Reading time ( words)
The second day of the virtual AltiumLive 2020 event got off to a great start with a class by Dr. Eric Bogatin. It was officially titled “You Must Unlearn What You’ve Learned,” but it could have just as easily been called “Designing Interconnect That Sucks Less.”
Eric is a signal integrity instructor, professor at the University of Colorado Boulder, and author of books like Signal and Power Integrity – Simplified.
But Eric is truly a teacher at heart. He could probably teach kindergarten or fifth grade and still get great results. During his signal integrity classes, he likes to tell stories about how his college students wind up teaching him points along the way.
He began his AltiumLive class with a story from his classroom at UC Boulder. He recently explained to his students how so many of the problems downstream stem from interconnect issues.
Eric told his class, “Once interconnectivity is established, interconnect just screws things up.”
One student replied, “You’re saying that our job is to design interconnect so that it sucks less?” Eric agreed that this sentiment describes their job to a T. It reminded me of the medical community’s Hippocratic Oath: First, do no harm.
He then shifted gears and got down to the business at hand—showing designers how to unlearn the bad habits that they’ve learned over the years. Eric started with switching noise, pointing out that many anti-noise techniques that worked 50 years ago don’t work now, and even if they do work, they may be very inefficient and/or expensive.
For instance, Eric explained that conventional wisdom once demanded, “You must use 0.1-, 1-, and 10-uF capacitors on your PCB to help fight noise.” The 0.1-uF cap had very little capacitance value, but it also had very low loop inductance, so it wouldn’t contribute much to switching noise.
Now, with today’s modern capacitors, you may not need to use a 0.1-uF cap because a 10-uF cap can have as low a loop inductance as the old 0.1-uF caps. But many design engineers still follow the 0.1-, 1-, and 10-uF capacitor rule that they learned decades ago.
As Eric explained, “The interaction of capacitance and loop inductance is where you find ways to lower switching noise and parallel resonance.” He also pointed out that you should never do something just because you saw someone else do the same thing, such as adding copper pour to a signal plane. “Unless you have a good reason, never add copper pour to a signal plane,” Eric said.
As Eric summed it up, what worked decades ago often doesn’t work today, so you better unlearn those old “legacy” habits.
Visit Real Time with… AltiumLive 2020 to catch the latest video interviews and event-related content.