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Original question:

FDC2214: FDC2214 output values

FDC2214: FDC2214-q1 DATAx ?

Cezar Morun1
Prodigy 250 points
Part Number: FDC2214

According to the FDC2214 datasheet (page 14):

DATAx = fSENSORx*2^28/fREFx

Does it mean that DATAx is not a function of conversion time (set in R_COUNTx)?

  • 0
    TI__Mastermind 21785 points

    Hello Cezar,

    Thanks for considering to use Texas Instruments in your design.  You are correct, DATAx is not a function of the conversion time.  Based upon your programming settings and the clock you use, the device will use a specific conversion time over which the sinusoidal signal from a given input pin is sampled and compared against the reference sinusoidal signal.  So long as you are not re-configuring the frequency settings or changing the clock source, that conversion time will always have the same duration for each conversion period.  However, the duration of each input sinusoidal period is expected to change with respect to the capacitance seen by the input pin.

  • 0
    Prodigy 250 points

    Thanks Patrick.

    One more question:

    How to estimate the measurement accuracy using a specific content of the relevant registers?

    In my application (power consumption is very important) I would like to have T_COUNTx as small as possible. However I am also looking to get at least 16 bit accuracy.

    Thanks again,

    Cezar

  • 0 in reply to Cezar Morun1
    TI__Mastermind 21785 points

    Hello Cezar,

    I dont think there is a T_COUNTx variable with this device; however, I think you meant you would like to minimize the conversion time and CHx_RCount.  Im presuming you would like to do this to quickly get your measurement and then enter into either the sleep or shutdown mode in between measurements.  Based on the details provided in the design example starting on page 41 and ending on page 42, I think you will need to use the max sampling frequency.  If you want the lowest possible RCount you will then need to use the highest fref frequency.  

    I need to check with my team to see how these settings might affect accuracy.  I will get back to you when I have more details.

  • 0 in reply to Patrick Simmons
    TI__Mastermind 21785 points

    Hello Cezar,

    According to what I was able to learn from my team and our existing collateral, for a given sensor frequency, resolution should improve with increasing reference frequency.  However, how good that resolution is depends upon your sensor frequency, which depends upon your LC tank design.  The latter part likely hinges on what kind of board space you have to work with and the consistency of that layout would then depend on manufacturing tolerance. 

    To see how the frequency ratio affects the expected resolution, I recommend looking  at this document.  Its for our inductive sensing devices, yet the same principles apply to our capacitive devices.  Also, I would recommend looking at this FAQ page.  It has a link to a power reduction application report as well as several other application reports.