Analysis of a Direct Interface Circuit for Capacitive Sensors

We present the theoretical analysis and performance results of a direct microcontroller unit (MCU) interface circuit for capacitive sensors based on the charge-transfer method, when stray capacitances are considered. The interface circuit can implement two alternative two-point calibration techniques that reduce the effects of stray capacitance, temperature, and MCU parameters that depend on the power supply voltage. The best measurement deviation achieved from 0degC to 50degC and for power supply voltage from 4.0 to 5 V is below 0.01 full-scale range (FSR) for the two subranges from 10 to 100 pF and from 100 pF to 1 nF and 0.08 FSR for the subrange from 2 to 10 pF.