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FDC2114EVM: Estimating resloution of capacitance measurement

N. Al-azzawi
Prodigy 10 points
Part Number: FDC2114EVM
Other Parts Discussed in Thread: FDC2114, LDC1314, FDC1004

Hello,

For PhD research purposes we are interested in the FDC2114 CDC. We have a strain capacitive sensors with no inductive component (essentially a stretchable two plate capacitor). We were planning to test FDC2114 with the FDC2114 EVM board. However it is very important for our application to achieve a measurement resolution of 0.01pF or even 0.001 pF if possible.

My understanding is that the resolution of the acquired FDC2114 measurement depends on the value of the sensor’s capacitance and the LC tank. Our sensors tend to have a base capacitance value between 20~30 pF and a range of change +/- 10~22pF. So my questions are as follows

  • For the EVM board which has a built-in LC tank with values L=1.8microH and C=33pF , is it possible achieve for our sensors a resolution 0.01pF or even 0.001 pF?
  • For any given sensor capacitance how can I estimate general range of resolution? assuming I'm using the EVM LC tanks.
  • Are the values in the table in page 3 of  This application note an approximation of measurement resolution for a 20pF sensor?
  • Can the method described in This post be used to approximate capacitance resolution for our strain capacitive sensors?
  • 0
    TI__Mastermind 40295 points

    What is the range (min to max) of the capacitance you wish to measure?

    The FDC2114 chip is very similar to an inductive sensor, the LDC1314, which has app note that addresses resolution and may be helpful: 
    Optimizing L Measurement Resolution for the LDC1312 and LDC1314.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hello,

    Thank you for your response,

    Depending on the produced sample the sensor value differs slightly, but the range should be covered between 10pF and 50pF max. Will the FDC2114 EVM broad be suitable to give desired resolution in this case?

  • 0 in reply to N. Al-azzawi
    TI__Mastermind 40295 points

    It is difficult to to say if the part will do what you want because there are other factors that will impact the resolution.
    External EMI is a big one. The resolution will also vary with your sensor's parameters.

    If you can support another sensor frequency, you might consider the FDC1004.
    It has a switch-cap front end and has a fixed sensor frequency of 25kHz and does not use a resonant sensor. 
    And it also has built-in active shield drivers for enhanced EMI management.

    It has16 bits of effective resolution, and the data sheet reports a measurement resolution of 0.5fF.
    The nominal measurement range is ±15pF, but this can be extended to a range of 0pF to 115pF with a programmable internal offset.

    You can find a comprehensive list of FDC1004 collateral at the FDC1004 FAQ page.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hello,

    Thank you for the recommendations,

    I do need further clarification regarding the following points:

    • Assuming I decided to use the FDC1004 for my application and I have fixed the nominal capacitance through the CAPDAC register to let’s say 40pF for example, does that mean that the +/- 15pF will be measurable from that point (meaning I can measure a range of min 25pF and max 55pF)? Will the output be simply decimated if my sensors value exceed the 55pF limit?

     

    • I’m curious about the sensor frequency term, up till now I have assumed that for the FDC2114, since it’s a resonance based circuit the sensor frequency means the resonance frequency of the equivalent circuit of the sensor connected in parallel with the LC tank (Csensor || Ltank || Ctank). But for the FDC1004 which has no resonance circuit what does the frequency of sensor represent? Is it the frequency of the signal the chip uses to charge and discharge the sensor capacitor?
  • +1 in reply to N. Al-azzawi
    TI__Mastermind 40295 points

    If you exceed the ±15pF range from the CAPDAC offset value it will saturate.

    The FDC2xxx is basically a data converter that counts the axis crossings (as opposed to crossings of a voltage threshold as the familiar A/D does) of the resonant waveform and compares the count to a reference. The sensor frequency changes as the sensor's capacitance changes.

    The FDC1004 uses a switch-capacitor interface to detect the change in sensor capacitance. The switch-cap interface does detect a change in the sensor signal, but frequency doesn't play a primary role.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Thanks a lot, 

    That was very helpful really appreciate it.