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FDC1004QEVM: Capacitance Sensing Accuracy

Xinghao Huang
Prodigy 10 points
Part Number: FDC1004QEVM
Other Parts Discussed in Thread: FDC1004

Hi Ti Support Team,

My name is Xinghao. I am a graduate student from the University of Southern California. I am currently using the FDC1004QEVM board to measure multiple capacitive strain sensors. However, I noticed there is a big difference between the capacitance measured by this board and our lab's LCR meter. For example, I tested a 2 pF +/0.25pF ceramic capacitor, I got 2.19 pF on my LCR meter at 20kHz frequency. But for FDC1004, I got around 8 pF for single ended measurement after connecting SHIELD 1 on my coaxial cables (the sensor's two ends are connected to CIN1 and GND). I also tried differential measurement by connecting one end of my sensor to CIN1 and taking CIN1-CIN2, and it gave me about 4 pF. It seems I still need to remove more parasitic capacitance because the manual says this board actually measures anything between the sensor and ground. In addition, CIN1 and CIN2 are probably interfering each other, but I am not too sure. Can you help me on this? 

Thank you very much. 

Best,

Xinghao

  • 0
    TI__Mastermind 40295 points

    Xinghao,

    The FDC1004 is better at measuring a change in capacitance, rather than an absolute capacitance. 
    It is not intended to serve as an LCR meter.

    You are correct in saying the FDC1004 measures the sum total capacitance on the device pins, including the sensor and all parasitics.
    For this reason, many FDC1004 applications use a calibration step to account for an offset in capacitance due to parasitics.

    The best approach is to make all connections as short as possible and minimize ground planes near the sensors and their connections.
    Use the active shields to help manage EMI. The active shields serve as "virtual grounds" but don't load the sensors or connections because they are driven with the same waveform as the sensors. 

    Many questions about the use of the FDC1004 are addressed by published app notes, with a comprehensive list given on the E2E FDC1004 FAQ page. 
    Please take a look at these, and if you have additional questions, please feel free to post them to this forum.

    Regards,
    John