A regulator-free 84dB DR audio-band ADC for compact digital microphones

A 20 kHz audio-band ADC with a single power-supply pad is implemented for a digital electret microphone. The designed low-noise preamplifier not only relaxes the ADC design requirement but also provides an excellent interface for the electret capacitor. A low power 4^th-order switched-capacitor (SC) ΣΔ modulator (ΣΔM) converts the analog signal into lb digital. Under the single power-supply pad, the switching noise effect on the signal quality is estimated via post simulations with simplified parasitic models. Performance degradation is minimized by time-domain noise isolation (TDNI) with sufficient time-spacing between the sampling edge and the output transition. A prototype ADC was implemented in a 0.18 μm CMOS process. It operates under a minimum supply voltage of 1.6 V with total current of 420 μA. Operating at 2.56 MHz clock frequency, it achieves 84 dB dynamic range and a 64 dB peak signal-to-(noise + distortion) ratio. The measured power supply rejection at a 100 mV_pp 217 Hz square wave is -72 dB without any supply regulation.