Re: [SI-LIST] : Decoupling between non-ground power rails, yes or no??

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From: Chris Padilla (cpad@cisco.com)
Date: Mon Jan 22 2001 - 08:38:34 PST


Michael,

While it is true you must follow image paths and provide short returns to
help improve emissions, I have found in my experience that if the Pwr/Pwr
decoupling is not done well, it can actually make things worse. Take
advantage of Pwr/Pwr sandwiches if you can although the preference is to
make Pwr/Gnd sandwiches. Distributed capacitance in some instances is
better than discrete solutions.

Most feel that Pwr planes to be quite noisy compared to ground planes so
care must be taken not to distribute this noise all over the PCB.

I personally find it disquieting to discretely decouple pwr planes. The
reason is that these delivery systems most likely will be switching at
different times. Imagine AC coupling a 5 V plane that is transitioning to
5 V at some 5/3.3 V decoupling cap at some frequency where the 2 power
planes look shorted together. Now further imagine that the 3.3 V plane is
transitioning from 3.3 V to zero at that same cap and at that same
frequency where the 2 power planes are shorted together! You are shorting
these two planes yet you want 5 V and 0 V at the same time at this
point! I have not the experience to describe this further but this
junction sounds like a clash of current running to and fro doing "bad
things" on your PCB. My emission scans showed definite problems when I had
these caps stuffed on my board--even a single one that I had forgottent to
remove!

I will admit that the discrete decoupling scheme on this board was not done
correctly from the beginning and my emissions saw to proving that point to me.

Hopefully this gets you "thinking" in the right direction and others will
chime in with more resolution than I have provided.

----->Chris

>Hi All,
>
>I recently saw something on a design that I
>felt was questionable and wanted to see what
>your thoughts were, especially as there appears
>to be a number of folks that have a focus on
>PDS and decoupling.
>
>While in the process of performing a PDS analysis
>of a new board, I noticed that there was a signi-
>ficant amount of decoupling between the power
>rails. For instance, there were caps between 5
>and 3.3, 3.3 and 2.5, etc. From discussions,
>it appears that this is there to facilitate
>return currents.
>
>So the obvious question is, Is this accomplishing
>what it is believed to be and is there in fact a
>better way to do this. Is a properly decoupled PDS
>(below target impedance through frequency range)
>all that is required. For the sake of argument,
>assume that all the planes are whole.
>
>Thanks for your opinions
>
>Best Regards,
>
>Michael C. Greim

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