18 July 2011

App Note 6

"Applications of new precision op amps." 8 pages.

This app note is a bit of a mixed bag. I'm not sure if I agree with the last sentence in the first paragraph. While the circuit in Figure 1 is an interesting beast, with its crazy opto-coupled MOSFETs and its 300-volt common-mode range, I'm just not sure if it's an "excellent example" of what to do with a new precision op amp. I think the platinum-RTD signal conditioner and the direct-connection thermocouple-driven battery charger are better examples of what to do with a precision op amp like the LT1001.

The best circuit would be the dead-zone generator in Figure 5, but why is the second LM301A labeled C1? Is he really using it (uncompensated) as a comparator? Bob Widlar would not approve. Why not use an LM311?

Also, the ultra-precision voltage reference is a neat application, but something about "ultra precision" and "chopper step-up transformer" in the same system leaves a bad taste in my mouth. Plus, C1 is another LM301A being used as a comparator. I like how Figure 7 nonchalantly indicates a Kelvin-Varley Divider in the circuit. No problem. Doesn't everyone have one in their desk drawer? (All kidding aside, the careful error budget on page AN6-7 is a skill more engineers should learn.) Note that page AN6-6 includes another call for the General Radio 1432-K precision resistor decade box (see App Note 3). He was a big fan.


Actually, the best circuit is the high-speed op amp in Figure 8, another brilliant discrete design, using 2N3688s, 2N5160s, 2N4440s, and a feed-forward path to improve for high-frequency performance. Finally, note the oscilloscope photo in Figure 9: is that an HP logo in the top left corner? Heresy!

Best quote (page AN6-1): "A2 is trimmed for a 93Hz clock output. This frequency inhibits power line-originated noise from interacting with the switching action because it is not harmonically related to 60Hz." I'm surprised that he didn't discuss the history of 93-cps modulated systems.

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