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Original question:

FDC1004-Q1: FDC1004 capacitance accuracy problem

FDC1004-Q1: Measurement probe capacitance effect

Aykut Altuntop
Prodigy 80 points
Part Number: FDC1004-Q1
Other Parts Discussed in Thread: FDC1004EVM, FDC1004

Tool/software:

Hi,

I am trying to measure the capacitance of my capacitive sensor which is in pF range. I want to use the board kind of like a multimeter but if I connect any cable longer than 15cm(Including coaxial cables) the measurement saturates I am assuming from parasitic capacitance. Is there any way to use FDC1004EVM for this purpose? Thanks.

  • 0
    TI__Mastermind 44150 points

    Hello Aykut,

    First, the FDC1004 is intended for measuring a change in capacitance and not really an absolute capacitance value.  

    Yes, it is likely that you are saturating the input due to the parasitic of the cable.  You can use the CAPDAC feature to create an offset for the cable capacitance up to 96.875pF.  See section 6.3.2 of the datasheet.   

    https://www.ti.com/lit/ds/symlink/fdc1004.pdf 

  • 0 in reply to Eddie LaCost
    Prodigy 80 points

    Hello Eddie, thank you for your answer. I understand what you mean but when I start the measurement at baseline and then touch my sensor, are you saying that it will not give my sensors capacitance, shouldnt the change in capacitance be equal to my absolute sensor capacitance in this case?

    Also the problem is I can compensate the cable capacitance when the cables are open but the offset wont give the same difference in different measurements. For example when I measure a 7.5pF capacitor as 11.5pF and calibrate accordingly, a 1.5pF capacitor is measured as 4pF. 

    I managed to minimize this difference when I tried a 2cm cable instead and used a longer usb cable to plug the device but still there is a difference in the measured values.

  • 0 in reply to Aykut Altuntop
    TI__Mastermind 44150 points

    Aykut,

    The FDC1004 will show that there is an increase in capacitance, but likely won't be very accurate for the absolute capacitance.  External parasitics from the PCB will also play a role here.  The capacitance change measured is relative to the system.  The idea behind these capacitive sensing systems is to have a repeatable change in capacitance that can be measured even if it isnt the exact absolute value.

    You could try measuring several different values and see if there is a consistent offset from the actual value that you could use as a correction factor.  We don't have any test data for this to share though.