MT-022 Datasheet, PDF (3/12 Pages) Analog Devices – ADC Architectures III: Sigma-Delta ADC Basics

MT-022
Slope clipping can be reduced by increasing the quantum step size or increasing the sampling
rate. Differential PCM uses a multibit quantizer to effectively increase the quantum step sizes at
the increase of complexity. Tests have shown that in order to obtain the same quality as classical
PCM, delta modulation requires very high sampling rates, typically 20× the highest frequency of
interest, as opposed to Nyquist rate of 2×.
For these reasons, delta modulation and differential PCM have never achieved any significant
degree of popularity, however a slight modification of the delta modulator leads to the basic Σ-Δ
architecture, one of the most popular ADC architectures in use today.
In 1954 C. C. Cutler of Bell Labs filed a very significant patent which introduced the principle of
oversampling and noise shaping with the specific intent of achieving higher resolution
(Reference 7). His objective was not specifically to design a Nyquist ADC, but to transmit the
oversampled noise-shaped signal without reducing the data rate. Thus Cutler's converter
embodied all the concepts in a Σ-Δ ADC with the exception of digital filtering and decimation
which would have been too complex and costly at the time using vacuum tube technology.
Occasional work continued on these concepts over the next several years, including an important
patent of C. B. Brahm filed in 1961 which gave details of the analog design of the loop filter for
a second-order multibit noise shaping ADC (Reference 8). Transistor circuits began to replace
vacuum tubes over the period, and this opened up many more possibilities for implementation of
the architecture.
In 1962, Inose, Yasuda, and Murakami elaborated on the single-bit oversampling noise-shaping
architecture proposed by Cutler in 1954 (Reference 9). Their experimental circuits used solid
state devices to implement first and second-order Σ-Δ modulators. The 1962 paper was followed
by a second paper in 1963 which gave excellent theoretical discussions on oversampling and
noise-shaping (Reference 10). These two papers were also the first to use the name delta-sigma
to describe the architecture. The name delta-sigma stuck until the 1970s when AT&T engineers
began using name sigma-delta. Since that time, both names have been used; however, sigma-
delta may be the more correct of the two.
It is interesting to note that all the work described thus far was related to transmitting an
oversampled digitized signal directly rather than the implementation of a Nyquist ADC. In 1969
D. J. Goodman at Bell Labs published a paper describing a true Nyquist Σ-Δ ADC with a digital
filter and a decimator following the modulator (Reference 11). This was the first use of the Σ-Δ
architecture for the explicit purpose of producing a Nyquist ADC. In 1974 J. C. Candy, also of
Bell Labs, described a multibit oversampling Σ-Δ ADC with noise shaping, digital filtering, and
decimation to achieve a high resolution Nyquist ADC (Reference 12).
The IC Σ-Δ ADC offers several advantages over the other architectures, especially for high
resolution, low frequency applications. First and foremost, the single-bit Σ-Δ ADC is inherently
monotonic and requires no laser trimming. The Σ-Δ ADC also lends itself to low cost foundry
CMOS processes because of the digitally intensive nature of the architecture. Examples of early
monolithic Σ-Δ ADCs are given in References 13-21. Since that time there have been a constant
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