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Original question:

FDC2214: capacitance over time

FDC2214: long wire optimization

Marc Friedheim
Prodigy 110 points
Part Number: FDC2214

Hello,

I'm trying to measure a capacitor over 2 seperate 1,5m long wires. Each wire is shielded and connected to GND.
At 20nF the results are good. At larger capacitances (e.g.) 40nF and, the signals amplitude can't be raised above 0,75V and the sinus shows a dip. Likely due to stability/resonance issues.

Do you have any recommendation on how to optimize signalling over longer wires since we do need to measure over quite a distance.

Thanks,

  • 0
    TI__Mastermind 40295 points

    Marc,

    You mentioned lower-tan-desired signal amplitudes.

    We have found monitoring pins can be difficult, and even a high-impedance probe can make it look like the waveforms are distorted, and/or low-amplitude.
    Please try re-probing the waveforms with a 1k leaded resistor between the probe tip and the test point.

    regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 110 points

    Hi John, already measured with 1k resistor between point and probe. It does work well if capacitor is connected close to the pcb but when the capacitor is 1,5m away using the shielded wires makes the signal distorted. The shield of the wires is connected to gnd. Should I use another approach measuring capacitance which is connected over 1,5m long wires? 

  • 0 in reply to Marc Friedheim
    TI__Mastermind 40295 points

    Marc,

    Could you describe your overall goals for your application?

    For example, do you need to measure the absolute capacitance, or a change in capacitance?
    what range of capacitances do you need to measure, and with what accuracy?
    What constraints do you faced on your choice of cables?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 110 points

    Hi John, we're trying to measure a interleaved layout capacitance over a distance. We mainly need to measure capacitive change. We now work for a 20nF sensor but also explore a 90nF one. Since we seem to be stuck already at 40,im consulting for possible solutions. No major constraints with regards to cables. In my experience with this chip, do far, grounded shielded cabled appear to perform best. I hope you get a clear picture of our situation and you can provide a or some possibilities? 

  • 0 in reply to Marc Friedheim
    TI__Mastermind 40295 points

    Marc,

    Our FDC  parts are better at measuring changes in capacitance as opposed to absolute capacitance, which aligns with your goals.

    The FDC2xxx series of devices can be sensitive to EMI; hopefully the choice of shielded cables will help with that, but the actual capacitive sensor may need to be shielded somehow to avoid EMI-related issues.

    The device will react to the total RLC at the input pins, so as a guess, the parasitic & distributed RLC of the cables will need to be considered as part of the overall sensor design. 

    Regards,
    John