20 December 2011

App Note 72 part 1

"A seven-nanosecond comparator for single supply operation: Guidance for putting civilized speed to work". 44 pages.

This app note is a major update to App Note 13, "High speed comparator techniques" from 1985. Jim admits this fact in the introduction, in his own humorous way...
This publication borrows shamelessly from earlier LTC efforts, while introducing new material. It approximates, affixes, appends, abridges, amends, abbreviates, abrogates, ameliorates and augments the previous work (an alliterative amalgamated assemblage).
Some of this material is also borrowed from App Note 47.

After a brief overview of the LT1394, Jim again starts with a tutorial section on probes, oscilloscopes, breadboarding, and bypass capacitors. The section begins with an updated "Rogue’s Gallery of High Speed Comparator Problems" updated with results using the new chip (Figures 3 through 15). The oscilloscope photos show the disastrous results of poor bypassing, improper probing, bad construction, and stray capacitance. Fuzz, ringing, overshoot, sluggish rise, and oscillations all rear their ugly heads. As before, this discussion is great reading.

The tutorial section discusses some cures for these common ailments. "Theory, techniques, prejudice and just plain gossip are offered as tools that may help avoid or deal with difficulties." After discussing some of his favorite pulse generators (HP-8110A, HP-8082A, and HP-215A), he discusses cables, connectors, and terminations. "Typically, inappropriate cable can introduce tailing, rise time degradation, aberrations following transitions, nonlinear impedance and other undesirable characteristics."

The next five sections, "About probes and probing techniques", "About oscilloscopes", "About ground planes", "About bypass capacitors", and "Breadboaring techniques" are borrowed directly from App Note 47, including the figures. It's still all good advice, though. Read it twice.

I'll cover the applications next time.


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