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.

rc-decay.png

Advantages:

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

Disadvantages:

  • 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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