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RE: Truncation of LSB's

Your seemingly simple questions have some complex answers. There are two well known ways to feed a data stream featuring a given bit-depth to a D/A unit featuring some lesser native bit-depth. The first way is to dither the incoming data to the lesser bit-depth rather than simply truncating it. This will remove the distortion of truncation at the price of increasing the channel noise floor. The other way is via sigma-delta-modulation(SDM), which can deliver the full dynamic range of the higher resolution original data, but only within a limited band. This is what most commercial D/A chips do, even when fed only 16-bits. These D/A chips typically feature only a 5-bit or 6-bit core, and utilize SDM to deliver the full resolution (SNR) of the original higher resolution data, within practical circuit limits, of course.

As far as various hybrid converter topologies are concerned, most of these have been tried over the years. The currently in production PCM1794A DAC chip is based on a hybrid converter core, while also utilizing SDM. The out of production PCM1704 R-2R DAC chip is actually a hybrid of two internal 23-bit R-2R cores coordinated to function as a single 24-bit complex converter. An architecture Burr-Brown refered to as 'colinear' in the data sheet. As a rule of thumb with the complex merging of multiple converter cores, each extra bit of resolution requires a doubling of converter circuitry. Simply paralleling converter outputs does not increase bit resolution. It only reduces the random noise floor by 3dB for every doubling of the number of converters.

High resolution discrete R-2R converters that reliably retain high accuracy are non-trivial to design and consistently manufacture. Such converters are, however, becoming in vogue among DAC vendors seeking to differentiate themselves in a crowded market. However, none of these which I'm aware objectively match the consistent accuracy of today's top multi-bit SDM converter chips. Subjective assessments may, and do, differ of course.
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Ken Newton



Edits: 06/03/17

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