3 Subtle Lessons on Shielded Cables

Engineers often follow simple rules of thumb that lead invariably to functional designs. However, there can be overriding conditions that render rules of thumb or even so-called best practices incomplete and sometimes misleading.

Image courtesy of Cloom

Take shielded cables. Without a detailed analysis, most engineers regard using shielded cable as inherently “good.” It heads off noise problems, after all. But experience shows it can also introduce undesirable effects. And when circuits don’t play by the rules of thumb, even experienced engineers can be stumped.

Here are three examples from when I worked as a part-time electrical engineer at Sundstrand Aerospace—as an engineering professor the rest of the time, I was usually the one called in to figure out what went wrong and how to fix it.

Professor, we have an unstable DC signal

I was at my Sundstrand desk when I received a frantic call from my engineering colleague Tim. He was behind schedule and the test stand he was working on was exhibiting erratic behavior. Consequently, our manager, Doug, was there by his side to “help.” For any engineer, that’s usually a very uncomfortable situation.

I walked downstairs to the test stand and asked Tim to explain the problem.

“We are ringing out an integrated test article (ITA) module which interfaces the unit under test (UUT) with the test stand. We are performing a wrap-around self-test. We are directing a DC voltage into an analog-to-digital converter (ADC). The output of the ADC is fed into a digital-to-analog converter (DAC). The scaling is designed so the DAC output voltage should match the applied DC test signal.”

I asked Tim to see his ITA board schematic. He was using a noninverting op amp circuit to deliver the test signal. I puzzled for a few moments and then asked, “Tim are you using shielded cable on the op amp’s output?” Tim replied, “Sure, we did not want to have noise problems!”

Bingo.

“Tim, you’re connecting a capacitive load across the op amp output and it’s oscillating. The DC shifts are produced by the asymmetrical clipping of the op amp’s output voltage.”

Doug was visibly upset and asked, “Tim, didn’t you put a scope across the op amp’s output?!” I tried to cover for Tim and responded, “This is a pretty obscure problem. I only recognized it because I had the same problem a few months ago.” With a sigh and a huff, Doug left immediately.

I sketched the below image for Tim to explain the reason why the capacitive load caused oscillation.

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