The final step is to level-decode the result into binary form. Together, these outputs can be read much like a thermometer.
#Analog devices analog to digital converter full
If for example the input is just above ¼ of full scale, all comparators biased to less than ¼ full scale will output a digital ‘1’, and the others will output a digital ‘0’. The comparator outputs simultaneously present 2 n-1 discrete digital output states. These values are spaced one least-significant bit (LSB=FS/2 n) apart. For a 4-bit flash ADC, the analog input is fed into 15 comparators, each of which is biased to compare the input to a discrete transition value. Flash ConvertersĬonceptually, the flash architecture (illustrated in Figure 2) is quite straightforward: a set of 2 n-1 comparators are used to directly measure an analog signal to a resolution of n bits. Tradeoff between decision cycles and comparators. the number of comparison cycles needed to perform a conversion. Figure 1 shows qualitatively how flash, pipelined, and SAR architectures differ with respect to the number of comparators used vs. All A/D converters require one or more steps involving comparison of an input signal with a reference. 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 (SD), a charge-balancing type. Yet the differences in their underlying architectures make one or the other a better choice, depending on the application. For example, the sigma-delta AD7722 and the successive-approximations AD974 have similar resolution (16 bits) and throughput performance (200 ksps). But in some cases the choice is more subtle. Sigma-delta converters are usually the best choice when very high resolution (20 bits or more) is needed. For example, pipelined converters are most popular for applications requiring a throughput rate of more than 5 MSPS with good resolution. In some cases the choice is simple because there is a clear-cut advantage to using one architecture over another. 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.Ĭonsidering architectures, for some applications just about any architecture could work well for others, there is a "best choice". Converter ArchitecturesĪn 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. 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. 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. Sources of more-detailed information on converter architectures can be found in the References and will be made readily accessible by direct links at appropriate points. Though not detailed or exhaustive, this overview is intended to raise issues that should be understood when considering converters of different architectures. The descriptions are augmented by three examples that illustrate tradeoffs and issues associated with architectural considerations. 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. 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. Analog-to-Digital Converter Architectures and Choices for System Design
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