Fully Differential ROIC for SNR Enhancement by using a MEMS Microphone Pair with a Differential AC Bias and a High-voltage DC Bias

Abstract

A fully differential readout integrated circuit (ROIC) is proposed to enhance SNR by using a micro electro mechanical system (MEMS) microphone pair. A differential clock is AC-coupled to the MEMS microphone pair as AC bias for chopper operation, while maintaining a high-voltage DC bias through high-resistance resistors for high sensitivity. The MEMS microphone pair connected to the fully differential ROIC amplifier with a demodulation chopper increased the measured SNRAW by 3.5~4.3dB compared to a single MEMS microphone with a chopper. The chopper operation applied to the pair of MEMS microphones reduced the measured audio band noise at the ROIC output by 14.8~16.5 times, compared to the no-chopper case. The fully differential ROIC consists of an amplifier, a demodulation chopper, a pair of resistor-capacitor low pass filters (RC-LPFs) and a unity-gain buffer. The ROIC implemented in a 0.35μm CMOS process takes a chip area of 2.5mm² and consumes 134μA at a supply voltage of 3.3V.