Delta-sigma ADCs in a nutshell
By Bonnie Baker - December 14, 2007
Delta-sigma converters are ideal for converting signals over a wide range of frequencies from dc to several megahertz with very-high-resolution results. Figure 1 shows the basic topology, or core, of a delta-sigma ADC, which has an internal delta-sigma modulator in series with a digital filter. As you explore delta-sigma ADCs, you will find that, although they have a variety of other features, they all possess this basic structure. This column and the next three Baker’s Best columns explore the basic topology and functions of these two modules.
The input signal to the delta-sigma ADC is an ac or dc voltage. This and the next three Baker’s Best columns use a single cycle of a sine wave as the input signal. Using a 1-bit internal ADC, the internal converter modulator in Figure 1 samples the input signal, producing a coarse, quantized output. The modulator converts the analog-input signal into a high-speed, pulse-wave representation. The ratio of ones to zeros in the modulator’s output pulse train mirrors the input-analog voltage. Although the modulator produces a noisy output, future columns will show that the circuit “shapes” this noise into the higher frequencies of the output spectrum. This action paves the way for a low-noise, high-resolution conversion at the output of the digital filter.
At the modulator output, the digital filter addresses high-frequency noise and high-speed-sample-rate issues. Because the signal now resides in the digital domain, you can apply a lowpass digital filter to attenuate the higher frequency noise and a decimator filter to slow down the output-data rate. The digital/decimator filter samples and filters the modulator’s stream of 1-bit codes and creates a slower multibit code.
Although most converters have only one sample rate, delta-sigma converters have two: the input sampling rate and the output-data rate. The ratio of these two meaningful variables defines the system’s decimation ratio. A strong relationship exists between the decimation ratio and the converter’s effective resolution. A future column will examine how the modulator, digital/decimator filter, and adjustable decimation ratio work.