The circuit offset often causes integration saturation in the traditional sigma-delta interface of capacitive MEMS accelerometers.To address this problem,a new type of capacitive digital interface circuit used for downhole explora-tion and oil detection is designed.This paper presents a MEMS-based 5 th-order sigma-delta capacitive accelerometer,where the 3 rd-order digital loop filter is realized using FPGA.This will reduce the ASIC analog circuit layout design and chip tes-ting difficulties and is easy to optimize the loop filter parameters,which can be used to improve the system stability and opti-mize the noise performance.The analog-frond-end amplifier (AFE)is realized by using a simple correlated double sampling (CDS),which is one effective method to reduce circuit offset of AFE.According to the Gaussian distribution of AFE output signal,one new type 8-bit instantaneous floating point ADC (IFP ADC)is designed.The IFP ADC is used to convert analog signal of AFE to digital signal to feed the 3 rd-order digital loop filter.The whole system provides a significantly low noise floor 53. 09ng/rt(Hz)overall a 200 Hz bandwidth.In this work,the AFE amplifier and ADC were successfully fabricated by using XFAB XH018 mixed-signal CMOS process.Furthermore,the sensitivity and noise floor of the AFE amplifier are 0. 69V/pF and 3. 20μV/rt(Hz)in open loop measurement,respectively.%MEMS加速度计接口电路主要采用传统sigma-delta架构实现，但这种方式中的电路失调电压很容易产生积分饱和现象．为解决这个问题，本文设计了一种可以用于钻井、石油勘探等微弱信号检测的新型数字电容接口电路．该设计在电容式MEMS加速度传感器基础上，采用FPGA实现数字三阶环路滤波器，构成5阶sigma-delta系统．采用数字环路滤波器降低了ASIC模拟电路版图设计与芯片测试难度，利于快速优化环路滤波器设计参数，改善系统稳定性和优化系统噪声性能．前置放大器采用一种相对简单的相关双采样技术，能够有效减小前置放大器的失调电压．根据MEMS加速度计前置放大器输出信号符合正态分布的特点，设计了带有一定预测功能的8-bit瞬时浮点ADC，实现模拟与数字环路滤波器互联．在200Hz带宽内，该接口电路系统噪声基底达到53．09ng／rt（Hz），满足系统噪声设计要求．前置放大器与ADC采用XFAB XH018混合信号CMOS工艺流片，开环测试表明，前置放大器的灵敏度和噪声分别为0．69V／pF和3．20μV／rt（Hz）．
展开▼
