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FDC2214; the way to calculate capacitance

Takao Yamamoto
Genius 4135 points
Other Parts Discussed in Thread: FDC2214EVM

Dear all,

The formula to calculate resonant frequency is f=1/2π√(L1*(C1+C2)).

I think that this formula is refer to the Figure 1 model.

On the other hand, how can we calculate the capacitance from resonant frequency from Figure 2 model?

Could you teach me the way to calculate?

*please look at attached file about Figure 1 and 2

resonant frequency.pdf

Best regards,

  • 0
    TI__Expert 8050 points
    Hello Yamamoto-san,

    The formula is valid for a differential configuration as shown in Figure 55 of the datasheet, i.e. C2 is in parallel with C1.

    For a single-ended configuration as shown in Figure 1 of your attachment, the calculation is a bit different. The LC tank is driven by a half-sine wave, and either node is active for half of the period. When one node is active, the other is grounded, and vice versa.

    In your Figure 1, let's call the top node IN0 and bottom node IN1. When IN0 is active, IN1 is grounded. In this case, C2 is in parallel with C1, and the frequency follows the above formula. However, during the other half cycle, IN1 is active and IN0 is grounded. In this case, C2 is virtually disconnected from the tank, so only C1 shows up in the formula. The overall period of oscillation is the average of the two periods.

    Figure 2 follows a similar analysis but involves a little more math.

    Best regards,
    Yibo
  • 0 in reply to Yibo Yu
    Genius 4135 points

    Dear Yibo,

    Thank you for your reply, but your answer is not correct to my experiment results.

    I mounted 100pF instead of SENSOR plate CH0.

    100pF capacitor is mounted CH0 and GND pin like attached file.

    *please see attached file

    At this time, I found that 92pF and 3.9MHz at GUI outputs.

    Therefore, I cannot think your answer that is average of two phase.

    For example, the answer will be 4.9MHz by using formula of Figure 1 like your recommendation.

    What do you think about these difference points?

    Best regards,

  • 0 in reply to Takao Yamamoto
    TI__Expert 8050 points
    Hello Yamamoto-san,

    Did you include the parasitic capacitance in your calculation?

    The EVM should have about 50 pF total capacitance (33pF SMD cap + parasitics). If you add the 100 pF as shown, the "effective" capacitance I calculated is 93 pF, which is very close to your measurement.

    Best regards,
    Yibo
  • 0 in reply to Yibo Yu
    Prodigy 90 points

    Hi Yibo,

    I am curious about how to calculate the effective capacitance of FDC2214EVM, could you please give me some instruction?

    Thanks,

    Yuchen