Analog Angle Article

What’s the difference between an ADC and a DAC?

No, this is not a “trick” question or brain teaser. And I assume that readers of this column know very well the basic function of an analog/digital converter (ADC) and its counterpart, the digital/analog converter (DAC).

But there is a philosophical difference, as well, that plays into how these converters are used and sometimes taken for granted. In simplest terms, it's this: an ADC is attempting to capture and convert a largely unknown signal into a known representation. In contrast, a DAC is taking a fully known, well-understood representation and “simply” generating an equivalent analog value.

In short, the DAC is in the deterministic world, while the ADC is in the world of random input signals and unknowns, as long as the input is within the defined range. In traditional signal-processing theory, as discussed by Harry L. Van Trees in his classic work Detection, Estimation, and Modulation Theory, Part I , signal processing has differing levels of challenge. For example, a signal which characteristics which are fairly well-known in advance (such as a analog signal under AM modulation) is much easier to evaluate than a signal with many unknown parameters (a radar return corrupted by noise, for example).

So, yes, the challenge for an ADC is much greater than it is for a DAC. To get the most out of an ADC, especially a higher-performance one (speed or precision) takes a well-designed analog signal-conditioning input channel, often with an ADC driver carefully matched to the ADC itself.

The DAC's life is much easier. But that relative ease shouldn't encourage complacency on the designer's part. It's too easy not to give the analog output of the DAC the attention it needs, regarding parameters such as slew rate, output drive (voltage, current, range) and protection against faults at its load. And that can lead to nasty circuit and system-level headaches, at both the prototype evaluation and in the field. ♦

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