A microcontroller-based interface circuit for lossy capacitive sensors

This paper introduces and analyses a low-cost microcontroller-based interface circuit for lossy capacitive sensors, i.e. sensors whose parasitic conductance ( G x ) is not negligible. Such a circuit relies on a previous circuit also proposed by the authors, in which the sensor is directly connected to a microcontroller without using either a signal conditioner or an analogue-to-digital converter in the signal path. The novel circuit uses the same hardware, but it performs an additional measurement and executes a new calibration technique. As a result, the sensitivity of the circuit to G x decreases significantly (a factor higher than ten), but not completely due to the input capacitances of the port pins of the microcontroller. Experimental results show a relative error in the capacitance measurement below 1% for G x < 200 nS, which is quite remarkable considering the simplicity of the circuit proposed. The measurement of a commercial capacitive humidity sensor subjected to condensation (in order to have a significant value of G x ) shows the effectiveness of the circuit.