Design briefs: Tear 'em apart!

I'm experiencing a severe case of information overload. Three cubic feet of clippings I've accumulated over the past umpteen years await filing, and seven file cabinets crammed with articles, catalogs, and correspondence need weeding and sorting.How did I get myself in so much trouble? I began with Yankee Packrat genes, added a professor's advice about never reinventing the wheel, and finished with a curiosity about the theory and practice of electronics. As a result, I saved many magazine articles that dealt with whatever I was working on at the time.

As I weed my files, I find many design "briefs" I've clipped from electronics magazines. Typically, these short write-ups describe clever circuits and software, but rarely do they contain test-related information.

But test engineers can learn from—and have fun with—design briefs by analyzing them as test problems and, when appropriate, by tearing them apart. Here are a few tips to help you sharpen your claws on the weak spots—or on engineers who submit a brief instead of a well-analyzed design.

Study inputs to circuits or software with an eye toward behavior at extremes and boundary conditions. What output would you expect, for example, when an input voltage goes to zero, a digital input goes to all ones, or software receives an unexpected combination of ones and zeroes?

Consider what may go unmentioned in a brief. Does a circuit demand an extremely stable or noise-free power supply? How much current does the circuit consume? What happens in battery-powered devices at new-battery and end-of-battery-life voltage extremes. What occurs over wide temperature ranges? How much processing capability will a subroutine require? Will a subroutine fit into available program memory?

Be wary of circuits that rely on undocumented device characteristics. For example, a circuit that relies on vendor X's 2N2222, which exhibits pronounced avalanche behavior, may not work with vendor Y's "equivalent" transistors. Screening components for undocumented behavior quickly gets expensive.

Does a brief rely on a vendor's sole-sourced component? If so, identify unique characteristics and look for equivalents from other vendors. In recent years, I've noticed that semiconductor manufacturers contribute many design briefs that feature their own unique components. If a vendor discontinues a component or goes out of business, do other vendors offer suitable replacements?

Finally, don't blame subtle shortcomings on the magazines that publish design briefs. Although editors do their best to weed out the howlers and blatant product pitches, it's up to you to determine a design's suitability and to find its weak spots.