Abstract:It is challenging to achieve high accuracy, low noise for the humidity measurements in the field of upper-air meteorological observation. To tackle this problem, a capacitance measuring circuit based on charge amplifier principle has been developed for sounding humidity sensors. It includes crystal oscillation, charge amplifiter, rectifying filter, A/D converter and MCU. The measurement results demonstrate that the circuit features several advantages, stray-immune, the linearity, and a high measurement accuracy of 0.04% error. This circuit can achieve a capacitance resolution of 100 PPM (RMS) from a nominal sensor capacitance of 100 pF, which indicates that the circuit is feasible for high altitude humidity sensing.