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Design007-June2020

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10 DESIGN007 MAGAZINE I JUNE 2020 There's a split in the PCB design community, and—as is often the case—the divide centers on data, specifically app notes. It's a lot like a boxing match. In one cor- ner, we have the PCB designers and design en- gineers who have been taught to follow app notes provided by the manufacturers of com- ponents and laminates. Many of these tech- nologists work for companies that stress the importance of following these app notes. As one senior designer told me, "What else are we supposed to use?" And in the other corner, we have veteran de- sign instructors like Lee Ritchey, Rick Hartley, Dan Beeker, and quite a few more. Their po- sition is that app notes should not be trusted unless you know that they're accurate. Design- ers and engineers don't automatically trust any other data, so they shouldn't trust app notes either just because an IC manufacturer says they're accurate. So far, neither side has managed to land a knockout blow, but there has been plenty of trash talk. There seems to be a lot of confu- sion regarding the role of app notes. Are app notes kissing cousins to datasheets (which most designers trust), or are they overly opti- mistic marketing collateral with results you're not likely to see in real-world operation? Apparently, the accuracy of app notes was not much of a problem 25–30 years ago when board speeds were much lower. IC mak- ers cranked them out with each part, and if they weren't exactly accurate, it didn't mat- ter. There was a lot of leeway back then, and a designer or design engineer could follow a bad app note and still wind up with a perfectly functioning PCB. But now, there's almost no leeway, and an app note that's a little off can have major con- sequences for a high-speed design. People who write app notes make mistakes, just like the The Shaughnessy Report by Andy Shaughnessy, I-CONNECT007 App Notes: To Trust or Not to Trust?

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