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FDC2114EVM: Capacitance change required wrt to two electrodes of capacitor, and not between electrode and ground.

Vijay Shirhatti
Prodigy 40 points
Part Number: FDC2114EVM

1.    I have connected a Capacitive sensor to Ch2 of FDC2114EVM such that both electrodes are connected to the terminals on board. The sensor is designed such that, application of pressure on one face should change the capacitance of the device. I am getting a steady capacitance of apprx. 10 nF initially. Ideally on application of pressure I should get a deviation from this value, but is not the case I observed. The capacitance is varying only when I take my hand near it, just as in case of proximity sensor. 

I can observe a clear change in capacitance change with pressure applied using a LCR meter. This EVM would give me the portabiity if it works.

Is it that the slow variation in capacitance due to applied signal, is seen as noise and is being filtered out by the board. 

2.   Also, are the 18uH and 33pF filter bank built on the board or do I need to put the values on my own.

3.  As the EVM feature says it has noise rejection capabiltiy, how do I ensure that the capacitance variation rejected is noise and not the signal.

  • 0
    TI__Genius 11340 points
    Hi Vijay,

    How are you applying pressure to the sensor? If I understand you right, when pressure is applied, you are able to see a change in capacitance with the LCR meter but not on the EVM GUI, is that right?

    The 18uH inductor and 33pF capacitor are on the EVM. You could change these values to change the resonant frequency if need be.

    Best,
    Jiashow
  • 0 in reply to Jiashow Ho
    Prodigy 40 points

    Yes Jiashow, When pressure is applied I can see the change with LCR meter but not with EVM. However further experiments showed there was indeed slight change in capacitance shown by EVM.

    Further query :

    1.  LCR meter is operated at 1V bias, 1kHz frequency. It is observed that EVM does not have this option to change bias voltage and measuring frequency.

    LCR meter gives capacitance of 10nF @ 1V, 1kHz. But the EVM shows capacitance of ~150pF with measured freq of ~4MHz. Is there any way to reduce the measurement frequency in EVM. We know that in a practical capacitor, inductance increases to a large value at higher frequencies, thus reducing the total impedance and hence the capacitance value. The LCR meter has a variable freq for the same purpose so that measurements for non-ideal capacitors can be taken at low frequencies.

    2.  Please confirm whether both terminals of capacitor can be floating or do I need to ground one of the terminal.

    Sorry for posing so many quaries at once.

    Regards,

    Vijay Shirhatti

  • 0 in reply to Vijay Shirhatti
    TI__Genius 11340 points
    Hi Vijay,

    In general, you can increase the values of the inductor and capacitor to reduce the sensor frequency.

    You should have one terminal of the sensor connected to IN2A and the other left floating.

    Best,
    Jiashow
  • 0 in reply to Jiashow Ho
    Prodigy 40 points

    Dear Jiashow,

    Sorry for miscommunication there.

    One terminal connected to IN2A is understood. Whether the other terminal should be connected to IN2B or Ground or kept open/floating.

    Also do you mean to add extra inductors or capacitors apart from those on the EVM. Or can I just tweak the inductor value in GUI considering the already available inductance in my sensor.

    Thanks & regards,

    Vijay Shirhatti

  • 0 in reply to Vijay Shirhatti
    TI__Genius 11340 points
    Hi Vijay,

    The other terminal should be left floating.

    You would need to physically change the inductor/capacitor in the sensor filter bank and update the new values in the GUI to reflect the actual component value.

    Best,
    Jiashow