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FDC2214EVM: Capacitance measurement setup

Yu-Lung KAO
Prodigy 10 points
Part Number: FDC2214EVM
Here is our measurement requirements.
1. sampling rate: 100~1KHz, higher sampling rate is good to have.
2. measured capacitance range: 5-10nF
3. accuracy: 1pF is good enough
4. single end capacitance measurement 
5. repeat single channel measurement
 
Do you have any suggestion for the LC value and register setting?
Here is my plan for the setting:
If I am wrong, please correct me.
1. Fsensor: I plan to make it 160Kz
2. divider setting and external oscillator for Fref: divider is 60 and 40MHz osc?  (Fref > 4xFsensor)
3. Idrive setting: ?
4. Inductor and capacitor for fsensor: 100uH and 1nF
5. settlecount: 10
6. Rcount: 8192
Thank you.
  • 0
    TI__Mastermind 40400 points

    Hello Yu-Lung,

    Thanks for posting on the forum. I am looking into your requirements and will provide detailed feedback tomorrow.

  • +1
    TI__Mastermind 40400 points

    Yu-Lung,

    1. 160kHz is okay.

    2. Fref needs to be 35MHz or less for single channel operation and the valid range of fsensor is less than fref/4, so you could really use a smaller divider.  This would also help with your sampling rate as the formula below is used to determine the activation time.  Conversion time will also be affected.

       

    3. I recommend the app note below.  This is in regards to the LDC inductive sensing devices, but the same applies to FDC.  In short, you will want to adjust idrive to obtain 1.2V to 1.8V sensor amplitude.  

    https://www.ti.com/lit/an/snoa950/snoa950.pdf 

    4. This is okay.

    5. Settle count should be set based on the formula below, so you will need to determine your amplitude and idrive setting first.

    6. This will likely work for your application, but you may find you need to adjust this during testing.  Higher count will give a better accuracy, but require more time.  Lower count will be faster, but less accurate.  You can alter this and determine what rcount works best for your requirements.

    Another thing to keep in mind is that our capacitive sensing devices are better at determining a change in capacitance rather than an absolute capacitance value, so I want to make sure that you are not looking to make an absolute measurement.

  • 0
    Prodigy 10 points

    Hi Eddie,
    Thanks for your suggestion.

    I measured 1.5nF/1.5nFx2/1.5nFx3 caps with FDC2214EVM.

    I measured with single end.

    But the result shown in the GUI is half of cap value.

    Do the GUI show the result of differential cap?

    Taank you.

      Cap measured(pF)
    1.5nF 1500 721
    1.5nFx2 3000 1362
    1.5nFx3 4500 1944
  • 0 in reply to Yu-Lung KAO
    TI__Mastermind 40400 points

    The EVM is set up to use single ended or differential.  The CH0 and CH1 sensors that are on the board are single ended.  

    Can you measure the signal amplitude?  You may need to adjust idrive if the voltage is not between 1.2V and 1.8V.

  • 0 in reply to Eddie LaCost
    Prodigy 10 points

    Eddie,
    I checked the Vpk is around 1V with Idrive =30 at L=100nH and C=1.5nF.
    but if I hook up the measured cap load(2.5nF), the Vpk is 1.5V.

    The amplitude is tuned with load or without load?

    I will try with larger inductor.

    Thank you.

  • +1 in reply to Yu-Lung KAO
    TI__Mastermind 40400 points

    The amplitude should be tuned to be within 1.2V and 1.8V ideally when the target is within the ranges that are expected for your application.

  • 0 in reply to Eddie LaCost
    Prodigy 10 points

    Hi Eddie,
    Thanks for your help,

    I found the capacitance value difference is from the measurement configuration.

    single-ended measurement will get half value of differential measurement, and the differential measurement result is same as multimeter,

    the Single-ended measurement will have INA to the metal pad of capacitor, INB is floating, and another pad of capacitor is grounded.

    the differential measurement will have INA to the one pad of capacitor, INB to the other pad of capacitor.

    Is there any way to have single-ended measurement get similar result as differential measurement?

    Thank you.