The invention provides an analog-to-digital converter. In one embodiment, the analog-to-digital converter receives a first audio signal from a microphone sensor, and receives a first channel selection signal and a clock signal, and comprises a toggle detection module, a first data processing module, a second data processing module, and a multiplexer. The toggle detection module detects whether the first channel selection signal is toggling between a logic low level and a logic high level to generate a control signal. The first data processing module processes the first channel selection signal to generate a second channel selection signal. The second data processing module converts the first audio signal from analog to digital to generate a second audio signal. If the control signal indicates that the first channel selection signal is toggling between the logic low level and the logic high level, the multiplexer outputs data bits of the second channel selection signal as an output signal of the analog-to-digital converter when the clock signal oscillates to the logic high level, and outputs data bits of the second audio signal as the output signal of the analog-to-digital converter when the clock signal oscillates to the logic low level.