A digital circuit that converts an analog signal into a digital signal is called an Analog-to-Digital Converter (ADC). The ADC takes in a continuous signal with values ranging from 0 to a certain maximum value, which is then converted into a discrete-time signal. This process involves the conversion of the amplitude and frequency of the analog signal into a binary number. The input to an Analog-to-Digital Converter typically consists of an analog voltage or current that needs to be quantized and digitized. The resolution of the ADC determines how many bits are required to represent the digital output, with higher resolutions resulting in more precise digital representations of the original analog signal.