In addition, I found this in the documentation SNOA952. Is this the equivalent circuit in single-ended mode?

Best regards!

  • 0
    TI__Mastermind 35885 points

    For your configuration you can assume the Cs can be considered to be in parallel with Co and Lo as long as Co and Lo are connected to earth ground.
    The consideration will be how close the sensor is to an object representing ground. 

    For example, if the sensor is out in "space" and has no nearby objects to act as ground, then its equivalent capacitance Cs will be small, and it won't contribute much to the resonant frequency.

    But if the sensor is near an object that acts as a ground (like a grounded metal object, or a human hand) then its equivalent capacitance is much higher and it will affect the resonant frequency.

    Please let me know if you have any more questions.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 151 points

    Thanks for your response.

    I use FDC2214EVM and connect it to the a laptop with a grounded power cord.

    And the sensor configuration is single-ended configuration,just like the below picture.

    In this case, can external inductor L0 and capacitor C0 be considered grounded?

    In addition, when the target is not grounded, how should this situation be analyzed?

    Regards.

  • 0 in reply to user6519346
    TI__Mastermind 35885 points

    L0 & C0 can be considered grounded in the last scenario you described.
    As for the ungrounded target, we don't have much advice to offer since the behavior will have a case-by-case dependence on the target's size & electrical properties. Approximating that scenario in an EM simulator (like FEMM) would be the best approach.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 151 points

    Okay. Thank you.

    Regards.