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FDC2214EVM: The tested value of capacitance compared with the actual value

Chun-Hsien Chen
Prodigy 10 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214, FDC1004

Hello,

I'm curious about the initial value/ error of the capacitance.

I've tried measuring a few different capacitors with both FDC2214 EVM and with E4990A impedance analyzer from Keysight. It turned out that there's a distinctive difference between the results from the two instruments, with the difference of around 20 pF when the capacitance is small and with 30ish pF when the capacitance is larger.

With the result mentioned above, I want to ask whether the difference is like a base value or a measurement error?  It's important for us to get a more accurate value.

Thanks!

Chun-Hsien

  • 0
    TI__Mastermind 40295 points

    Chun-Hsien,

    In general, our capacitive sensors are better at detecting a change in capacitance as opposed to an absolute capacitance. 
    They were never intended to serve as the equivalent of capacitance meters.
    In particular, we do not recommend the FDC2xx devices, and advise the use of the FDC1004 instead.

    Users have struggled to finalize designs with the FDC2xxx because of EMI and the FDC1004 has active shields that aid in EMI management.
    You can find out more about the FDC1004 at the FDC1004 Product Folder and the E2E FDC1004 FAQ Page.

    The FDC2xxx devices use an LC-resonant oscillator that shifts in frequency due to a nearby object changing the capacitance of the sensor.
    The tolerance of the LC oscillator will control the nominal sensor frequency and therefor the reported nominal capacitance of the sensor.
    You might be able to improve the precision by picking sensor inductors and capacitors with low tolerances, minimizing PCB parasitics, and having a comprehensive calibration protocol. 

    The FDC1004 uses a switch-cap front end instead of an LC resonant circuit, so it is inherently more precise.
    The nominal 16b measurement range is ±16pF, which can be extended to 0pF to 110pF with internal offsets. 
    If this will suit your application, you can find out more at the URLs given earlier.

    Regards,
    John