High Accuracy Data Acquisition System And Its Application In Capacitive Position Sensor Research

The roles of ultra-sensitive inertial sensors are more and more important in resources exploration and recovery of earth gravitational field. High accuracy data acquisition system is needed in capacitive position sensor circuit and feedback voltage read-out circuit, which are the two key techniques of ultra-sensitive inertial sensors.The measurement frequency band of inertial sensors is commonly between 10-4Hz to 1Hz, with the effect of low frequency 1/f noise, the resolution of inertial sensors is limited. In this thesis, in order to increase the low frequency resolution of capacitive sensor and reduce the noise of feedback voltage read-out circuit, the research of High accuracy data acquisition system is presented.Firstly, designed, developed and tested a data acquisition system based on a high precision 24-bit sigma-delta analog-to-digital converter, AD7712, and a Field Programmable Gate Array(FPGA), and ultra-sensitive bandgap reference. The function and noise performance are tested particular. The output voltage noise resolution of the data acquisition system reaches 0.6×10-6V/Hz1/2.Secondly, based on the principle of correlated double sampling, according to the noise spectrum and the output sample rate requirement, the key parameters of the low frequency noise elimination system is determined. Using the FPGA to make the digital signal processing, including digital CDS demodulation, filtering and decimation, the ensemble system hardware and software is developed. The function and noise performance are tested. The-3d B frequency of the low frequency noise elimination system reaches 10-4Hz, and the resolution reaches 0.6×10-6V/Hz1/2. The related progress makes a good base for the improvement of the low-frequency band resolution of space inertial sensors in the future.Finally, the low frequency noise performance of a transformer bridge based differential capacitive position sensor of a space inertial sensor is also analyzed and calculated. The low frequency noise of the differential transformer bridge capacitive sensor is affected by the noise of the AC amplifier, the phase sensitive detector, the low pass filter, and the data acquisition system. It's also affected by the low frequency noise of the excitation signal. The noise contributions of each noise sources are tested by data acquisition system. It is found that the low frequency noise of the capacitive sensor is limited mainly by the excitation signal Vp. This sets the direction for the next step to reduce the low frequency noise of the system.