A differential interface for trace moisture sensor

The proposed interface is a capacitance to frequency convertor (CFC), which is simple and effective solution for reading capacitive difference between the sensor capacitance and reference. The output is a square wave signal, where the signal frequency is proportional to the sensor differential capacitance. Proposed interface integrates the advantage of two basic measurement techniques, bridge and oscillator, provides accurate and digital output in the form of pulses. Initially, experiments and analysis have been performed with discrete ceramic capacitance and then to assure the effectiveness of the circuit, a capacitive humidity sensor based on sol-gel thin film porous γ-Al₂O₃ has been used. In trace moisture (10-100ppm) measurement capacitance difference of the sensor is very small in the range of few pF, requires measurement of differential capacitance. Proposed circuit is insensitive to stray capacitance between sensors nodes and ground and feature high resolution and excellent linearity as well as compatible with microcontroller. The effect of central frequency on the sensitivity of the circuit has also been analyzed.