Analog to Digital Converter ADC Architectures and Choices for System Design
How important are the differences between sigma-delta and successive-approximation architectures in choosing an analog-todigital (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 a r en o tc omp r e h e n s i ve ; a n dva r i at i o n s wi t h i ne a c ho ft h e 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.
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 dataacquisition, 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. Sigmadelta 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 successiveapproximations (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.
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