The invention provides an analog-to-digital converter. In one embodiment, the analog-to-digital converter receives a first audio signal from a microphone, and comprises a coding selection module, a pre-amplifier, a 1-bit ΔΣ modulator, and a signal coding module. The coding selection module receives a mode signal and a channel selection signal, and generates a control signal according to the mode signal and the channel selection signal. The pre-amplifier amplifies the first audio signal to obtain a second audio signal. The 1-bit ΔΣ modulator converts the second audio signal from analog to digital to obtain a third audio signal. The signal coding module encodes a fourth audio signal according to the third audio signal as an output signal of the analog-to-digital converter when the control signal indicates that the mode signal selects an encoding mode, and forwards the third audio signal as the output signal of the analog-to-digital converter when the control signal indicates that mode signal does not select the encoding mode.