Analog Dialogue 33-8 (1999) analog-to-digital-converter-architectures-and-choices

Analog Dialogue 33-8 (1999)
1
Analog-to-Digital
Converter Architec-
tures
and Choices for
System Design
By Brian Black
How important are the differences between sigma-delta and
successive-approximation architectures in choosing an analog-to-
digital (A/D) converter? They can often be an important factor in
initiating the selection of a converter for a specific application. We
describe here four major circuit architectures used in A/D
converter
(ADC) design and outline the role they play in converter choice
for various kinds of applications. The descriptions are augmented
by three examples that illustrate tradeoffs and issues associated
with architectural considerations.
Though not detailed or exhaustive, this overview is intended
to raise issues that should be understood when considering
converters of different architectures. Sources
of more-detailed
information on converter architectures can be found in the
References
and at Internet sites indicated at appropriate points.
As one might expect in a survey of this kind, these descriptions
are not comprehensive; and variations within each of the
architecture families make generalizations less than fully accurate.
Nevertheless, such generalizations
are useful for the system
designer to keep in mind when conducting a high level overview
of a proposed system’s requirements.
CONVERTER ARCHITECTURES
An overwhelming variety of ADCs exist on the market today, with
differing resolutions, bandwidths, accuracies,
architectures,
packaging, power requirements, and
temperature ranges, as well
as hosts of specifications, covering a broad range of performance
needs. And indeed, there exists a variety of applications in data-
acquisition, communications, instrumentation, and interfacing for
signal processing, all having a host of differing requirements.
Considering architectures, for some applications just about any
architecture could work well; for others, there is a “best choice.”
In some cases the choice is simple because there is a clear-cut
advantage to using one architecture over another. For example,
pipelined converters are most popular for applications requiring a
throughput rate of more than 5 MSPS with good resolution. Sigma-
delta converters are usually the best choice when very high
resolution (20 bits or more) is needed.
But in some cases the choice
is more subtle. For example, the sigma-delta AD7722 and the
successive-approximations AD974 have similar resolution (16 bits)
and throughput performance (200 kSPS). Yet the differences in
their underlying architectures make one or the other a better
choice, depending on the application.
The most popular ADC architectures available today are
successive
approximations
(sometimes called SAR because a
successive-
approximations (shift) register
is the key defining element),
flash
(all decisions made simultaneously),
pipelined
(with multiple flash
stages), and
sigma-delta
(
Σ∆
), a charge-balancing type. All A/D
converters require one or more steps involving comparison of an
input signal with a reference. Figure 1 shows qualitatively how
flash, pipelined, and SAR architectures differ with respect to the
number of comparators used vs. the
number of comparison cycles
needed to perform a conversion.