Define Sigma-Delta Analog to Digital Converters?

Sigma-Delta ADCs have the benefit of being implemented almost totally in the digital domain.  They are extremely reliable, highly stable, and very functional.  Due to their largely digital composition, they can be integrated onto similar silicon like a DSP.  Most ADCs are relies on the principle of sampling and transforming a signal directly at the sample rate; though, this needs precise component matching and low noise circuit design. 

Sigma-Delta ADCs make use of a low-resolution ADC (1-bit quantizer) with a sampling rate many times higher as compared to the Nyquist frequency.  This results in a conversion noise which is spread over a very wide bandwidth.  As the desired output bandwidth is a tiny fraction of the internal bandwidth, a filter can be utilized to remove the noise.  Mainly, the digital filter trades high bandwidth for better noise performance.