[SI-LIST] Re: return currents

Larry,

What you said about the current distribution at high frequency(>100Mhz)
is true, but my questions is whether the current distribution is important
or the voltage variation on the power plane is more important than current
distribution?

Let's say, there are 2 points on the power plane and the most (>93%) current
flows
on the surface near digital circuit. The current flow near analog circuit is
very small (<7%), but it has big loop (large inductance) and it produces same
voltage drop between those 2 points as large current on the surface of digital
circuit.
IF the anolg circuit is sensitive to the voltage noise, then it is a problem.

Jim, another thing you need to be aware is that low frequency may be a killer
for your anolog circuit. In your current stack up, it is diffcult for to
control
the lower frequency current return path on layer 4.

Thanks.

Zhiping


--
   Zhiping Yang, Ph. D.
   Hardware Engineer
   Cisco Systems
   270 West Tasman Drive
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Larry Smith wrote:

> Jim - I don't believe that the high frequency return currents on your
> digital traces will have much effect on your analog traces even though
> they share the same ground plane (layer 4).
>
> The skin depth at 100 MHz is about 0.26 mil compared with the 0.7 mil
> thickness of half oz copper.  The skin depth is essentially the depth
> that the magnetic field penetrates into the copper.  At 100 MHz, very
> little magnetic field (approximately 1/[e^(.7/.26)] = 7% ) will
> penetrate through the copper plane.  Even less of it will reach an
> analog trace.  At higher frequencies, the penetration will be even less.
>
> How sensitive are your analog signals?  For digital signals, I would
> not worry about 7% magnetic field penetration.
>
> regards,
> Larry Smith
> Sun Microsystems.
>
> > Delivered-To: si-list@xxxxxxxxxxxxxx
> > From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] return currents
> > Date: Wed, 26 Sep 2001 07:32:10 -0400
> > MIME-Version: 1.0
> > Content-Transfer-Encoding: 8bit
> > X-archive-position: 946
> > X-listar-version: Listar v1.0.0
> > X-original-sender: james.f.peterson@xxxxxxxxxxxxx
> > X-list: si-list
> >
> >
> > hello,
> >
> > Stackup :
> >
> > 1 - gnd
> > 2 - digital sig
> > 3 - digital sig
> > 4 - gnd
> > 5 - analog sig
> > 6 - analog sig
> > 7 - gnd
> > 8,9,10,11 .....
> >
> > notice that the digital signals from layer 3 and the analog signals from
> > layer 5 will probably have return currents on layer 4.
> >
> > question :
> > will the digital return currents cause noise in the analog section, since
> > they both share layer 4 for return currents? (My first guess is yes, but
> > someone mentioned that the skin depth  for the return currents is small so
> > they can share layer 4 without effecting each other.)
> >
> > thanks for your input.
> > Jim
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