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How to connect a capacitor?

Capacitors are the backbone of a board power distribution network, or PDN.  However, just as important as having the capacitors connected to the PDN is how they are connected.  If you think that connecting them with inch-long 5-mil traces is a good idea, you might want to reconsider (or maybe you are still living in the ’70s?).  Obviously that is an extreme example, but there are a number of nuances to connecting capacitors to the board to maximize their effectiveness as part of the PDN.  I actually analyzed and quantified some common capacitor mounting methods in the following article: http://www.pcdandf.com/pcdesign/index.php/current-issue/241-designer-s-notebook/9233-designers-notebook-1401

Among the variables explored are the connections to the planes, and that doesn’t just mean the connections themselves, but the plane configuration.  This can have a huge effect on the mounted inductance of the capacitor.  The separation between the power and ground planes is probably the most important.  Planes which are very close together are actually much more effective in minimizing the mounted inductance of the capacitor than planes which are further apart but closer to the capacitor (like in layers closer to the top).  In fact, that is one of the many tradeoffs you can analyze in the PDN editor in HyperLynx LineSim when trying to plan your PDN design.

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About Patrick Carrier

Patrick CarrierPatrick Carrier is a Product Marketing Manager for PCB analysis tools, including signal integrity, power integrity, electromagnetic compatibility, and thermal analysis. Visit The HyperBlog

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