多重感應器讀取之可調式增益介面電路三軸加速度計單晶設計

Abstract

本論文提出一單晶片採用分時多工之多種感測器讀取之可調式增益介面電路三軸加速度計設計。而此感測系統在標準0.18um製程下製作。此讀出電路的輸入等效雜訊在100Hz的頻寬下為65.4 nV/√Hz，功率消耗為14.16μW，靈敏度為100mV/g，加速度計操作範圍設計在-4g到4g。在本設計中，三軸加速度計的電容變化值相近，所以只需一組讀出電路。X，Y，Z三軸的電容變化在施加1g的加速度下模擬分別為0.37,0.4,0.33fF。而一個擁有較小電容值變化的感測器也在此設計中。此感測器的電容值變化在施加1g的加速度下為15aF。本讀出電路設計由四級組成。第一級為一個使用雙相關取樣的電容轉電壓轉換器。此級可以減小放大器的偏移、閃爍雜訊、開關的電荷注入，和kT/C雜訊。此外，在第一級的控制訊號可以決定要讀取哪一個感測器。第二級也使用雙相關取樣去放大第一級的輸出信號，並且減小放大器的非理想效應。第三級為一個積分器，而積分器的增益是和積分電容成反比。因此，藉由調整積分電容可以完成可調增益的功能，使不同的感射器擁有相似的輸出振幅。第四級為採樣和維持。藉由從第一級產生的四組控制訊號，它可以分開從積分器輸出的混和訊號，始分時多工得以完成。

A monolithic three-axis accelerometer with tunable gain analog front end circuit using time division to read multi-sensors signals in 0.18μm CMOS MEMS process is proposed. The circuit input referred noise is 65.4 nV/√Hz under 100Hz, the power consumption is 14.16μW, the sensitivity is 100mV/g and the linear range is +/- 4g. In this design, the three-axis accelerometer capacitance changes are designed with similar mechanism, so a single read out circuit can be used. The X, Y, Z axis capacitance changes under 1g acceleration are simulated to be 0.37,0.4,0.33 femtofarad respectively. An additive sensor with smaller capacitance change under 1g acceleration of 15 attofarad is also designed in. The universal readout circuit schematic composes four stages. The first stage is a capacitance to voltage convertor combining with correlated double sampling to reduce the interference from amplifier offset, flicker noise, charge injection, and kT/C noise. In addition, the control signal can decide which sensors to be read. The second stage also uses CDS technique to amplify signals from the first stage and to suppress the non-ideal effects. The third stage is an integrator, and the gain is inversely proportional to integration capacitance. In this way, tunable gain can be achieved by adjusting integration capacitance to make two sensors with similar output amplitudes. The fourth stage is sample and hold (S&H). It can separate the mixed signal from the integrator by using four different control signals generated in the first stage.