Experimental study of single loop sigma-delta and multi stage noise shaping (MASH) modulators for MEMS accelerometer

This paper presents a comparative study of two 4^th order electromechanical sigma-delta modulators for a micromachined, capacitive accelerometer: 1) a multi-stage (MASH22) architecture and 2) a single loop (SD4) architecture. Both architectures share the common features that they improve the bandwidth, dynamic range and linearity of the MEMS sensor and provide a digital output, compared to an open loop sensor. The comparison was performed in terms of stability, noise shaping and parameter sensitivities due to fabrication tolerances. Both architectures were investigated by system level modelling (in Simulink) and hardware implementation using surface mount PCB technology. The results show that: a) both modulator architectures achieve the same noise floor level of -110dB, b) the 4^th order MASH22 is unconditionally stable, while the SD4 is only conditionally stable, c) MASH22 achieves higher overload input level compared with the SD4, and d) the SD4 architecture confirms its immunity to sensing element parameter variations whereas the MASH22 performance is degraded in such a case.