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

From: Michael Nudelman ([email protected])
Date: Mon Jan 22 2001 - 14:16:45 PST

Vinu:

The difference is that every "tying" cap is referenced to a GND plane; thus,
if you consider a GND plane a 0V potential and there is no direct capacitive
link between any non-GND-planes, teh effective capacitance bewteen the
planes is 0.

To facilitate explanation, imagine a capacitive system of three plates. If a
middle plate is floating, then the capacitance between the 1-st and the
third plates practically (ideally) does not depend on the second plane being
present or absent (provided the distances bewteen plates are greater than
the plates' thicknesses).
Now, let us Ground the 2-nd plate. This plate then will "shield" either 1st
and 3rd plates from each other, since the potential of the second plate is
always 0 and does not change with the 1-st (or 3rd) plate's potential
(capacitive load) and so the opposite plate will not see the changes,
rendering the capacitance bewteen them 0.

This is the difference.

Mike.

-----Original Message-----
From: Vinu Arumugham [mailto:[email protected]]
Sent: 22 января 2001 г. 04:44
To: Larry Miller
Cc: 'Zabinski, Patrick J.'; [email protected]
Subject: Re: [SI-LIST] : Decoupling between non-ground power rails, yes
or no??

If there are multiple voltage rails on a board that are properly decoupled
to
the same ground, there is a significant amount of capacitance between the
voltage rails, even without explicitly adding caps between them. Why does
that
not cause a problem?

Thanks,
Vinu

Larry Miller wrote:

> This instability is not a surprise. Putting caps between the voltages
> introduces a lead term (treble boost) into the voltage regulator feedback
> loop which can raise havoc by pushing the loop bandwidth out farther than
it
> was designed for.
>
> Offhand, I would say that this is overall a Bad Idea.
>
> Larry Miller
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Zabinski, Patrick
> J.
> Sent: Monday, January 22, 2001 9:04 AM
> To: [email protected]
> Subject: RE: [SI-LIST] : Decoupling between non-ground power rails, yes
> or no??
>
> Michael,
>
> directly, but I did want to bring up one cautionary
> note.
>
> 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.
>
> Pat
>
> > 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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