Sigma-delta ADC based adaptive readout ASIC for digital audio sensor

Digital audio sensors have been used in microphone- embedded portable electronics. This paper shows a mixed-signal adaptive readout ASIC for digital audio sensor, which includes a programmable-gain-amplifier, a switched-capacitor sigma-delta modulator, digital signal processing, and power management circuits. The digital part comprises a digital low-pass filter, a digital-gaincompensation and a digital automatic-gain-controller. The clear advantage of this system is to obtain high signal-tonoise ratio (SNR) and wide input range simultaneously. In addition, both a normal operation mode and a sleep mode are realized to address a flexible current budget. The ASIC is fabricated in a 0.18-um CMOS process. Experimental results show the expected adaptive readout function over the input range from 94 to 136 dB SPL. In the normal mode, a system SNR of 68 dB (A-weighted) at 1 kHz 94 dB SPL input is achieved under 600 uA. Further, an impressive acoustic overload point (AOP) of 136 dB SPL is guaranteed. In the standby condition, the sleep mode with current less than 20 uA is obtained, so that the battery life can be extended. Compared to other existing solutions, this adaptive readout ASIC shows the best combination of achievable SNR, AOP and current budget for digital audio sensors.