How delta-sigma ADCs work, Part 1

Analog techniques have dominated signal processing for years, but digital techniques are slowly encroaching into this domain. The design of delta-sigma (DS) analog-todigital converters (ADCs) is approximately three-quarters digital and one-quarter analog. DS ADCs are now ideal for converting analog signals over a wide range of frequencies, from DC to several megahertz. Basically, these converters consist of an oversampling modulator followed by a digital/ decimation filter that together produce a high-resolution data-stream output. This two-part article will look closely at the DS ADC’s core. Part 1 will explore the basic topology and func tion of the DS modulator, and Part 2 will explore the basic topology and function of the digital/decimation filter module. DS converters: An overview The rudimentary DS converter is a 1-bit sampling system. An analog signal applied to the input of the converter needs to be relatively slow so the converter can sample it multiple times, a technique known as oversampling. The sampling rate is hundreds of times faster than the digital results at the output ports. Each individual sample is accumulated over time and “averaged” with the other input-signal samples through the digital/decimation filter. The DS converter’s primary internal cells are the DS modu lator and the digital/decimation filter. The internal DS modulator shown in Figure 1 coarsely samples the input signal at a very high rate into a 1-bit stream. The digital/decimation filter then takes this sampled data and converts it into a high-resolution, slower digital code. While most converters have one sample rate, the DS converter has two—the input sampling rate (fS) and the output data rate (fD).