An Analog Readout Circuit With a Noise-Reduction Input Buffer for MEMS Microphone

This brief presents a programmable-gain analog readout circuit with a noise-reduction input buffer for single-ended micro-electro-mechanical-system (MEMS) microphone sensors, aiming at applications requiring low cost and high SNR. An internal feedback path is adopted in the source-follower-based input buffer to suppress the noise contribution from its bias circuit, while improve the linearity and power supply rejection ratio (PSRR). In addition, another feedback path taking advantage of the readout circuit's high-voltage bias output for the MEMS sensor is employed to realize an effective gain of larger than unity for the input buffer, which can suppress the noise contribution of the programmable capacitive-feedback preamplifier. Fabricated in a 0.18- mu text{m} CMOS technology, the proposed readout circuit achieves a 3.7- mu Vrms (A-weighted) input-referred noise. After packaged with a commercial MEMS microphone sensor, the overall system achieves an A-weighted SNR of 66 dBA, an acoustic overload point (AOP) of 119.5 dB sound pressure level (SPL) and a PSRR of 68 dB@1kHz. The readout chip consumes 115 mu text{A} under a 2-V power supply and occupies an active area of 0.25mm².