Reconfigurable Closed-Loop Digital ΔΣ CapacitiveMEMS Accelerometer for Wide Dynamic Range, HighLinearity Applications

Closed-loop capacitive MEMS accelerometer issimilar to a ΔΣ ADC where the MEMS sensor introducessecond-order dynamics in the loop. The sensor has limitedsignal gain, hence provides very limited noise shaping capabilityto other noise contributions within the loop. Furthermore,fabrication non-idealities can cause unwanted stability issues inhigher order closed-loop operation. We present a 4th order ΔΣclosed-loop system with a digital 2nd order ΔΣ modulator. Thedigital ΔΣ modulator allows fine accuracy in choosing thecoefficients for the ΔΣ modulators. It also provides flexibility inthe placement of poles and zeros for compensating theunforeseen phase delays caused by fabrication non-idealities.Our system is able to achieve an SNR > 120dB in simulationand > 90dB from experiment with an evaluation board. Thedigital implementation of ΔΣ modulator allows us to buildreconfigurable closed-loop system with the potential for evenhigher order system which can be optimized to suit differentMEMS sensor characteristics and performance.