Analog to digital converter is electronic circuit which make analog signal digital. in other words we can say that ADC is basic and very important circuit in modern era of life. when we speak ,microphone produce analog signal. this signal is then amplified and then before transmission, it is converted to digital form.
Analog signal is continuous signal as like sine wave, sawtooth wave etc while digital signal is discrete signal in the form of 1 and 0. Any of the input or analog signal is converted to digital if transmitted. There are a lot of methods to convert the analog signal into digital form. Mostly used methods are comparators. In comparator one reference voltage is given while on other terminal signal is provided. This comparator gives output if signal voltage is greater or equal to the reference as 1 and 0 vice versa. Here also keep in mind that resolution of digital signal is also important. It mean the signal you are converting into digital form, how many bits signal do you want. one bit, 2 bit,4bit,8bit,16bit 32 bit 64 bit etc. Now a days laptop and window are operation on 64 bit processors.