Goodday!
I am currently designing a humidity sensor module that is based on a capacitive sensor element (MK33-W). The full range of the capacitor is approx. between 250 to 370 pF, where 0,45pF/%RH is the measurement output, the rest is manufacturing inaccuracy of sensor.
Combining properties from both the sensor and the FDC datasheet my calculation came to set the CHx_SETTLECOUNT to 0,0956 (rounded up to 1). This number seems quite small, and I am not sure, if my values are selected correct:
Vpk = 1,6V
Csens = 300 pF sensor + 10 pF parasitic
Fclk from external source = 4MHz
Fref (CH0_FREF_DIVIDER set to 400) = 10kHz (recommended frequency for MK33-W)
IDRIVE set to minimum = 16uA (settlecount is inversely proportional to idrive, higher current leads to smaller settlecount)
The required output resolution should correspond to 0,1pF.
I also suppose that a selection of 1mH + 27nF LC tank (around 9,6 kHz oscillation freq.) is suitable for single ended configuration.
My calculation is theoretical without actaul measurement, because I am looking into different solutions to convert capacity to digital value. I would like to appreciate some guidelines to define the accuracy output of my system. Temperature compensation with calibration is considered, so I am mainly interested in the ability of the FDC component.
Best Regards
Istvan Szeker