A low-cost microcontroller interface for low-value capacitive sensors

Microcontrollers with embedded timers can measure resistances or capacitances by determining the charging or discharging time of an RC circuit. The microcontroller-based interfaces proposed for capacitive sensors have not been analyzed in detail, and basic information such as capacitance range, stray capacitance compensation, and accuracy is not available. This paper analyzes the performance of these interfaces when measuring capacitances in the picofarad range. The effects of stray capacitances are evaluated and reduced by applying the three-signal calibration technique. For the PIC16F873 microcontroller, the absolute error achieved is below 4% FSR for 1 pF < C_x < 10 pF, and below 1.5% for 10 pF < C_x < 100 pF.