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FDC1004Q Periodically Grounding CIN

Zachary Hammond
Prodigy 30 points


Hello,

I am using the FDC1004Q capacitance-to-digital converter to provide position feedback of a linear actuator.  I would like to occasionally "touch off" or short the sense electrode to the grounded electrode to home the actuator. I am wondering if this is within the application scope of the chip or if it is detrimental to the chip's performance. I would also like to know what I should expect the measured capacitance to be when the sense electrode is grounded.

Thank you for your help.

Zack

  • 0
    TI__Expert 7400 points
    Hi Zachary:

    Let me check with the designers about this specific case, and I'll let you know as soon as possible.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Prodigy 30 points
    Thank you, David. I look forward to hearing your response.

    In the meantime I was thinking about ways I can still manage to touch-off without grounding the CIN pin in the case that doing so will damage the chip. I was thinking about two approaches to this. One would be adding a capacitor in series with the sense capacitor and the other is adding a resistor in series with the sense capacitor.

    If I use a capacitor, the CIN pin is always isolated from ground through a capacitor. The larger this capacitor is, the less effect it has on the equivalent capacitance of the two capacitors in series. On the other hand, the larger the capacitor is, the more current will be consumed in charging the capacitor. I think this method will not simply provide a capacitance offset because there will be a non-linear relationship between measured capacitance and the sense capacitance. I have heard that this can dilute the sensitivity of the capacitance measurement.

    If I use a resistor, the CIN pin will have to source current equal to I=V/R through the resistor when the sense electrode is grounded. I think during normal operation, this resistor will produce a constant offset in the capacitance measurement, which can be corrected with the CAPDAC. Hopefully, a resistor value can be selected such that the measurement offset is small and the current through the resistor is within the specification of the excitation driver. I also think that adding this resistor may help provide extra ESD protection, which may be a concern in my application.

    Any input you have on the thoughts above or other methods of allowing the actuator to touch-off would be greatly appreciated.
  • 0 in reply to Zachary Hammond
    TI__Expert 7400 points
    Hi Zachary:

    I confirmed with the designer and yes, by shorting the CINx when it's selected will cause damage to the device. If CINx is not selected, connecting it to GND isn't a problem.

    As you mention above, it is possible to use a resistor in series with your primary electrode. You would need to make Rs >50kohm to limit the current and protect the chip. If the time constant R1*Cs, where Cs is the sensor capacitance, is <<10us, where 10us is the sampling clock period, the insertion of R1 would not degrade the measurement. The designers also confirmed this.

    Hope that helps.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Prodigy 30 points

    Thank you. I appreciate your help.