A digital microphone is provided. The digital microphone includes an acoustic sensor, a bias generator, first and second attenuators, a buffer, an amplifier, an ADC, and a controller. The acoustic sensor transfers an acoustic signal to a voltage signal. The bias generator provides a bias voltage to the acoustic sensor. The first attenuator attenuates the voltage signal by a first attenuation value. The buffer buffers the voltage signal to generate a buffered voltage signal. The amplifier amplifies the buffered voltage signal to generate an amplified signal. The ADC converts the amplified signal to a data signal with a digital format. The second attenuator attenuates the data signal by a second attenuation value. The controller determines whether the amplified signal is larger than a reference value and adjusts the bias voltage and the first and second attenuation values of the first and second attenuators according to the result determined by the controller.