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LDC1612: Capacitive coupling

Sami Lakka76
Prodigy 220 points
Part Number: LDC1612

Hi,

I have an application where we need to measure displacement of a 2mm diameter conductive target. The measurement is done from relatively close, approx 1-2mm and the displacement is about 0.4mm. 

The problem is that the target is attached to skin, and we see with some existing coils quite a lot of capacitive coupling. Are there any methods to filter the capacitive effects from the measurement, e.g. by using two coils?

  • 0
    TI__Genius 14270 points
    Hi Sami,

    Yes, there are several approaches - one is to use a differential configuration. Another approach is to create a larger parasitic capacitance between the LDC ground and the target; in this way the parasitic capacitance effects are attenuated.

    Note that with a small target of only 2mm, a larger sensor will see a reduced response.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 220 points

    Thanks Chris,

    Can you comment on the size of the coil that we should target? I assume that the coil size should be around the same as the target size.

    Regards,

    Sami

  • 0 in reply to Sami Lakka76
    TI__Genius 14270 points
    Hi Sami,

    You had said the target is 2mm in diameter and up to 2mm away; I would recommend a smaller sensor; you can get to a 5mm diameter 2 layer design with 11 turns/layer and ~420pF sensor cap.

    You will need to consider the desired resolution and sample rate. With a small target you will not be able to construct a differential sensing configuration. You will be able to measure relative position changes quite accurately. Can you increase the size of the target? I would recommend Cu or Al target (or even silver :) ) composition of at least 80um thickness if possible

    Regards,

    ChrisO