An analog-to-digital converter (ADC) is a device that converts an analog signal into a digital signal. The ADC takes in a continuous-time analog signal, which can be a voltage or current, and outputs a discrete-time digital signal, typically represented as a binary number. The input to the ADC can be any type of analog signal, such as a sound wave, temperature reading, or light intensity measurement. The ADC must accurately capture the entire range of the input signal and convert it into a digital representation that can be processed by digital systems.