RC Decay

The figure below shows a Resistor-Capacitor (RC) Decay circuit that conditions a capacitive sensor. The time it takes for the voltage to decay to a threshold is a function of the capacitance, CSEN. A PIC® MCU produces a single pole step response (voltage) with an RC decay. This circuit measures the decay time, where the threshold is proportional to VDD. R1 has a low temperature coefficient to minimize temperature errors. The PIC MCU provides the switching and control needed.



  • Ratiometric correction of VDD and temperature
  • High accuracy
  • Simple timing measurement


  • PIC® MCU resolution limitation
  • Digital noise
  • Threshold needs to be ratiometric

Sensor Examples:

  • Capacitive humidity sensor
  • Capacitive touch sensor
  • Capacitive tank level sensor

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