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FDC2214: When connecting the FDC2214 board and the capacitance sensor with an FPC

Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

Hi, all

We received the following questions from our customer.

[Customer's questions]
(1) When water droplets on FPC (Flexible printed circuits), we recognize that the sensor's capacitance value changes greatly,
  but as shown in the experiment below, there was almost no change with FDC2214.
  Does the FDC2214 take any measures against such water droplets?
  The LCR meter measures the impedance to calculate the capacitance, but the FDC2214 calculates the capacitance by changing the resonance frequency.
  Why is FDC2214 less susceptible?

・When the FDC2214 mounting board and the capacitance sensor are connected via the FPC, there is a change of about 1 to 2 pF when water droplets fall on the FPC.
・When the LCR meter and sensor are connected via FPC, when water droplets fall on the FPC, it changes by about 10pF to 20pF.

(2) As the resistance value on the FPC increases, the measured capacitance decreases. Please tell me the reason why the measured capacitance value decreases.
it would be helpful if you could explain it with a calculation formula.

Best regards,

  • Toshi,

    The FDC2214 doesn't really function like an LCR meter.
    The FDC2214 uses a PCB inductor to form a resonant circuit with the capacitive sensor, and this sets the frequency for the sensor.
    It might be worth experimenting with the sensor frequency to try and optimize for droplet detection.
    Unfortunately we don't have any specific equations to help with this. 

    If you are using our EVM, you can change the sensor frequency by adjusting the capacitors and/or inductors as shown in Figure 44 of the FDC2214 EVM Users Guide.
    You can use the well-known equation ω = 1/√(LC) for calculations, remembering to account for any parasitics on your PCB and wire harnesses.

    Just so you know, we are recommending the FDC1004 over the FDC2xxx devices.
    The FDC1004 has built-in active shield drivers, which can be a very useful feature for managing EMI. The FDC2xxx devices do not have this feature
    The FDC1004 also does not use a resonant circuit as the input. It uses a fixed 25kHz frequency.
    You can find more info on the FDC1004 on its product page and on the E2E FDC1004 Capacitive Sensing FAQs page.