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You are here: Home » Blog » Power Integrity (PI/PDN)

How to specify bypass capacitors

April 2, 2013 by Rolf Ostergaard

EDN Europe March issue had signal integrity as the key theme. I think that is great and we need more stories like that, but that is not a big surprise I guess. Two stories caught my eye: “Signal-Integrity Issues on the Rise” and “Why your 4.7-uF ceramic cap becomes a 0.33-uF cap”.

The first is a journalist interviewing test equipment manufacturers and SI gurus. As with any story written by a journalist interviewing experts, there are a few slips in the text. Some are amusing and some are just plain wrong. Examples include: “EMI and crosstalk is closing the eye” – Yes, crosstalk will contribute to that. EMI will not unless you have a very uncommon setup that I can only imagine.

The second article is by an FAE from MAXIM who clearly did some actual work himself to understand the situation with capacitor ceramics. If you look at this plot, you get the point I think:

Showing why you should be careful when you specify bypass capacitorsThe central thing to learn from this article may not be that ceramic capacitors are DC voltage sensitive, because you probably already knew that. And the ones with high capacity in a very small package are often the worst. And X5R is worse than X7R. And Y5V is very bad. No big surprise either.

The thing to really pay attention to is the fact that any ceramic that satisfies the temperature characteristic can be called X7R. No matter how it behaves across DC (bias) voltage. So you really need to specify a specific manufacturer and part number to get what you want.

Very good article – well worth the read. And thanks to Phil for bringing it to my attention – please be precise when you specify bypass capacitors.

Filed Under: Power Integrity (PI/PDN) Tagged With: Capacitors, Ceramics, PDN

About Rolf Ostergaard

Rolf V. Ostergaard, M.Sc.EE. has worked with signal integrity in many different projects since working for 3Com in 1998 as a colleague to Lee Ritchey in Silicon Valley. While building a consulting business focused on advanced electronics and embedded software in Denmark, Rolf has been helping numerous companies with signal integrity and power integrity both as design, simulations, coaching, measurements, and troubleshooting. He started conducting training in SI in 2004 and has trained hundreds of engineers, which lead to founding EE-Training to further expand this.
You can hire Rolf to do signal integrity training and consulting worldwide and remote.

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