[SI-LIST] Re: return currents
- From: Larry Smith <ldsmith@xxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, james.f.peterson@xxxxxxxxxxxxx
- Date: Wed, 26 Sep 2001 10:28:41 -0700 (PDT)
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
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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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