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FDC1004: Question

user5929126
Prodigy 150 points
Part Number: FDC1004


If the target 1 to be measured (capacitance known) and the capacitor 2 (capacitance known) are connected in series, can the FDC 1004 measure the capacitance correctly?

(Example)
When target 1 is 150 pF and capacitor 2 is 100 pF
Is it recognized as 60 pF of synthetic capacity as reading on the IC ?

Imagewise, the potential between target 1 and capacitor 2 can not be determined.
I feel that I can not measure correctly with short time charge transfer.
Best Regards,
Koji Kuboyama
  • 0
    TI__Mastermind 23810 points
    Yes, that should work; however, make sure that the CAPDAC setting is correct. The CAPDAC introduces an offset to the capacitance measurement since the FDC1004 only supports +-15pF of dynamic range.
  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    Thank you for answering.
    It is an additional question.
    If offset setting is performed with the measurement target and capacitor connected in series, the linearity between the offset setting value and the measured value can not be seen.
    Is this a characteristic of the IC?
    Or is it the effect of series connection?
  • 0 in reply to user5929126
    TI__Mastermind 23810 points
    It is possible that if the capacitors are different types or if they are physically spaced further away from each other, that there could be some non-linear behavior. We haven't done as much testing with series capacitive connections, so you could easily test it by repeating the same experiment with a 60pF capacitor by itself.
  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    Thank you.
    One more point, if the capacitors are not connected in series, does the offset setting have linearity? Or can there be non-linear behavior with or without series connection? Or, in normal use, is it linear behavior?
  • 0 in reply to user5929126
    TI__Mastermind 23810 points
    The CAPDAC setting should be fairly linear, but it isn't perfect. There will also be some small gain and offset error of the CAPDAC as you sweep through the possible settings.
  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    In short, the use of capacitors in series is irrelevant to the linearity of the offset, right?
    If only gain setting and offset error are affected...
  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    I tested by changing the value of the offset. As the offset setting value increased, the offset error increased. Is this a characteristic of the IC?
  • 0 in reply to user5929126
    TI__Mastermind 23810 points

    Can you share your data? When I ran a quick test here, I did not see that behavior. Here is my data:

    Offset (pF) Capacitance (pF)
    -5 -4.40
    -10 -9.40
    -15 -14.40
    0 0.60
    5 5.60
    10 10.60
    15 15.60
  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    It is my measurement data.
    Capacity to measurement point[pF] Offset[pF] Capacitance[pF]
    1.859 14 15
    1.859 13 14.096
    1.859 8 9.407
    1.859 4 5.659
    1.859 2 3.783
    1.859 1 2.846
    1.859 0.5 2.377
    1.859 0 1.859
    1.859 -0.5 1.390
    1.859 -1 0.922
    1.859 -1.8 0.171
    1.859 -3.125 -1.071
    1.859 -6.25 -4.001
    1.859 -9.375 -6.913
    1.859 -12.5 -9.862
    1.859 -15.625 -12.696
    1.859 -18.75 -15.000
    1.859 -21.875 -15.000
  • 0 in reply to user5929126
    TI__Mastermind 23810 points

    Your data looks pretty different from mine. If you disregard the over +-15pF measurements (which is out of range), the data has a consistent slope, meaning that you could apply a gain correction factor.

    However, I don't understand why you are seeing the gain error in the first place.

  • 0 in reply to Clancy Soehren
    Prodigy 150 points
    Currently, measurement tests are being conducted, and the test conditions are as follows:
    1.The capacity of the measurement object is about 150 pF.
    2.Because it is out of the measurement range, adjustment to the offset range by series connection of capacitors.
    3.Adjustment within the measurement range by setting the offset.
    From the above, it is determined that the linearity of the offset is important.
    Does the linearity of the offset be affected by the set gain?
  • 0 in reply to user5929126
    TI__Mastermind 23810 points

    It looks like the figure I attached in my last post did not load correctly. My point is that your data has a systematic gain error that could be calibrated out. It might be interesting to see how the data looks for a single offset setting with varying capacitance.

    However, my data shows very little error and does not match your data. Do you see the same results when using a single capacitor (instead of the series capacitors that you are currently using)?

  • 0 in reply to Clancy Soehren
    Prodigy 150 points

    It is a measurement result of a single capacitor.

    Connection capacity[pF] Capacity to measurement point[pF] Offset[pF] Capacitance
    [pF]
    10.31 1.934 8 15.000
    10.31 1.934 5 15.000
    10.31 1.934 2 13.470
    10.31 1.934 1 12.532
    10.31 1.934 0.8 12.344
    10.31 1.934 0.5 12.064
    10.31 1.934 0.2 11.783
    10.31 1.934 0.1 11.688
    10.31 1.934 0 11.595
    10.31 1.934 -0.1 11.501
    10.31 1.934 -0.2 11.409
    10.31 1.934 -0.5 11.126
    10.31 1.934 -0.8 10.844
    10.31 1.934 -1 10.657
    10.31 1.934 -2 9.721
    10.31 1.934 -3.125 8.665
    10.31 1.934 -5 6.908
    10.31 1.934 -6.25 5.734
    10.31 1.934 -9.375 2.833
    10.31 1.934 -10 2.220
    10.31 1.934 -12.5 -0.126
    10.31 1.934 -15.625 -2.958
    10.31 1.934 -16 -3.405
    10.31 1.934 -18.75 -5.748
    10.31 1.934 -21.875 -8.766
    10.31 1.934 -25 -11.541
    10.31 1.934 -28.125 -15.000
  • 0 in reply to user5929126
    TI__Mastermind 23810 points

    Thanks for the additional data set. It looks like you are still seeing error here as well. Let me run a few more tests and I will get back to you by the end of the week.