Comparisons of Feed-Forward and Multiple-Feedback Sigma-Delta Modulators for MEMS Accelerometers

Beijing Microelectronics Technology Institute, 100076 Beijing, China

Abstract

This paper investigates two different architectures of a 5^th order electro-mechanical sigma-delta modulator: a feed-forward (FF) architecture and a multiple-feedback (MF) architecture. And a comparison was performed in terms of stability and noise shaping ability, sensitivities to parameter variances due to fabrication tolerances and loop gain, and nonlinearity in feedback force. Both architectures were modeled in Simulink and investigated at system level. The results show that: a) both architectures are stable and achieve the similar noise floor level of -170dB within 250Hz in the ideal condition; b) both architectures have good ability in fabrication tolerance; c) the performance of the MF architecture will degrade heavily with the loop gain decreasing and become unstable if the loop gain beyond one optimal value, while the FF architecture is insensitive; d) the FF architecture controls the proof mass well and achieves better SNDR, whereas the MF has a 56dB degradation in consideration of nonlinearity in feedback force.