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FDC2114: Measure capacitance that is part of a different circuit

Yaswanth Rangineni
Prodigy 10 points
Part Number: FDC2114

Hello,

I am trying to use FDC EVM board to measure a capacitor that is part of a larger circuit operating at a higher frequency.  I am able to manage filtering so that FDC is not seeing any high frequency noise. However, I see that the FDC values are getting impacted when the parallel capacitors in the larger circuit change.  Has anyone tried using FDC for such kind of application? 

For example, in circuit below C2 is the capacitor that I need to measure. However, the measured value from FDC is changing when C1 or L1 change.  Any suggestions on how to make the FDC in-sensitive to everything else except C2? 

  • 0
    TI__Mastermind 35585 points

    Yaswanth,

    Thank you for your inquiry and your interest in TI products.

    It may not be possible given the constraints of your design, but I suspect the best approach would be to adjust the values of L1 & C1 so they cause minimal loading on C2. 
    As an alternative, would it be possible to add some resistance between C2 and RF in, or between L1 and RF in to reduce the impact of the L1 & C1 reactance on C2?


    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Thank you John for your response. 

    We don't have liberty to change L1 or C1.  Is it possible to make changes to L & C on the IN1A & IN1B such that it can shift the total resonance to a different spectrum window where L1/C1 does not impact the measurement of C2? 

  • 0 in reply to Yaswanth Rangineni
    TI__Mastermind 35585 points

    Yaswanth,
    The approach you propose could work, as long as the shifted values for L & C on IN1x support resonant frequencies, Q, and Rp that fall within the recommended ranges in the data sheet.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hi John,

    What would be the best way to simulate this to optimize the L&C on IN1x? TI website is only showing the ibis model which we cannot use for any regular SPICE simulation to do a circuit level analysis. Is there a model that TI can share which will still protect TI proprietary information, act like a black box in SPICE and I can use that for my simulation to optimize and see how the FDC would respond?  Without that, the iterative/empirical method to just try out some values is inefficient.   

  • 0 in reply to Yaswanth Rangineni
    TI__Mastermind 35585 points

    Yaswanth,

    I agree simulation is much more efficient than trial-and-error on the bench.
    We do not provide Spice models for our FDC or LDC devices at this time.

    It might be possible to tune the L's and C's in simulation by creating a circuit with L1, C1, the other L's and C's, and the filters, and doing an AC sweep using a current source between INxA and INxB.
    Once you have the desired frequency response, you could confirm behavior in the time domain with a TRAN sim, again with a current source driver, set to the desired drive level. Hopefully this will speed up confirmation of the design on the bench with the FDC2114.

    I hope this helps. Please let me know if you have any questions.

    Regards,
    John