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FDC2214-Q1: Calculation of the correct value for the CHx_SETTLECOUNT register

Simon Schnider
Prodigy 85 points
Part Number: FDC2214-Q1
Other Parts Discussed in Thread: FDC2214

Hi everybody,

I’m working with the FDC2214 and did a first few measurements. In the datasheet of the FDC2214 I saw the very helpful chapter 8.2.4 Recommended Initial Register Configuration Values. But for me it’s not clear, how to calculate the value of CHx_SETTLECOUNT (Page 44, Point 3 (a)). The formula includes the following variable:

Vpk -> I set to 1.68V

fREFx -> I set to 40MHz

C -> means this the capacitive of the sensor (copper plate) or the additional capacitive?

IDRIVEx -> I set to 15 (from Point 2, Page 43)

So my question is, which value I do have to set for C and did I set the other variables correct. I don’t understand how you get the result of 7.5.

 

Best Regards

Simon Schnider

  • 0
    TI__Genius 14270 points

    Hello Simon,

    The settlecount is used to provide time for the sensor signal to stabilize. You need to use he total sensor capacitance, which includes the sensor plates and any other parasitics. Keep in mind that the you need to handle the maximum capacitance, so you should use the include the fixed cap tolerances and senosr plate with highest possible target interaction.

    You can probe the signal to see how long the sensor amplitude takes to stabilize. The extra 10pF from the probe will even provide a bit of extra margin.

    Regards,

    ChrisO

  • 0 in reply to Chris Oberhauser66
    Prodigy 85 points
    Hi Chris,

    Thanks for your quick answer. So I have to measure the time until the signal has been stabilized and then I can calculate the value of the CHx_SETTLECOUNT register by the following formula (data sheet page 16) - is this correct?

    Measured_stabilisation_time(in second) = (CHx_SETTLECOUNT*16)/40MHz

    Because I'm not sure about the capacity of my "sensors". I'm using two channels - at every channel I connected a copper tape (500mm x 10mm). I need 100 samples per second (for each channel) and the resolution is not so important.


    Best Regards

    Simon
  • 0 in reply to Simon Schnider
    TI__Genius 14270 points
    Hello Simon,

    Sorry for the delay in response. I recommend simply measuring the settle time with a scope and use that value.

    Regards,

    ChrisO