9.8 Analog-to-Digital Conversion: Sigma-Delta

The operation of sigma-delta ADCs is entirely different from successive-approximation ADCs. Their distinctive characteristics are high precision and low speed, which makes them ideal for many applications in metering. The sigma-delta ADCs in the MSP430 work at around 1 ksps rather than 100ksps for the SAR ADCs. Unfortunately their operation is far more sophisticated than SAR ADCs. I avoid most of the mathematics but a picture of their operation is vital to understand how to use a sigma-delta ADC. Bonnie Baker recently wrote a brief series on Delta-Sigma ADCs in a Nutshell, which you might like to read [62]. You can see that even the name is not standardized: It is often delta-sigma instead of sigma-delta. Greek letters are frequently used as abbreviations to give ?? or ??.

The basic idea behind a sigma-delta converter is to reduce the ADC itself to the simplest circuit possible. This is a 1-bit ADC no more than a comparator which must take samples at a much faster rate than the desired output to compensate for its poor resolution. It therefore produces a very rapid stream of single-bit samples, whose mean value represents the magnitude of the analog input. These bits are processed digitally to extract their average and reduce the frequency of samples to the desired rate.