27 July 2011

App Note 13 part 1

"High speed comparator techniques." 32 pages.

At 32 pages, this app note is the longest one so far, the first with a table of contents, and the first with separate named appendices (there have been "box sections" before, but never appendices, and this note contains five!).  In addition to just being long, this work is also an important piece of foreshadowing for future projects.  I cannot cover this app note in a single day, so I'm going to spread it out over three days.

After a short introductory lament ("Comparators may be the most underrated and underutilized monolithic linear component"), this note covers design, construction, and instrumentation techniques for high-speed comparators (such as the ten-nanosecond LT1016 featured here), and it lays the foundation for his longest (and best?) application note, the fabled forthcoming App Note 47.

The first section of this note is a "Rogue's Gallery of High Speed Comparator Problems" along with thirteen scope photos of various ailments.  The frustration in these comments is really close to the surface (which I totally understand and absolutely agree with).  Imagine how many application complaints are eventually traced to poor bypassing, incorrect scope probes, poor grounding, suboptimal construction, and excessive parasitic capacitances!  The fact that five pages of this app note are dedicated to "You're doing it wrong!" is quite telling.  Nevertheless, this rogue's gallery should be required reading.  Read it twice.

Page AN13-8 has a short (the first!) discussion of oscilloscopes.  He doesn't name any names, but he does admit that "90% of the development work was done with a 50MHz oscilloscope."  I'll bet dollars-to-donuts that he's talking about his Tektronix 547 and 556 (you can see them in Figures 3 though 15).  He sums it up in a good quote: "In general, use equipment you trust and measurement techniques you understand.  Keep asking questions and don't be satisfied until everything you see on the oscilloscope is accounted for and makes sense."

Best quote (from page AN13-1): "In developing such [fast] circuits, even the most veteran designers sometimes feel that nature is conspiring against them.  In some measure this is true.  Like all engineering endeavors, high speed circuits can only work if negotiated compromises with nature are arranged.  Ignorance of, or contempt for, physical law is a direct route to frustration."

I'll discuss the applications section tomorrow.


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