Research On Integrated Technology Of Interface ASIC For Capcitive Silicon Gyroscope

Compared with traditional gyroscope, micro-machined gyroscope based on MEMSand CMOS technology has many advantages, such as low price, small size, low powerand high reliability. It is not only widely used in civil fields such as automotiveelectronics, medical equipment and mechanical movement, but also adopted in militaryareas like tactical missiles, MAV and so on. As the adoption area expands, capacitivemicro-machined gyroscope faces higher requirements.The ASIC in capacitive micro-machined gyroscope has some problems to solve.First, both driving mode and detecting mode in interface circuit have nonlinearitycharacteristic which has bad influence on sensitivity and stability. Second, phase noisehas bad effect on the driving stability, while analysis and optimization technique ofmultiple noise sources are lacked in recent researches. Third, non-ideal factors such asquadrature error and offset error have bad effects on improving the performance ofmicro-machined gyroscope. The signal coupling with feedback of quadrature errorinfluences the detecting accuracy. To solve these problems, this paper has conducted adepth study on capacitive micro-machined gyroscope nonlinearity, model of multiplenoise sources and compensation of quadrature error and offset error.Above all, this paper studies the nonlinearity of various parts of the interface circuitin micro-machined gyroscope through theoretical analysis. The nonlinearity in drivingmode is mainly generated by the transformation process of driving voltage to drivingdisplacement. The nonlinearity in detecting mode is caused by secondary demodulationprocess which includes the transformation processes of detecting displacement topreamp output voltage. Quantitative analysis about the reasons of nonlinearity is madein this paper. The result shows nonlinearity is caused by the inconsistency betweenreference signal and the positive and negative transmission characteristics indemodulation circuit. According to quantitative analysis, one phase-sensitivedemodulation circuit with high linearity is proposed.Besides, based on the impact of multiple noise sources, a phase noise couplingmodel for self-excited driving circuit with multiple noise sources is built. From thismodel, it can be known that the main sources include low-frequency noise, noise near the resonant frequency and noise near the multiple harmonic. Among them,low-frequency noise and noise near the resonant frequency are the most important twoparts. From the analysis, phase noise can be reduced by reducing the low-frequencynoise. Low-frequency noise is made up by thermal noise pre-charge amplifier and1/fnoise of pre-charge amplifier. The result of quantitative analysis shows the injectingcharge causes phase varies. The phase variation is proportional to the amount of chargeinjected while inversely proportional to the equivalent capacitance and amplitude ofdriving voltage. Meanwhile it is related to the time of charge injecting. After aboveanalysis, optimized design of charge amplifier which can reduce the phase noise indriving circuit in driving mode is completed.Nextly, in order to solve the quadrature error problem, a behavioral model of thecancellation of quadrature error based on theoretical analysis is built. Under theguidance of the model, a novel compatible no-feedback electrode quadrature errorcancellation circuit is proposed. This circuit can eliminate the quadrature error signaleffectively while avoiding the increase of interference.Finally, a driving circuit based frequency modulation principle is proposed. Thiscircuit can eliminate the coupling of the driving signal to displacement. Meanwhile, it iscompatible with gyroscope with low Q. A detecting circuit based on secondarydemodulation principle and interface ASIC chip of silicon capacitive micro-machinedgyroscope is proposed. The chip includes capacitor-voltage converter, filter, phaser,AGC, mixer, charge amplifier, phase sensitive demodulator and so on. The ASIC chip isfabricated by CMOS IC process. The area is5.05×3.7mm2. The testing results of themicro-machined gyroscope system made by this ASIC chip and sensitive structure showthe measuring range is±800o/s, zero-position stability is30o/h, and nonlinearity is0.1%and noise is0.003/s/Hz1/2.