Download AN132 Fidelity Testing for A-D Converters (2011, Linear Technology, 8 pages, 3.65 MB pdf file)
The last app note that was (co-)authored by Jim Williams does not tell you much about ADC testing (for a comprehensive discussion of this subject please see Kester chapter 5). Instead it focusses on the low distortion Wien bridge oscillator that is used for the test. In the description of the circuit it is mentioned that it was “adapted from work by Winfield Hill of Harvard University”. This probably refers to chapter 5.17, “Wien bridge and LC oscillators”, p. 297, figure 5.42 B of The Art of Electronics (AoE) by Horowitz and Hill. More interesting, however, is the comparison with Jim Williams own Wien bridge circuit from 1990, see AN43, p.33, figure 48. It already shows the replacement of the FET (that is used in AoE to control the gain) by a LED driven CdS photocell, and several other ideas to reduce the distortion.
This app note was released by Linear Technologies in October 2011, several months after Williams died (June 2011). It is not a “typical” Jim Williams app note. Professor Kent Lundberg, who is writing the Reading Jim Williams blog, has not included it in his list of Williams app notes. He gives the reason for this in his (very exhaustive) bibliography on page 3: “Although this note bears his name (as coauthor) and discusses an appropriate topic, (a high-purity sine wave oscillator), Jim’s signature touches are absent.” He further explains in a footnote: “The oscilloscope shots are not from his Tektronix 556, there’s no hand-drawn cartoon, there are no voluminous appendices, and there are three pages on computer-screen captures” and concludes “It just doesn’t feel like Jim”.
In principle I agree with all of this, in addition there would certainly be a comprehensive list of references if the app note had been finished by Williams himself. However, the app note shows the schematic of a low distortion oscillator complete with description and measurement results (3ppm THD, 5.8 ppm THD+N) and therefore should be useful to everybody who is interested in (audio frequency) oscillators. It would certainly be interesting to read Lundberg comment on the circuit from a control theory point of view. I hope he will do so in his blog even if he does not recognize AN132 as a typical Jim Williams app note. Anyway, even if the app note is kind of incomplete, Linear Technology should be applauded for releasing this interesting circuit.
If you want to see other, recent (2010) examples of Williams mastery of analog electronics, please look at AN120 and AN128. These are 100 percent complete, 100 percent Jim Williams app notes, showing all of his “signature touches”.
You might also be interested in the other Application Notes by Jim Williams
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