Mems Micro-accelerometer Readout Circuit Design

MEMS accelerometer has the advantage of small size, light weight, high resolution and low temperature drift and is widely used in the areas like car, inertia navigation, micro gravity test and the oil prospect which have more resolution requirements. The parameters of MEMS accelerometer are quietly different in different integrated process, and have high uncertainty and a variety of non-linear. The size of micro-accelerometer is very small, the variation of capacitance is very weak, generally around 10-15F, the impact of integrated process, various noises and the parasitic capacitance is serious, for these factors the design of readout circuit is a challenge.In this paper, the readout circuit structures of MEMS accelerometer are in-depth research. After a comprehensive comparison of the performance advantages and disadvantages, the paper presented a low-noise and high precision differential readout circuit structure, which can be a very good structure to eliminate the bias caused by the mismatch problems.First of all, according to the characteristics and micro-structure of MEMS accelerometer, the dynamic model and the capacitance model was built, then analyzed the relationship between the external acceleration a, the displacement x of movable plate and the variationΔC of capacitance.According to the impact of parasitic capacitance and various noises, a low-noise amplifier was designed. The paper analyzed the circuit noises and its mechanism, and chose appropriate structure and device parameters to reduce the noise. The input referred thermal noise of the low-noise amplifier is only 2.85nV/rtHz, the total input referred noise is 9.78nV/rtHz, the gain is 79.8dB, the phase margin is 61degree. The impact of common-mode signal and power supply ripple has been well suppressed, and the amplifier has good low-noise performance.The circuit of sampling demodulation is the switch capacitor circuit with a dummy transistor. It not only eliminats the channel charge injection effect, and also suppresses the clock feedthrough effect. Low-pass filter circuit is a second-order Bessel switched-capacitor filter, it has good delay characteristics and steep transition zone. The output buffer circuit can reduce spikes of waveforms in the readout circuit.Based on 0.35μm, 5V CMOS process, the paper simulated each functional module and the overall circuit. Simulation results show that the MEMS accelerometer readout circuit meets the design requirements of low noise and high precision. The frequency range of the acceleration signal which can be measured is 0~20kHz. The variation of capacitance can be amplified 100 times, and the magnification can be controlled by adjusting the capacitance value around differential amplifier.Finally, the layout of the readout circuit is designed based on 0.35um, 5V CMOS technology.