Digital Drive

RE: Truncation of LSB's

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This Post Has Been Edited by the Author

In Reply to: RE: Truncation of LSB's posted by sideliner on June 04, 2017 at 09:54:17

First of all, I want to clearly state that I'm not a D/A architect. With that said, there are several reasons why an D/A designer might choose to utilize multiple smaller R-2R arrays rather than a single larger array. For one, multiple arrays enables moving the point of maximum glitching of the overall converter unit away from the waveform's zero crossing point, where it would otherwise be located with a single array. For another, you have to consider the primary obstacle to high resolution R-2R D/A unit design, which is the accuracy of the resistor elements (and the stability of that accuracy over changes in temperature and humidity).
For example, say you want to create a 20-bit single array R-2R converter unit. The dynamic range of such a converter is about 120dB, a ratio of roughly 1,000,000 : 1. Which means that the resistors in the array would need to have the same level of accuracy. In percentage terms, this is a 0.0001% tolerance for every resistor comprising the array. You can begin to see the technical challeges. An D/A converter comprised of dual 19-bit arrays, however, can each utilize resistors of only twice the tolerance. The engineering and manufacturing problem isn't the number resistors needed as much as the very high accuracy required of them.
Under the no-free-lunch principle, multiple arrays would need to be tightly gain matched to each other, not to mention the additional control logic required to coordinate their operation. These issues rapidly gain complexity with the number of arrays being integrated.
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Ken Newton

Edits: 06/04/17 06/04/17 06/04/17

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