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FDC2112: Increase the sensitivity of FDC2112

Part Number: FDC2112
Other Parts Discussed in Thread: FDC2212

Hi team,

  1. One of my customer is using FDC2112 to detect the change of capacitance between 5pF~20pF with a resolution requirement of at least 0.2 pF per bit. But the actual resolution in their application is only about 1pF. They have already set the gain to the maximum value 16 and do u have any other advice about how to improve the resolution? The circuit they are using is the typical application as attached.
  2. Actually I don't know about the device very well and  in my opinion, the resolution of FD2112 is about 250/2^12/16=3.8nF. Is it right?   So I am very confused about the correct way to calculate the resolution.
  3. The customer also found that if they decrease the capacitance of C from 33pF to 16 pF and the resolution has increased. Could u tell me the reason and what is  the minimum capacitance of C ?
  • Hello, 

    In addition to changing the gain, the RCOUNT setting for the channel has an impact on the resolution obtained. A higher RCOUNT setting will give a lower sample rate but better SNR for the measurement and in turn, better resolution. Additionally, the FDC2212 is a higher resolution device and gives access to more bits for a finer measurement result. 

    The resolution of the device depends on the sensor design, the reference clock, and the register settings used for the application. using the max input capacitance to calculate the expected resolution of the device isn't the correct way due to these dependencies. For a given design, you need to consider how many of the bits are actually changing for the range of the measurement to know what the resolution per bit is.

    The FDC2112 operates on a resonant architecture based on the LC tank attached to the sensor channel. Decreasing the capacitance of the LC tank means that the sample frequency of the sensor is increased. This allows for more samples to be collected during the sampling window which is why they experienced the increased resolution in their test. 

    Best Regards, 
    Justin Beigel