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The I-Connect007 team met with design expert Lee Ritchey to pick his brain on EMI (electromagnetic interference) problems and what can be done to minimize them. Lee explains the issues are almost always tied to power delivery, as well as the abundant amount of misinformation surrounding this topic.
Barry Matties: Thank you for speaking with us, Lee.
Andy Shaughnessy: We want to discuss finer pitches and features and EMI and EMC because we have talked to people having EMI issues, particularly in Germany on the automotive side. You mentioned that you weren’t seeing many EMI problems due to different ways of doing busses and other tricks that helped relieve those issues.
Lee Ritchey: I believe that might have been happening because they have big distributed networks in cars, and if you don’t do it right, you turn the whole car into an antenna. To Andy’s point, I used to do six to 12 EMI problems a year, but I did only one last year. The source of most EMI is a ripple in the power supply, which is caused by trying to drive these wide data busses for DDR, VME, etc., and the power supply hasn’t been designed correctly. When we replace a parallel bus with a differential pair, several things happen all at once. The incidence of wide busses switching from zero to one goes away, and that’s the villain in almost all cases.
Most of what I work on has nothing but differential pairs in it. Big networking products that you would think would be a problem are flying because of inherently quiet differential signaling. To give you an example, everyone has differential twisted pairs in your house. The energy on it is an EMI band, but we don’t have any issues, even with unshielded wires. The reason is that the fields cancel each other on the two wires. That does not happen when you have a single-ended circuit.
People having problems have something in their product, such as a parallel bus, with the power delivery system designed incorrectly. That used to be how we made easy money. I’d get a call and say, “Did you follow the app notes?” If the answer was, “Yes,” then I knew what their problem was. However, being a good consultant, I won’t tell you over the phone because then you think it’s free advice. You fly me out at great expense, we fix it, and you’re happy. It’s hard to get this concept through, but if you follow the app notes, you will likely have an EMI problem. If your problems are typical applications, then you will always have an EMI problem. Almost all of them say to use 0.1-uf and 0.01-uF capacitors in all sorts of different ways. Neither of those capacitors is able to deal with EMI. That’s my giveaway. I know what to fix. You don’t have the right capacitors in your power system.
To read this entire interview, which appeared in the February 2020 issue of Design007 Magazine, click here.