Look on any of the major analog manufacturers' websites and you'll find lots of 12-bit D/A converters. Do any of them really qualify as 12-bit devices? I mean really.
A little history is in order here.
The earliest commercially available D/A converters were plastic-packaged modules... typically 2 inches by 3 to 4 inches by ½ to 1 inch thick, containing a circuit board populated by discrete devices, sometimes hand-selected, or at best a few low-level ICs. They were big and bulky, but they were the only game in town. And many of them were 12-bit accurate over temperature, including gain, offset, linearity, and all other errors. Just to clarify, 1 least-significant-bit (LSB) at 12 bits is 1 part in 4096 or 0.024 percent.
One company producing such devices was Analogic, headed by a brilliant engineer and entrepreneur named Bernie Gordon. Analogic was making a lot of money selling these converter modules, and wanted to protect their market. Bernie waged a marketing campaign in the trade press against the inevitably lower-price monolithic D/A converters, telling anyone who would listen that it was not possible to produce a true 12-bit current-switching D/A converter in monolithic technology, and it would never be possible.
In the 1970s, various companies tried to produce D/A converters with 12-bit resolution and accuracy. In 1977, Donald Comer of Precision Monolithics, Incorporated (PMI, later acquired by Analog Devices) presented a paper at the ISSCC on a monolithic 12-bit D/A converter. The paper is available on the IEEE site if you are a member. In the corner of the chip, the metal mask included a sketch of a heart with a bite taken out of it, and the initials "B.G." in the bite. The message was: "Eat your heart out, Bernie Gordon. We have done what you said was impossible."
It did not take Bernie long to respond. In a letter to George Rostky, editor of Electronic Design magazine, the leading trade magazine of the day, Bernie made several points.
Bernie Gordon's paper describing his converter and a letter to George Rostky. Click here to see a larger version with all pages.
Reprinted with the kind permission of Analogic.
One was that the PMI D/A converter used an additional layer of thin-film resistors deposited on the chip, and that disqualified it from being a monolithic device. That was kind of a weak argument, since all ICs use a deposited layer of interconnect metal.
The next point was that the PMI D/A converter was not quite complete, needing a reference, an output amplifier for current-to-voltage conversion, and bypass capacitors. This was definitely true, and adding a 12-bit-accurate reference would require some discrete components and some kind of manufacturing adjustment.
However, Bernie made some valid points regarding temperature drift and long-term drift. Whether they were actually needed is arguable. Bernie's position was that if you needed a 12-bit accurate device at the time a system was sold, you probably also needed a 12-bit-accurate system a year or two later. Of course, most instruments are re-calibrated and performance-certified periodically to take out the biggest errors, but re-calibrating the linearity of a laser-trimmed or Zener-zap-trimmed D/A converter isn't really feasible.
As time went on, the price-performance ratio of 12-bit IC D/A converters was too attractive to resist, the market decided they were "good enough," and shifted away from the more-expensive modules. Analogic turned its attention to developing and manufacturing systems (ironically, many of which included the much-maligned IC converters).
I looked through a few websites to see if anyone has yet made a "true" 12-bit D/A converter... guaranteed over temperature, time, and including all error sources. I didn't find any. Some are pretty close, at least hitting the linearity specs, and even holding them over temperature. Even 16-bit linearity is being met by some devices. Getting the full-scale accurate to 0.024 percent with an on-chip reference is a lot harder.
Will we ever get there? Was Bernie Gordon right in 1977?
Dan Sheingold (ex-Analog Devices) and Walt Kester of Analog Devices provided some fact-checking and filled in a few details for this post.