A 20kHz audio-band ADC with a single pair of power and ground pads is implemented for a digital electret microphone. Under the limited power/ground pad condition, the switching noise effect on the signal quality is estimated via post simulations with parasitic models. Performance degradation is minimized by time-domain noise isolation with sufficient time-spacing between the sampling edge and the output transition. The prototype ADC was implemented in a 0.18µm CMOS process. It operates under a minimum supply voltage of 1.6V with total current of 420µA. Operating at 2.56MHz clock frequency, it achieves 84dB dynamic range and a 64dB peak signal-to-(noise+distortion) ratio. The measured power supply rejection at a 100mVpp 217Hz square wave is -72dB.