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Original question:

FDC2214: the way to estimate resolution

FDC2214EVM: Calculate Resolution and refer it to a capacitance

6041604
Prodigy 40 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Hello,

referring to I'd like to calculate the resolution of my measurement and express it then as capacitance value.

I.e. I want to measure a sensor capacitance of around Cmin = 183pF to Cmax=200pF with a p-p noise of 4,95fF. How can I translate the resolution into a capacitance value?

Thanks in advance!

Lukas

  • 0
    TI__Genius 17440 points

    Hello Lukas,

    Can you clarify your question? The E2E question you linked to has a detailed explanation of how to measure the system's resolution in units of capacitance.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 40 points

    Hello Kristin,

    thanks for the fast response. I got an idea how to calculate the resolution in bits. But what’s the capacitance this resolution refers to? Could you help me with this, maybe for the example I posted? I measured the values mentioned with the FDC2214EVM single channel measurement on channel 3, with the basic configurations (didn’t change sth in the configurations, besides choosing single channel measurement).

    I’d like to give the sensor value +/- the error/accuracy given by the measurement I’m doing with the FDC2214EVM.

    Thanks,

    Best regards

  • 0 in reply to 6041604
    TI__Genius 17440 points

    Hello,

    The output of the FDC2214EVM is actually a frequency measurement, not a capacitance measurement. The frequency measurement will always have some noise, even if it is minimal. You can use the values in the LC tank to calculate the sensor capacitance on the electrode. This is likely the capacitance value you mentioned above. The peak to peak noise is the measured changes in the frequency measurement. Although the noise may not fully result from changes in capacitance, you can convert it from frequency to capacitance, which represents your noise floor in your capacitance measurement. Your SNR should be at least 10 for robust sensing. So, 10 times the capacitive noise floor is the minimum capacitance shift that you can measure. 

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 40 points

    Hello,

    thanks. I have some more questions:

    1.) Could you tell me how the measurement works? The datasheet only tells "The core uses a reference frequency (fREF) to measure the sensor frequency. fREF is derived from either an internal reference clock (oscillator), or an externally supplied clock." How exactly is the measurement done? Is it realized with a counter?

    2.) How can I determine the frequency measurement accuracy? For an alternative measurement I use an IC that gives a rectangular signal with a frequency determined by C. I did a reference measurement and received a relative error of 1%. I'd like to make a statement about whether the FDC2214 is better suitable for my application or not.

    Best Regards

  • 0 in reply to 6041604
    TI__Genius 17440 points

    Hello,

    1. Unfortunately that information is proprietary and I can't share exactly how the measurement is done. The digital output codes are a ratio of the sensor frequency to the reference frequency.

    2. It's very difficult (and for most applications, not particularly useful) to determine the frequency measurement accuracy. It's much more useful to measure your noise floor and the minimum signal response from your target. Using this information you should be able to determine the accuracy of your target's position calculation using the FDC2214 versus the target's actual physical position.

    Best Regards,