[SI-LIST] Re: return currents

Jim, Larry, Zhiping -
I'm not a mixed analog/digital guy, but what would be the effect of putting
the analog stuff on layers 1&3 and the digital on 5&6?  Or putting the
analog on the top and bottom?  Emissions shouldn't be an issue if the
microstrips are laid out correctly (assuming you're not an intentional
radiator).  The slow stuff will be on top, and the fast stuff will be in the
middle.

1  analog
2  ground
3  analog
4  ground/pwr
5  Dig
6  Dig
7  ground/pwr
8 etc
************
Top analog
.
.
.
n Dig sandwich
.
.
bottom  analog
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zhiping Yang
Sent: Wednesday, September 26, 2001 1:28 PM
To: ldsmith@xxxxxxxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; james.f.peterson@xxxxxxxxxxxxx
Subject: [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
   Mail Stop:SJCG/2/2
   San Jose, CA 95134             |          |
   email: zhiping@xxxxxxxxx      :|:        :|:
   Tel  : 408 525 5690          :|||:      :|||:
   Fax  : 408 526 5504       .:|||||||:..:|||||||:.
*****************************************************

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
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> >              http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >              http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >              http://www.qsl.net/wb6tpu
> >
> >
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
>                http://www.freelists.org/archives/si-list
> or at our remote archives:
>                http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>                http://www.qsl.net/wb6tpu
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: