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The Sigma-Delta ADC Converter
Sigma-delta ADC converters are used almost exclusively in applications that require high-resolution output, such as high-fidelity audio or industrial measurements. To achieve the required high resolution, the analog signal must be sampled at rates much higher than the Nyquist rate. In general, the complexity of the circuit design is proportional to the sampling rate. For this reason, sigma-delta ADCs have traditionally been used for sampling low-frequency signals—that is, signals with relatively low Nyquist rates, such as speech in mobile phones.
A sigma-delta ADC consists of an analog and a digital section (Figure 1). The analog section, which includes a sigma-delta modulator, samples the input at a high rate and produces a binary output whose average value over time tracks the analog input. The digital section, which includes decimation filtering, finds the average value by running the output of the sigma-delta modulator through a low-pass decimating FIR filter. Later in this article, we will exploit the binary nature of the filter’s input to implement a very efficient filter architecture.
http://www.mathworks.com/company/newsletters/digest/2007/sept/sigmadelta.html