Research On CMOS MEMS Accelerometer And Integration Design Of Low-Noise Interface Circuits

With the development of Microelectronic Mechanical System (MEMS), inertial instruments have experienced significant progress over the past decades. The advantages of low-cost, low-power, small size, high reliability make MEMS-based inertial sensor have a wide range of applications in both civil and military realms.CMOS MEMS accelerometer is fabricated by mainstream CMOS process, which can intensively reduce the cost. The CMOS MEMS technology also allows using metal interconnection to connect the MEMS sensor and interface circuits. And the distance between these two parts can be less than 50um. Hence, the parasitic capacity and resistance can decrease greatly, which will improve the performance of the accelerometer. However, low sensing capacitance, big offset and low robustness have made the sensing system design for CMOS MEMS sensor a huge challenge.The aim of this paper is to design low-noise interface circuits for low-mass low-capacity CMOS MEMS accelerometer. First, the behavioral level model of the sensor is built for the simulation of the whole sensing system. The acceleration variation can be converted to electronic signal by this model. Then, main noise sources and nonlinear factors in interface circuits are analyzed, and the noise model used to get the overall input-referred noise floor is presented in this paper, which can be used to realize noise minimization design of the interface circuits. In the circuit design, continuous-time voltage-mode capacitive sensing topology is chosen to avoid noise-folding. The chopper stabilization technology is utilized for low-noise sensing. The main part of the interface is a three-input high gain wide bandwidth differential difference amplifier (DDA), which can realize both DC and AC offset compensation. This paper also uses periodic DC reset to stabilize the DC bias at the sensing node. Finally, an open loop sensing system is designed, which can successfully sensing capacitive variation, on the order of 10-4-10-3fF, for a given CMOS MEMS accelerometer fabricated by Carnegie-Mellon University. With 0.1V modulation signal, the system can sense 0-10g acceleration, with a sensitivity of 205mV/g and a resolution of 0.01g. Under this condition, the overall error is less than 5.5%, and there is almost no sensing error within the range of 0-6g. If a modulation signal of 1V is applied, the system sensing resolution can reach 1mg, which has met the demand of current commercial accelerometer, with a measuring range of 1g.