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FDC2114EVM: EVM parasitic capacitance issue

Nikhil Jadhav
Prodigy 50 points
Part Number: FDC2114EVM
Other Parts Discussed in Thread: FDC2214, FDC1004

Hi, I have ordered FDC2114EVM for evaluation purpose. I want to measure 1pF to 100pF capacitance variation. I am observing below behavior on EVM:

1) When I don't connect anything on any channel, it gives by default 50pF on each channel. When I connect any capacitance value to any EVM channel, EVM output will be 49-to-50pF higher than what is connected. For example; when I connect 5pF on ch# 2, it gives 54pF.

Am I doing something wrong here? or Is there any setting to reduce this parasitic capacitance?

Regards,

Nikhil Jadhav

  • 0
    TI__Mastermind 33170 points

    Nikhil,

    Have you made any changes to the EVM, or are you using it in its default configuration?
    Could you send snapshots of your GUI Configuration page?

    Just so you know, The FDC2xxx devices can be very challenging to shield from EMI. 

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Hi ,

    I am using EVM in default configuration. Please refer below snapshots of GUI configuration.

    I have selected channel# 2 and took few readings on graph. Also, attached graph snap and data log file for your reference.

    Regards,

    NikhilConfiguration-1Configuration-2Configuration-3Graph0160.data.csv

  • 0 in reply to Nikhil Jadhav
    TI__Mastermind 33170 points

    Nikhil,

    As the FDC2214 EVM Users Guide  shows the EVM uses 33pF capacitors and 18uH inductor in parallel with the PCB sensor.
    The 33pF caps combine with the sensor capacitance, causing the ~50pF readings you are seeing.

    It is unlikely the built-in EVM sensors will give the desired 1pF to 100pF range you are looking for.
    You will most likely need a different sensor design, as well as revised values of the PCB inductance and capacitance. 
    The FDC2214 data sheet shows the equations to support those component calculations.
    There are jumper connections on the EVM that support easy connections to an external sensor.
    Please be sure to remove the on-board EVM sensors if you decide to use external sensors.

    You haven't mentioned your end application, but the app note Capacitive Proximity Sensing Using FDC2x1y has some useful guidance on how to use the FDC2214.

    Please be aware the FDC2xxx devices can be sensitive to EMI, so please plan for robust shielding if this will be a potential concern.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 50 points

    Hi John,

    Thanks for explanation. I have only two queries now:

    1) Is there any recommended EVM configuration if I want to measure capacitance variation of 3 to 4pF? (Added parasitic capacitance is okay.)

    2) Can I set required capacitance measurement range in EVM so that I can have better accuracy?

    Regards,

    Nikhil Jadhav

  • +1 in reply to Nikhil Jadhav
    TI__Mastermind 33170 points

    Nikhil,

    It is very difficult to recommend an overall EVM configuration because the sensor parameters - L,C,Q, fres - interact with the device register settings, and the overall device configuration. Some of this is discussed in the FDC2214 device data sheet and also in the app note mentioned earlier in this thread.

    You can maximize the accuracy by choosing a lower sensor resonant frequency as opposed to a higher frequency. 
    Note that if you choose a very low frequency, some of the sensor component values can get a bit extreme and be challenging to implement.

    Another factor to maximize the accuracy is to maximize the conversion time by setting RCOUNT_CHx register to its maximum value (0xFFFF).
    This details are discussed in the device data sheet.

    A great alternative to consider would be the FDC1004. It has16b resolution, a measurement range of ±15pF, but with single-ended capacitance measurements, this can be extended to a max of 115pF using the device's CAPDAC feature. The sensor is based on a fixed-frequency 25kHz switch-cap filter rather than a resonant LC tank, so that can greatly simplify the sensor design. And unlike the FDC2xxx devices, the FDC1004 has built-in active shield drivers which can really help reduce & manage EMI. 

    Regards,
    John