Analysis And Optimization For Harmonic Distortion Of Sigma Delta Accelerometer

Miniaturization and digitization are becoming the major trends of accelerometerdevelopment. Sigma-delta accelerometer use sigma-delta modulation technique toachieve high-accuracy analog-to-digital conversion, and it is convenient to connect tothe digital circuit modules, so sigma-delta accelerometer is being widely researched andused in these years. However there are many bottleneck problems in the research onsigma-delta accelerometer, the Harmonic Distortion is just one of the most importantfactors to hinder the improvement for the performance of accelerometer. In someapplications, such as seismic prospecting, the HD of accelerometer should be effectivelysuppressed. Therefore the research on HD of accelerometer can make great sense toimprove the performance of accelerometer.In this paper, a structure of forth-order feedforward Sigma-delta accelerometer isproposed based on the analysis for the principle of accelerometer and sigma-deltamodulation, and the ideal model of the system is constructed in Simulink. As the resultsof simulation, the SNDR of system is94.6dB and ENOB is about15.42bits. Then itanalyses for the nonlinearities of some modules in accelerometer, such as sense element,static force feedback module, integrator, switch, etc. The nonlinear models are alsoconstructed in Simulink and verified by simulation. At last the whole nonlinear modelof system is constructed and the HD is analyzed by simulation, as a result the noisefloor comes to-140dB, while the HD is about-110dB.On the basis of system level design in Simulink, the interface circuit ofSigma-delta accelerometer is designed and simulated in transistor level with0.5μmCMOS process, and the linearities of switches and op amps are optimized. Thesimulation indicates that the noise floor of system is about-120dB, while all the HD isreduced to below-100dB, it means that the system can suppress the HD to some extent.Finally the HD of electrical modules is analyzed to verify the theoretical analysis, andthen some optimization method can be obtained.