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Original question:

FDC1004QEVM: Human Body Detection Capability of FDC1004QDGSRQ1

FDC1004QEVM: FDC2214

vijay s b
Prodigy 50 points
Part Number: FDC1004QEVM
Other Parts Discussed in Thread: FDC1004

Tool/software:

Hi John,
How to measure change in capacitance which is in terms of fraction of picofarads using FDC10004QEVM?

Thanks

  • 0
    TI__Mastermind 40295 points

    Vijay,

    The FDC1004 should be able to measure some sub-picofarad capacitances pretty easily.

    That being said, what are you trying to do exactly?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    John,
    I'm trying to measure change in capacitance to quantify liquid in a metal container with two electrodes on surface of container.

    but could not observe change in capacitance, may be change is in terms of femtofarad. is it not possible to measure with this? 

    Thanks
    Vijay

  • 0 in reply to vijay s b
    TI__Mastermind 40295 points

    Vijay.

    A capacitive sensor won't work through a conductive surface. 
    The reason a conductive surface won't work is the capacitive sensor works via an electric field, which can't penetrate the conductive material.
    The material will need to be non-conductive, like plastic.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    John,
    is there a possibility if metal container itself an electrode and top of it an insulator and then again an electrode. it will form a capacitor and that should provide a  change in capacitance along with varying fluid quantity which may be femto farads.  

    Thanks

  • 0 in reply to vijay s b
    TI__Mastermind 40295 points

    Vijay,

    Can you provide a diagram of your proposal?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Hi John,

    below is the representation.  

    Thanks

  • 0 in reply to vijay s b
    TI__Mastermind 40295 points

    Vijay,

    For a capacitive sensing system to work, there needs to be some interaction between the liquid to be measured and the electric field created by the capacitive sensing device driving the electrode. Near the electrode, the electric field in the liquid will be strongest, with it weakening as it propagates through the liquid to a point of lower potential, like a ground plane or another electrode of the opposite sign. In other words, the interaction should result in an electric field gradient as it passes through the liquid

    For the system you have drawn, the metal container will result in the same potential at all points around the liquid, so there will no change in potential - and no electric field gradient - thru the liquid. 

    One alternative that might work is to use electrodes that are immersed in the liquid, but away from the walls of the tank.

    Please be aware that this will work best if the liquid is non-conductive, like distilled water, as opposed to a conductive liquid like salt water.

    Regards,
    John