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FDC2214EVM: Single-ended level sensing, configuration of "surrounding ground plane"

Dominic Germana
Prodigy 20 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Hello,

I'm implementing a level sensing proof of concept prototype using the FDC2214EVM.  Referring to the FDC2214 datasheet, it shows in section 10.2 a "typical application" of level sensing implemented with single-ended sensor electrodes "with a ground plane surrounding the electrodes." I've done some prototyping with differential sensor electrodes, but I would like to proceed with single-ended, because ultimately my application will multiplex several sensor electrode sets to a single FDC2214, and single-ended sensors will reduce the number of signals I need to multiplex.

My question is, what is the function of the surrounding ground plane (vs just floating single-ended electrodes with no reference ground nearby), and what is the optimal configuration of the ground plane relative to the sensor electrodes for maximum sensitivity.  For example, should I have a size-matched ground electrode next to each sensor electrode, separated by a couple mm, as I would if doing a differential sensor configuration, or should the ground be completely surrounding the other electrodes with minimal separation, or some other configuration?  It's difficult to tell what the intended configuration is from the datasheet figure 56.

Thank you very much!

Dominic

  • 0
    TI__Genius 14270 points
    Hi Dominic,

    The ground planes act as a shield for the sensors. In this way, the sensitivity in some directions can be minimized. For example, for a level detection application, you only want the fluid level to change the capacitance. Without the ground shield, movement on the opposite side of the sensor could disrupt your measurement.

    There is a tradeoff in sensitivity when there is more ground shielding. Use as much ground to protect from interference. Position the ground as far away as possible (for example, use a thicker PCB) to maintain sensitivity.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 20 points

    Thank you for the reply Chris.  In my prototype, I used a single layer copper board and used a utility knife to cut the copper into sensor sections.  I found that having grounded copper next to the sensor electrodes actually increased the sensitivity.  I assume this is because ground is the reference for the measurement signal, so having ground next to the electrode was sort of like having a differential sensor configuration for half of the signal cycle.  I understand from your response that if I had a ground plane on another layer "behind" the sensor electrodes, that ground plane would have provided a shielding effect from items behind the shield, but at the expense of reducing the sensitivity of the sensor electrodes.

    By the way, for a next prototype, I will consider adding additional shielding.  I assume active shielding provides better shielding performance than just ground.  Does active shielding also decrease the sensitivity?  I was thinking that based on one of the TI documents I read, it would actually sort of focus the area of sensitivity and therefore increase the sensitivity in the desired direction while decreasing sensitivity (shielding) in other directions.

    Thank you again,

    Dominic

  • 0 in reply to Dominic Germana
    TI__Genius 14270 points

    Hello Dominic,

    I recommend using the ground only for shielding from undesired stimuli. You can improve the performance of the FDC2214 by splitting the cap sensor into 2 equal sized sections, and tying one section to INxA and the other to INxB. This is better than using ground - the FDC actually drives the sensor as a half-rectified sine-wave; one channel is always tied to ground! In this way, INxA is your sensor and INxB is its ground; when the phase inverts, then INxB is the sensor and INxA is the ground. This design allows for you to increase your sensor area and provide better performance.

    Use ground behind both sensor sections if needed for shielding. Finally, place a pair of ferrite choke of ~400ohms between the sensor and the inductor.

    Refer to for some more information.

    Regards,

    ChrisO