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

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From: Zabinski, Patrick J. ([email protected])
Date: Mon Jan 22 2001 - 09:04:16 PST


Not exactly answering your question
directly, but I did want to bring up one cautionary

In at least two past board designs in our group, folks
have decoupled power planes directly to one another
(e.g., +5 to +3.3) in addition to decoupling them to
ground. In both of these board designs, oscillations
were found in the board. When we removed one set of
decoupling capacitors (i.e., breaking the GND-to-3.3-to-5-to-GND
loop), the oscillations stopped.

We did not have the time to dig into the root cause,
but I have since made it a rule to avoid having strong
decoupling between power supplies.


> 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 Sonus Networks

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