Signal Conditioning Circuit Design Of Sounding Capacitive Sensors

The capacitive humidity sensor reflects changes in humidity by altering capacitance value. Polymer material on the surface of humidity capacity adsorbs ambient water vapor molecules, which contributes to changes of dielectric constant that causing changes of capacitance value. There is a certain determined relationship between relative humidity and humidity sensitive capacitance value, and humidity condition in the environment can be obtained indirectly by detecting the capacitance value of the sensor. Capacitive sensor is outstanding for its small size, low cost, good stability, high sensitivity, thus having been widely used in the field of meteorology. The capacitance value and the amount of change in capacitance of the sensor is much less than the stray capacitance of the surrounding environment, and how to achieve high-precision detection of weak capacitive signal under the condition of noise interference has being the key to improve the performance of the sensor in the meteorological observation areasIn this thesis, an extremely low capacitance detection system based on the basic principles of the charge amplifier was designed, aiming at the requirements of high accuracy and low noise in the field of high-altitude meteorological observation. By researching domestic and foreign capacitance detection circuits and analyzing several mainstream detecting circuits, the charge amplifier circuit was selected for the extremely low capacitance detection, and thus the detection circuit principle and anti-stray were analyzed. The hardware system design included:sine wave signal generating circuit, the detection circuit of the charge amplifier, rectifier and filter circuit, A/D analog-to-digital conversion circuit and MCU control circuit or the like. The feasibility of the crystal in frequency stabilization was analyzed by theory and simulation, and combined with the practical application demand of the steady increase, a sine wave crystal oscillator circuit was designed for amplitude-frequency stability. Parameters of key components of the capacitance detection system were optimized to improve the performance of the detection circuit.The conclusion of debugging and experiment shows that the developed detection circuit has a strong anti-stray, its non-linear reaches0.46%, the accuracy is better than0.04%, resolution is10fF, and it works stably. The research has a certain significance of improving requires of accuracy and low noise performance in the meteorological observation.