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FDC2212: Designing for temperature stability

Philip Trapasso
Prodigy 30 points
Part Number: FDC2212
Other Parts Discussed in Thread: FDC1004

I am using the FDC2212, in differential mode with an external 25 MHz oscillator and C0G tank (LC) capacitor, to measure the capacitance of a concentric tube sensor. Overall I am very happy with the sensor, but the measurements change as function of the ambient temperature, even when a fixed capacitor C0G is soldered to the input. The expected change in capacitance from the fixed capacitor is about 0.45 pF, but I am seeing changes of 1.5 pF.

I have confirmed through experimental testing that the composition of the inductor core has an effect on temperature stability of the measurement system, with iron core being the most stable. I then switched the external oscillator from 25 ppm/c to 0.2 ppm/c, but surprisingly, the temperature stability of the system did not improve. My question is: what other factors effect the temperature stability of the measurement system?

  • 0
    TI__Expert 6480 points
    Hi Philip,

    The dominating factor should be the capacitor and a C0G capacitor has a temperature rating of about 30ppm/K. Using an external oscillator was the right move as the internal oscillator is much less stable. Are you measuring across ambient temperature? About what range?

    Also, just for more knowledge, what sort of application is this for? Is there any way to try and use a reference sensor to help mitigate the drift?

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 30 points
    Rachel,

    Thank you for the response, and confirming that I am on the right path. The temperature range for my application is 60 C to -40 C, but the testing that I have done so far is between room temperature and -40 C.

    The concentric tube sensor will be used to measure the level of cryogenic liquids. Your point about using the reference sensor (presumably the other channel?) is interesting, and something I'm willing to explore. Can you elaborate on how I would go about using the reference to mitigate drift?

    Phil
  • 0 in reply to Philip Trapasso
    TI__Expert 6480 points
    Hi Phil,

    The ratiometric approach shown in this app note may be helpful: www.ti.com/.../snoa925.pdf. This is written for the FDC1004 device but the idea can also be applied to the FDC2212.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 30 points
    Rachel,

    Thank you for your help with this topic. I think I have all the information I need to move forward. You can close this thread.

    Phil