An analog-to-digital converter (commonly abbreviated as ADC, A/D or A to D) is a device which converts continuous signals to discrete digital numbers. In general, an ADC is an electronic device basically employed for transforming an input analog voltage or current in to a digital number proportional to the magnitude of the voltage or current.
An analog signal is continuous in time and it is required to convert this to a flow of digital values. For that reason it is essential to define the rate at which new digital values are sampled from the analog signal. The rate of new values is called the sampling rate or sampling frequency of the converter.
A continuously varying bandlimited signal is sampled i.e. the signal values at intervals of time T, the sampling time, are measured and stored and then the original signal is exactly reproduced from the discrete-time values by an interpolation formula. Though, this accurate reproduction is only possible if the sampling rate is higher than twice the highest frequency of the signal.
As a practical ADC cannot make an instantaneous conversion, the input value must inevitably be held constant during the time that the converter performs a conversion i.e. the conversion time. An input circuit called a sample and hold performs this task in majority of the cases. This is done by means of a capacitor to store the analog voltage at the input, and using an electronic switch or gate to disconnect the capacitor from the input.