Noise Analysis And Optimization Of Sigma-delta Digital Accelerometer

Due to the extensive prospect in research and application, as well as the significant value in military and civil area, micromechanical accelerometer is becoming the focus of micro sensors. Among the diverse accelerometer, with the high resolution, low consumption, simple structure, low noise, and high compatibility with standard CMOS process, Sigma-delta capacitive micromechanical accelerometer is currently widely used in discrete-time system. Nowadays, with the development of information technique, integration, close-loop function, direct digital output are becoming the focus of research. High-performance interface circuit is the key to achieve high-performance accelerometer. Therefore, it has significance in theory and application to research Sigma-delta micromechanical accelerometer.Noise tolerance is the most critical performance of high resolution micromechanical accelerometer. The structure and working principle of Sigma-delta micromechanical accelerometer are analyzed in this paper. It lucubrates on the noise of switch, aliasing of thermal noise, noise of operational amplifier, noise of integrator, and the quantization noise of the fifth-order distributed feedback Sigma-delta micromechanical accelerometer, that includes detailed analysis of noise principle, the model of noise model, and the interaction of different noise. The equivalent output noise of accelerometer is calculated, which based on the theoretical analysis. And test results of the chip prove the accuracy of theoretical analysis and calculation.In this paper, the noise optimization of the behavioral model of Sigma-delta micromechanical accelerometer is achieved on the Simulink platform in Matlab. Fourth-order feedforward structure is employed, instead of fifth-order distributed feedback structure, in order to enhance the loop gain while guaranteeing the stability. The gain of preceding stage is increased by reducing the feedback capacitance. The correlative double sampling circuit is used to restrain the low frequency interference. The foldover component of thermal noise is lessened by reducing the bandwidth and equivalent input noise of operational amplifier. This design is achieved by0.5μm CMOS process, and the supply voltage is5V. The entire circuit is designed in Cadence, and the analog simulation is performed by Spectre. The simulation results show that all blocks of Sigma-delta micromechanical accelerometer function well. The equivalent close-loop output noise of accelerometer is approximately100nV/Hz1/2, and the SNR is94.5dB with500Hz bandwidth.