Direct interface circuit for capacitive humidity sensors

This paper proposes and analyses a direct interface circuit for capacitive humidity sensors. Such a circuit relies on directly connecting the sensor to a microcontroller without using either a signal conditioner or an analogue-to-digital converter, thus resulting in a simple, compact, low-cost and low-power interface circuit. In spite of its simplicity, the interface circuit performs satisfactorily when measuring commercial capacitive humidity sensors (Philips H1 and Humirel HS1101). The non-linearity error of the interface circuit itself (0.1% full scale span (FSS)) is much smaller than that of the sensor (5.6% FSS for the H1 and 2.1% FSS for the HS1101) and, therefore, the circuit does not limit the accuracy (in terms of linearity) of the measurement, as is to be expected from a well-designed interface circuit. Furthermore, the effective resolution equals 9 bits for a measuring time of 50 ms, which are fully acceptable for humidity measurements.