Sigma-delta ADCs/ Over sampling Converters

Sigma-delta ADCs/ Over sampling Converters:

It consists of 2 main parts - modulator and digital filter. The modulator includes an integrator and a comparator with a feedback loop that contains a 1-bit DAC. The modulator oversamples the input signal, converting it to a serial bit stream with a frequency much higher than the required sampling rate. This is then transformed by the output filter to a sequence of parallel digital words at the sampling rate. The characteristics of sigma-delta converters are high resolution, high accuracy,

Low  noise and low cost.

Typical applications are for speech and audio.

A Sigma-Delta ADC (also known as a Delta-Sigma ADC) oversamples the desired signal by a large factor and filters the desired signal band. Generally a smaller number of bits than required are converted using a Flash ADC after the Filter. The resulting signal, along with the error generated by the discrete levels of the Flash, is fed back and subtracted from the input to the filter. This negative feedback has the effect of noise shaping the error due to the Flash so that it does not appear in the desired signal frequencies. 

A digital filter (decimation filter) follows the ADC which reduces the sampling rate, filters off unwanted noise signal and increases the resolution of the output. (sigma-delta modulation, also called delta-sigma modulation)