A multifunction low-power preamplifier for MEMS capacitive microphones

A multi-function two-stage chopper-stabilized preamplifier (PAMP) for MEMS capacitive microphones (MCM) is presented. The PAMP integrates digitally controllable gain, high-pass filtering and offset control, adding flexibility to the front-end readout of MCMs. The first stage of the PAMP consists of a source-follower (SF) while the second-stage is a capacitive gain stage. The second-stage employs chopper-stabilization (CHS), while SF buffer shields the MCM sensor from the switching spurs. The PAMP uses MW poly bias resistors for the second-stage, exploiting Miller effect to achieve flat audio-band response. The gain and high-pass filtering corner can be adjusted by digitally controlling the capacitor banks in the PAMP. The offset-control feature of the PAMP is implemented using a narrow-band low-pass gm-C filter. The PAMP occupies 950µm × 950µm in 0.35µm CMOS technology and draws a 50µA total current from a 1.8V single supply. The PAMP achieves SNDR of 44dBA/Pa (elec. meas.) and 27dBA/Pa (acoustic meas.) and a conversion range from 50dBSPL to 120dBSPL.