A 1.8 V 828 μW 80 dB digital MEMS microphone

A single-package digital MEMS Capacitive Microphone (MCM) system is presented. The system consists of a MCM, which is wire-bonded with its readout interface (RI). The MCM sensor is fabricated using a combination of surface and bulk micromachining, employing diaphragm-stiffening to achieve piston-like diaphragm-movement and attaining required sensitivity with a smaller diaphragm-area. The RI is designed in 0.35 μm CMOS and it consists of a preamplifier (PAMP), a sigma-delta modulator (SDM), integrated biasing and digital control, converting the MCM capacitive variations into a single-bit over-sampled digital bitstream. The PAMP employs a two-terminal bootstrapped source-follower buffer to make the readout insensitive to the MCM parasitics, subsequently feeding a third-order single-loop single-bit modulator running at 2.5 MHz. The electrical measurements of the standalone RI demonstrate 55 dB A-weighted @ 1 Pa SNDR at the analog PAMP output and 80 dB A-weighted dynamic-range at the digital output, which corresponds to a conversion range from 40 to 120 dB SPL. The SNDR for acoustic measurements is 33 dB A-weighted @ 1 Pa, limited by the higher MCM thermal noise floor and reduced sensitivity (-53 dB V @ 1 Pa). The frequency characterization of the system for the complete audio-band demonstrates the effect of the system package towards higher frequencies (>9 kHz), giving rise to Helmholtz resonance, and reduction in sensitivity for low-frequencies (<400 Hz) because of acoustic short-circuiting inside the MCM due to flow-by slots. The complete system consumes 460 μA of total current for a 1.8 V single-supply. The total system dimensions are 4.5 × 2 mm2 (excluding the package), demonstrating the viability of a low-area, low-power and high dynamic-range implementation of digital MCM. © 2011 Springer Science+Business Media, LLC.