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FDC2114: Design Tradeoffs when Selecting Resonant L & C

Kaushik Jayaram
Prodigy 20 points
Part Number: FDC2114

Hello,

Sorry in advanced if this question is a bit broad. I'm using the FDC2114 in a position sensing robotics application where I have two electrodes sliding over one another which forms a variable capacitor (10s of pF nominally) that is directly proportional to the position of an appendage in X or Y. Weight and positional accuracy are the two main design drivers here.

From a weight perspective I want to make my resonant tank inductor as small as possible physically, but this will limit my maximum inductance value and increase series resistance. Since this will lower my Q-factor I suspect I would take a hit in capacitance accuracy, and sensitivity.

Long story short, when I'm selecting my L & C value to set my resonant frequency, what do I need to consider in order to get the most accurate/sensitive design, while minimizing the weight of my magnetics?

Cheers,

Parker

  • +1
    TI__Mastermind 18545 points

    Hello Parker, 

    The capacitance of the LC tank will change how much the capacitive change of your sensor will impact the frequency of the sensor. If you have a smaller capacitance on the LC tank, the shift in your sensor will cause a greater shift in your sensor frequency, which will give your more effective resolution. When choosing your inductor, you can use this to help set the operating frequency of the LC tank but make sure to have your sensor frequency below the SRF of the inductor: 

    If you are minimizing your inductor you will also have to keep in mind the maximum sensor frequency of 10MHz. Having a Q factor too low could lead to less resolution as the sensor frequency measurement could have increased noise so you are correct in your statement. If you can keep the Q factor above 10, then it should not be a problem. 

    Best Regards, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Prodigy 20 points

    Hi Justin!

    Yes that all makes sense, thank you for the clear response. I will select my capacitance on the LC tank to obtain the desired sensitivity, and then select an L to guarantee I remain within the acceptable sensor frequency range (10KHz to 10MHz).

    Since I am trying to minimize L, C may need to increase to keep Fsensor < 10 MHz, which will reduce my sensitivity. To combat this I will likely increase the physical size of my sensor electrode to increase its change in capacitance during movement.

    Thanks,

    Parker