[SI-LIST] Re: EMI point to point versus Differential

Well . . ., 
  It may be hard to come up with a simple answer to this.  With a proper
low impedance ground return path for point to point traces, the mirror
image of the trace along the ground return path becomes the "other
conductor" and you end up with what is essentially a diff pair or a
close approximation.  However, if your grounding and routing system is
not well laid out, even with diff pairs you can carry common mode energy
(even energy related to the generated signal) from one area of the board
to another.  I have seen both designs (point to point & diff pair) work
exceedingly well for EMI control and both not work at all for EMI
control.  So there is no magic EMI bullet as far as point to point or
diff pair is concerned.  And the orders of magnitude should be the same
with properly designed grounds and other layout considerations (e.g. 3W
rule and related).

The most important factor in either case is the grounding system of the
product, but opinions vary on the implementation of this.  There was a
great single vs multipoint ground discussion last June and I'm sure
there have been others.

Best Regards,
Dave Heald



"hamdi, amine" wrote:
> 
> Howard Johnson has a section about that in his High Speed Digital Design
> book.  See Sec4.1.2 .  If you do not have this book, please feel free to let
> me know, I will give you more feed back based on his approach.
> 
> Amine Hamdi
> EMC Corporation
> Hopkinton MA  01748
> (508)249-4834
> hamdi_amine@xxxxxxx
> 
> -----Original Message-----
> From: Suresh Sivasubramaniam [mailto:cacofonix98@xxxxxxxxx]
> Sent: Friday, October 26, 2001 6:39 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] EMI point to point versus Differential
> 
> Hello SI gurus,
> 
> I am interested in a quick comparison of the magnitude
> of EMI for the case of point to point wiring to that
> of differential signaling for PCB traces. Can someone
> point me to some formulas with which to do this
> analysis ?
> 
> Thanks much
> 
> Suresh S
> 
> __________________________________________________
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