Research On Interface ASIC For Capacitive SIGMA-DELTA Accelerometer

Micromachined accelerometer is the key research field in institutes and corporations because it is widely applied in both civil and military fields. Sigma-delta (ΣΔ) interface for capacitive micromachined accelerometer has attracted considerable attention since it is simple, has a large bandwidth, can be easily implemented by CMOS technologies, and provides direct digital output with closed-loop operation. With the development of MEMS, integration, intellection and numeralization have become the main tendency of accelerometer. The research onΣΔinterface circuit of micromachined accelerometer has significant theoretical value and applied benefit.With focus on the deficiencies of the theoretical research and circuit design inΣΔinterface circuit of micromachined accelerometer, the models of system and noises are built, and interaction of noise sources and stability of system are analyzed. The sensing element's equivalent electrical model is improved. And theΣΔinterface ASIC of accelerometer and key technique are researched.Considering the distortion produced by nonlinear character of quantizer, the system model ofΣΔaccelerometer is set up on the basis of describing function method model of 1-bit quantizer. Noise sources in the system are analyzed detailed and improved electronic and quantization noise models are established. Interaction between noise sources is considered in electronic noise model, parameters of sensing element and nonlinearity of quantizer are considered in quantization noise model. Simulated and tested results indicate that the noise model can predict noise level exactly. Interaction of electronic and quatizaiton noise in second-order and higher-order system are analyzed based on the system model. Analytical results reveal that in second-order system, the quantization noise is affected by electronic noise badly. This makes quantization noise higher than electronic noise in signal band, which cannot be avoided by increasing sampling frequency. In high-order system, the noise shaping ability will not be weakened as the very high low-frequency gain. The noise interaction is revealed theoretically in this work.Based on the system model, the closed-loop stability and signal dependent stability are researched. In the research about closed-loop stability, the demand on compensation coefficient of second-order and higher-order system and the effect of compensation coefficient on noise performance are analyzed, showing that there is a compromise between stability and noise performance. In the research about signal dependent stability, the system stablility in large signal condition is researched, the input signal conditions of second-order and fourth-order systems are established. The analytical results indicate that in second-order system, once the compensation coefficient is chosen to make system steady, the modulator will not overload with the increasing input signal. However, in higher-order system, when input signal is too large, the overload will happen, making the integrator saturated.The cause why the bend of sense finger under exterior force makesΣΔclosed-loop feedback disabled is studied based on the analysis about limitation of traditional equivalent model. Adopting sense finger dividing method, the sense finger is partitioned into five segments. Dynamic equations of the two segments apart from proof-mass furthest are established based on the fact that the further parting from the proof-mass, the more bended the sense finger is. The influence of the bend of sense finger on electrostatic force is analyzed, combining traditional model, the improved equivalent electrical model including the factor of sense finger bend is presented. The simulation results show that in the systemic simulation, utilizing traditional model can not reflect the failure of modulator, the output shows the modulator works properly. However, using the improved model established in this work could capture the failure ofΣΔclosed-loop feedback, providing guarantee for validity of circuit simulation. And the model can be applied in circuit grade Spice simulation.The design ofΣΔinterface for micromachined accelerometer is completed based on theoretical analysis. Fourth-orderΣΔinterface and second-order mixed output interface are designed in this work. A novel capacitance-voltage converter is designed to solve the problem of charge leakage in traditional charge integrator. Lead compensator, electrical integrator, quantizer and clock generator are all designed. The ASICs are fabricated in 0.5μm two metal two poly CMOS technology, and the ASICs and accelerometer sensors are both tested. The tested results indicate that the power dissipation of fourth-orderΣΔinterface is 40mW, electric noise is 7μV/Hz^1/2, full range of the sensor constituted with ASIC and sensing element developed by Peking Univercity is±2g, open-loop nonlinearity is 0.211%, sensitivity is 1.25V/g, open-loop noise density is 12μg/Hz^1/2, and closed-loop noise density is 80μg/Hz^1/2. Power dissipation of second-order mixed output interface is 50mW, noise density of closed-loop digital output mode is 80μg/Hz^1/2, noise density of closed-loop analog output mode is 9.8μg/Hz^1/2, nonlinearity is 0.06%, and bias stability is 0.129mG.The theoretical research in this paper is of great significance forΣΔinterface circuit of capacitive accelerometer. The designed and manufactured ASIC in this work is very valuable and presents great potential in further application.