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FDC2214EVM: Problems measuring the range of capacitance 30 ... 2000 pF, in a single-ended configuration.

Georgy Tverdokhlebov
Prodigy 20 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Good day.
When testing the FDC2214 EVM, I had problems measuring capacitance in the range of 30 ... 2000 pF. Schemes for an asymmetric configuration.

There were two problems:
1) The measured capacity was about 2 times less. But I sort of figured out this problem (https://e2e.ti.com/support/sensors/f/1023/p/489881/2108249?tisearch=e2e-sitesearch&keymatch=FDC2214#2108249).
2) At the minimum measured capacitance (30 pF), I set the parameter Idrive so that the signal amplitude on the LC is no higher than 1.8 V.

After an increase in the measured capacitance (the pictures are shown below), the signal amplitude decreases and when the capacitance exceeds 300 pF, the generation practically ceases.

Cmin (~30 pF)

+100 pF

+100 pF

+300 pF

+300 pF

+1000 pF

+1000 pF

Please tell me whether I made the settings correctly or if I need other values ​​C and L for my task.


Thanks in advance.

  • 0
    TI__Genius 17440 points

    Hello Georgy,

    What you're seeing is a large change in the Rp value of the sensor. That's described in more detail in section 1.2 of this application note. Different Rp values require different drive current settings to keep the sensor oscillation amplitude within the optimal range (1.2V - 1.8V). You have two options to mitigate this problem:

    1. You can change the drive current setting multiple times keep the sensor oscillation amplitude within the optimal range for the full sensor capacitance range. The problem with this approach is that changing the drive current will introduce offsets in the digital output code, which can reduce the sensing resolution. 

    2. You can add a parallel resistor to the LC tank to minimize the shift in Rp. This will reduce the Q factor of your sensor, which may increase the noise floor. This application note may help you measure your sensor Rp value. I suggest selecting a resistor value close to your minimum Rp (which occurs at your maximum sensor C).

    I also strongly recommend using a differential configuration for this application, not a single-ended configuration. In a single-ended configuration, INxA and INxB will see significantly different loads, because the sensor capacitance is only connected to one side. This can lead to significantly different Rp values during each half of the sensor oscillation, which can cause one side to be well outside of the optimal amplitude range. For example, if you use a much stronger drive current to compensate for the 1000pF sensor capacitance, then the other side of the LC tank may oscillate well above 1.8V. This can cause accuracy issues at best and long term reliability issues at worst.The image below illustrates the situation.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 20 points

    Good day.
    Thank you for your quick response.
    I tried the second option you proposed. According to the documentation recommended by you, I measured Pp (at L = 18 μH and C = 2000 pF) it turned out to be ~ 2.3 kOhm, but this only slightly improved the performance, the signal still goes out with increasing capacitance.


    Regarding the differential configuration. I have no way to use it, since the second lining of the sensor can only be grounded.

    What can give a change in the denominations LC of the bank?

  • 0 in reply to Georgy Tverdokhlebov
    TI__Genius 17440 points

    Hi Georgy,

    Unfortunately if you cannot use the differential configuration then I cannot recommend using the FDC2214 for such a large change in sensor capacitance. Either INxA or INxB will be significantly outside of the recommended amplitude range, which will prevent the FDC device from effectively driving the sensor.

    Changing the LC values of the tank will not solve the changing Rp problem.

    Best Regards,