Abstract
Closed-loop capacitive MEMS accelerometer is similar to a ΔΣ ADC where the MEMS sensor introduces second-order dynamics in the loop. The sensor has limited signal gain, hence provides very limited noise shaping capability to other noise contributions within the loop. Furthermore, fabrication non-idealities can cause unwanted stability issues in higher order closed-loop operation. We present a 4 th order ΔΣ closed-loop system with a digital 2 nd order ΔΣ modulator. The digital ΔΣ modulator allows fine accuracy in choosing the coefficients for the ΔΣ modulators. It also provides flexibility in the placement of poles and zeros for compensating the unforeseen phase delays caused by fabrication non-idealities. Our system is able to achieve an SNR > 120dB in simulation and > 90dB from experiment with an evaluation board. The digital implementation of ΔΣ modulator allows us to build reconfigurable closed-loop system with the potential for even higher order system which can be optimized to suit different MEMS sensor characteristics and performance.
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