Analog to Digital Converter

In real world, all signals like light, sound etc, are analog signals. These signals have to be converted into digital form so that they can be manipulated by digital equipment. Device used to convert analog signal into digital signal is called Analog to Digital Converter (ADC). An example of an analog to digital converter is a Scanner – It takes a picture (analog) as input and convert into digital picture. ADC is an electrical circuit that converts continuous time and continuous amplitude signal into discrete time and discrete amplitude signal.

Let us first discuss basic concept of analog to digital conversion. The process of digitizing the domain(time) is called sampling and the process of digitizing range(voltage/current) is called quantization.

Sampling : An ADC circuit samples analog signal from time to time. Then, each sample is converted into a number based on its voltage level. The frequency at which sampling occurs is called sampling rate or sampling frequency. e.g if sampling frequency is 22000 Hz, it means, in one second 22000 input points will be sampled and distance between two adjacent time points is 1/22000 seconds. Higher the sampling frequency, more perfect will be the analog signal produced by DAC (when it is required to reconstruct the analog signal from digital samples). But more memory will be needed to store these samples. So there is always a trade off between memory required to store samples and accuracy of signal. But to reproduce analog signal from digital samples, there should be some minimum number of samples. And

According to Nyquist sampling theorem, sampling rate must be at least twice the highest frequency component to avoid aliasing.

                                      Fs = 2Fmax

Quantization: Quantization is the process of converting continuous value signal into discrete value signal so that signal takes only finite set of values. Unlike sampling (where we saw that under some conditions, it is possible to reconstruct the signal), quantization results in some loss of information called quantization error. One of basic choice in quantization is the number of discrete quantization levels to use. Fundamental tradeoff in this choice is the resulting signal quality vs data(bits) needed to represent each sample. With L levels, number of bits required to represent each level,

                     
              N = logL/log2.

Analog to Digital Converter with 32 levels(5 bits)