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FDC2214: Frequency resolution calculation

Felix Luedeke
Prodigy 20 points
Part Number: FDC2214

Hi,

I'm developing a capacitive wetness sensor which uses the FDC2214. For my calculations and documentation I would like to take a closer look at the resolution of my system.

Therefore I would like redo the calculations and plots (like Fig.5) shown in this appnote (I assume the LDC161x and the FDC221x are very similar) for my use case. Are there formulars for the calculation of the frequency resolution? 

Best regards,

Felix

  • 0
    TI__Genius 17440 points

    Hello Felix,

    You are correct that the LDC161x and the FDC221x are very similar, and the resolution app note you referenced is also applicable to the FDC221x devices. However, I would not recommend redoing the graphs shown in the app note. It's more useful to use the general principles in the app note and measure the resolution that you can achieve in your system.

    To maximize your effective resolution, use the following principles:

    1. Maximize RCOUNT (use the slowest sample rate possible)
    2. Set your sensor frequency between 400kHz and 6MHz
    3. Keep your sensor oscillation amplitude between 1.2V - 1.8V 
    4. Use a 40MHz external oscillator with low jitter and drift across temperature. The external oscillator on the FDC2214EVM is a good option.

    To measure your system resolution, follow these steps:

    1. Break off the default sensors on the FDC2214EVM and connect your own sensors.
    2. Open the Sensing Solutions GUI and configure the FDC2214 for the above specifications (drive current, RCOUNT setting, oscillator selection, etc).
    3. Go to the Data Streaming tab.
    4. Once streaming data, click on "Show Statistics." This will show the standard deviation for the measurements. 
    5. Without a target present, calculate your noise floor by multiplying the standard deviation by 6. For robust sensing, we recommend an SNR of at least 10. So 10x your noise floor would be the minimum output code shift you can reliably measure (which you can use to calculate your maximum resolution with equation 2 in the datasheet).

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 20 points

    Hello Kristin,

    thank you for the fast answer. My sensor has to use frequencies around 100khz with a variation of approx 5%, so the scales of the graphs are quiet bad for my application.

    Further I'm more interested in the theoretical achivable resolution and the mathematical connection between frequency, RCOUNT and resolution, even if this is not relevant for the real application. My goal is to find a (theoretical) tradeoff for my system.

    Best regards,

    Felix 

  • 0 in reply to Felix Luedeke
    TI__Genius 17440 points

    Hello Felix,

    Some of the resolution calculations are based on the internal architecture of the device, so I'm unable to share those formulas. For the resolution based on quantization error, you can use equation 2 in the datasheet. For resolution based on RCOUNT, you can use the information on page 17 of the datasheet pertaining to ENOB. This blog post may also help.

    Best Regards,