An analog-to-digital converter (ADC) circuit is configured to receive an analog input signal and convert the analog input signal to a digital output signal. The ADC circuit includes a first circuit that is configured to convert the analog input signal into a first digital signal that includes a first subset of bits of the digital output signal and further provide a residue signal based on the first digital signal; and a second circuit, coupled to the first circuit, and is configured to determine a discharging time duration by simultaneously amplifying and discharging the residue signal.