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FDC2212: Capacitance

Part Number: FDC2212

Hi,

Good Day. I have a customer who is working with FDC2212. Please see below his query for your reference. Thank you very much.

Kindly provide the details on drift in capacitance measurement due to following:   1) Effect of 3.3v variation on Frequency measurement  %  2) Effect of ambient temperature on Frequency measurement in terms of %   3)  In the given illustrated example in data sheet page 42  Freq calculated is 5.15Mhz ,  is it ok if we get 4.2 Mhz  4) Does the frequency of tank circuit depend on the register configuration settings or only the tank circuit L C  18uh - 33pf . We are using differential mode

Best Regards,

Ray Vincent

  • Hi Ray,

    Thank you for posting to the Sensing forum.

    1) Effect of 3.3v variation on Frequency measurement  %

    Figure 9 in the datasheet shows the reference frequency variation across Vdd for several different temperatures, as the reference frequency is used to measure sensor frequency.

    2) Effect of ambient temperature on Frequency measurement in terms of %

    Figure 8 in the datasheet shows the internal reference clock frequency as a function of temperature. The digitized output of each channel is proportional to f_sensor / f_ref.

    3)  In the given illustrated example in data sheet page 42  Freq calculated is 5.15Mhz ,  is it ok if we get 4.2 Mhz

    Yes that is fine, as 4.2 MHz is within the device specifications for sensor excitation frequency.

    4) Does the frequency of tank circuit depend on the register configuration settings or only the tank circuit L C  18uh - 33pf . We are using differential mode

    The sensor frequency will depend on both the register configurations as well as the L and C values. Equations 8 and 9 on page 19 of the device datasheet have further details on this.

    Best regards,

    Nicole

  • Thanks . These queries were posted by me. For queries 1 & 2 , In terms of %  , how much tolerance do we consider in design.

    Yes that is fine, as 4.2 MHz is within the device specifications for sensor excitation frequency. Device Specification excitation frequency range is  10Khz to 10Mhz ?    Does this not affect capacitance measurement if frequency drift is from Design value?

    What is the expected variation in capacitance tolerance ,  actual vs measured in %

  • Hi Shekhar,

    Seen in the attached graphs above, the frequency drift as a result of Vdd variations is very minimal. Additionally, the shift as a result of temperature appears to be a drift of approximately 0.14% from -40C to 125C. 

    For the effect of frequency on capacitance measurements, please refer to Equation 7 of the datasheet:

    What is the expected variation in capacitance tolerance ,  actual vs measured in %

    The variation in capacitance tolerance will depend heavily on individual system parameters, and as a result there is no set value that is published in the datasheet. 

    Best regards,

    Nicole

  • Hi , Thanks for your reply. Can you suggest a method where we can check for Actual vs measured capacitance  so we get error  tolerance .

  • Hi Shekhar,

    You should be able to use Equation 7 from the datasheet to calculate the expected sensor capacitance, using the parallel sensor capacitance and the sensor frequency. This value can be compared with any capacitance values that are taken during measurement.

    Best regards,

    Nicole