FDC2114: wrong calculation of capacity values

Hello,

I am using the FDC2114 for capacitive measurements. I measure capacities in the range from 22 pF to 100 pF and I observe a deviation from the actual capacities I cannot explain. Can you please provide detailed feedback as the FDC2114 device is not working as expected?

For my sensor capacity, I use "reference capacitors", meaning that I insert capacitors from 22 pF to 100 pF instead of a real sensor.

I face some issues concerning the calculation of the capacitance out of the conversion result from the DATA register. Here are the conversion results, the sensor frequency and the capacity I calculate out of the sensor frequency and the comparison with the reference / actual capacity.

For the calculation of fSENSOR, I use formula (8) from the data sheet with the following register / frequency configuration: FIN_SEL = 2, f_REF = 2,4 MHz, OUTPUT_GAIN = 0, OFFSET = 0.

I provide an Excel file with a section of the circuit diagram, the data from above and a graphical representation of these data.

Capacity_values.xlsx

The issue I am concerned about in these data is the fact that the calculated capacity is below the actual capacity for some capacitors and it is above the actual capacity for some other capacitors.

Of course, my readout of the DATA register and therefore the calculated capacity values do also include parasitic capacities. But this doesn't explain the graph I see above. I checked with a LCR meter that parasitic capacities add roughly the same capacity to each actual capacity.

I also tried a second LC tank for my measurements. These are the measurement results:

A section of the circuit diagram, the measurement results from the table above and a graphical representation is also provided in the Excel file from above. For the measurements with the second LC tank, I also observe the behaviour that some calculated capacities are below the actual capacity and some capacities are above the actual capacity.

Can you please look in detail at the provided information and give detailed feeback about my issues?

Best regards,

Andrea