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Resolution

Miguel Rey Fraga
Prodigy 40 points
Part Number: FDC2214-Q1
Other Parts Discussed in Thread: FDC2214

Hello,

I would like to confirm 3 issues:

the resolution in Farads of the chip. In máximum ratings I see a max capacity of 250nF. Is the teorical resolution then 250nF/2^28=0.93fF?

the mínimum capacity posible to measure. If we use the recomended C in parallel of 33pF and L of 18uH, then the max frecuency of oscilation in sensor side is 6,5Mhz. Any capacity to be measured will be added to this 33pF and will decrease the frecuency. The mínimum capacity to be measured is the resolution then? The max is 250nF?

Capacity variation for a 1500 counts variation. If we use the recomended C in parallel of 33pF and L of 18uH, and Fref=40MHz, and CHx_FIN_SEL=1, then are the capacity values like this?:

• DATAx = 30000000 => 3.74182168049954E-11F --> 37.4182pF

• DATAx = 30001500 => 3.74111755114163E-11F --> 37.4111pF


thanks in advance,

 

Miguel

  • 0
    TI__Genius 17440 points

    Hi Miguel,

    Resolution

    While the FDC2214 is used to measure capacitance, what it is really doing is measuring the frequency of the LC tank. The output codes are digital representations of the ratio of the sensor frequency to the reference frequency. When we hold the inductance constant, we can use the shifts in sensor frequency to determine how much the capacitance has shifted. Therefore, the resolution of the device is really measured in Hz, not in Farads. With the 10MHz maximum sensor frequency and 28-bit resolution, our resolution is 0.00375Hz. This allows the device to detect sub-femtoFarad changes in capacitance. 

    The maximum capacitance you see is based on the minimum sensor frequency that the FDC2214 can drive (10kHz). This value is dependent on the inductor in the LC tank. To determine the minimum or maximum possible capacitance for a given inductance, use equation 7 in the datasheet.

    Minimum Measurable Capacitance

    The minimum measurable capacitance is based on the noise floor of your system. This is most easily measured in Hz using the EVM. Navigate to the data streaming page and start streaming the data. Click on the Show Statistics button and record both the average and the standard deviation. The average plus 6*(standard deviation) is an estimate of your noise floor. We recommend that your desired capacitance resolution yield a frequency shift at least 10 times as large as the noise floor. You can determine this using equation 7.

    Finally, your calculations look correct to me for the 1500 output code variation.

    Regards,