[SI-LIST] Re: return currents
- From: "Chris Rokusek" <crokusek@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 28 Sep 2001 14:35:42 -0700
Jim,
Concerning the skin depth argument...
The actual frequency domain representation of _practical_ digital signals
cannot be simplified as an
evelope with fundemental at clock frequency and its harmonics. Because most
digital signals are far
from periodic (meaning they don't switch every clock cycle) they contain energy
well below the
fundamental clock frequency. E.g. a 100MHz data line that only switches
exactly once every 100
cycles will have lots of energy at 1MHz.
Chris Rokusek
Innoveda
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Larry Smith
> Sent: Wednesday, September 26, 2001 10:29 AM
> To: si-list@xxxxxxxxxxxxx; james.f.peterson@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: return currents
>
>
>
> 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
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> >
> >
> > 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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