Capacitive Micromachined Accelerometer Processing Circuit

Accelerometer is a sensor that used to meaurse acceleration. It can be used to meter intertial parameters including tilt angle, intertial force, impact and vibration. MEMS accelerometers have broad applications in fields such as inertial guidance, medical instruments, and consumer electronics.Capacitance sensitive MEMS accelerometer has been concerned because of it's stability and high sensitivity. The performance of capacitance measurement circuit has a direct impact on the performance of accelerometer system. The improvement of capacitance measurement ciruit performance is important in determing the performance of accelerometers.Both analog circuits based and digital system based MEMS accelerometer system has designed for a capacitance sensitive MEMS accelerometer. Both the two accelerometer system works and the factors impact on system performance were analyzed. These will provide theoretical basis for the choosing and designing of detection circuits for MEMS accelerometers.The major work of this thesis contains:(1) A single-carrier modulation-type measurement circuit is designed, which includes carrier generator, charge amplifier, demodulator, low-pass filter and amplifier. The circuit structure of each module has been analized. The parameter of the circuits has been decided according to theory. The factors impact on the system performance such as carrier signal, phase control accuracy and parasitic effect has also been analized. Program of temperature compensation based on DSP has been designed by test the temperature feature of accelerometer system.(2) A capacitance measurement system based on digital circuit is designed, which includes carrier generator, demodulator, phase controller and IIR low-pass filter. Principle of CORDIC algrithem has been analized and implemented base on FPGA platform. The demodulation algrithem in digital system has been optimized and the experimental results before improvement and after improvement have been compared. A automatical phase controller module has been designed and implemented in FPGA. All the module have been simulated by using MATLAB.(3) A performance test scheme is made with reference to test guides on both analog and digital accelerometers. The measurement shows analog has a sensitivity of 278mV/g, nonlinearity of 1223ppm , and zero-draft of 5.5×10^-4g(1σ) . The measurement shows digital has a sensitivity of 2.37×10⁷quants / g, nonlinearity of 69Appm , and zero-draft of 1.5×10^-4g(1σ) .The performance of digital accelerometer system is better than analog accelerometer system.