An analog-to-digital converting (ADC) apparatus is disclosed. The ADC apparatus includes a coarse comparing module, at least one pre-switching detection module, at least one fine comparing module, and an encoder. The coarse comparing module compares an input signal and a plurality of first reference signals to generate a previous comparing result and a coarse comparing result in sequence. The pre-switching detection module generates a plurality of previous selecting signals according to the received previous comparing result. The encoder generates a previous encoding result according to the coarse comparing result. The fine comparing module selects a selected reference signal to be compared with the input signal from a plurality of second reference signals according to the previous selecting signals and the previous encoding result, so as to generate a fine comparing result.