My previous Tips column, “Oversample on Purpose,” (DN 10.20.08, http://designnews.hotims.com/22267523), explained how averaging oversampled data can improve the resolution of an analogtodigital converter (ADC). A deltasigma converter also uses oversampling to produce results with as many as 32 bits, a resolution thought impossible not long ago. (Engineers often use “delta sigma” and “sigma delta” interchangeably to describe this type of ADC). A deltasigma ADC includes a modulator section, which, in turn, comprises a difference amplifier, an integrator, a comparator and a 1bit digitaltoanalog converter. Those components form a closed loop that integrates the sum of an unknown signal and the DAC’s +Vref or Vref signal. A comparator controls the DAC based on the voltage from the integrator and a reference voltage at the comparator. (For more operational details, see Useful Links, below right).
The modulator produces a “weighted” string of 1’s and 0’s. So, oversampling the signal at, say, 64 times the signal’s highest frequency component yields a string of 64 bits. The more 1’s in the string, the higher the voltage and the more 0’s, the lower. But the 64bit stream doesn’t mean you get a 64bit ADC! Digital circuits create a data value from the bit stream. Those values then go to a digital lowpass filter that removes noise that comes from several sources. In some converters, the filter will produce a sinc response, [sin(px)]/px, that helps attenuate powerline noise and harmonics.
At the end of the digitalfiltering process the ADC has more values than you need. If you oversample a signal at 64 times the Nyquist frequency, you won’t need 64 values to reconstruct it. So, a decimation step eliminates, say, three out of four or seven out of eight values. Thus, the ADC produces values at 16 or eight times the Nyquist frequency. Typically, deltasignal converters offer high resolution, but they operate best with lowfrequency signals. Texas Instruments’ ADS1282 ADC, for example, produces either 24 or 32bit results at either a 4,000 or 250 samples/sec rate.
The modulator portion of a deltasigma converter acts like a lowpass filter for the unknown signal and as a highpass filter for quantization noise. (Remember, the converter has discrete voltage steps). So, the modulator provides some “noise shaping” capabilities. But the simple firstorder deltasigma modulator described here doesn’t offer enough noise shaping to allow for highresolution results. Thus, IC manufacturers include from second to fifthorder modulators that increase a converter’s signaltonoise ratio. The TI ADS1282 noted earlier provides a fourthorder modulator. For more on modulators, go to http://designnews.hotims.com/22267524.
Because a deltasigma ADC oversamples at a high rate, the frequency of its Nyquist limit increases, too. This means the converter can often use an inexpensive and simple RC antialias filter to remove highfrequency signal components.
Useful Links

Find an excellent nonmath introduction to deltasigma converters: http://designnews.hotims.com/22267525

“An Overview of SigmaDelta Converters:” http://designnews.hotims.com/22267526

“Are S? ADCs Greek to You?” http://designnews.hotims.com/22267527