[SI-LIST] Re: current return-path for differential pair on PCB

I have always believed the current flows BOTH on the continuous, 
relevant, closely spaced plane underneath the traces and also on the 
traces themselves.

Consider the transition time between states --- the rise and fall 
times are typically fast and the current couples to the planes.
During the (short) time the signals have stabilized at their 
respective levels, the coupling dies out and the current flows around 
the trace-pair.

This is why we follow the design rule that differential traces are 
routed close to continuous, relevant planes AND ALSO close to each other.

You will hear other opinions on this forum!!

Doug Brooks
UltraCAD Design, Inc
http://www.ultracad.com



At 03:40 AM 11/29/2008, Istvan Nagy wrote:
>hi
>for a differential pair, like PCIe or SATA, some people (some of my 
>colleagues) say, its not needed to have a continous good current 
>return path since the differential voltage is referenced only to the 
>other signal-trace in the pair.
>
>I think the current return path has to be continous even for 
>diffpairs, because:
>1.) Common mode noise:
>if the return path is not continous, then it creates loops for the 
>common mode noise, so the common mode noise will be bigger. If the 
>CM noise is bigger than it introduces jitter into the differential 
>signal. which is bad.
>2.) Impedance:
>if the return path is poor, the the return current has to flow 
>around to get a path between the source and destination, so on some 
>areas the return current doesnt flow unterneath the signal traces so 
>basically their single-ended (odd mode) impedances (so the diff 
>impedance too) will be different than its calculated in the 
>imp.calculator (polar, or MMTL), which assumes a perfect ground plane.
>
>am i right, or wrong? why?
>
>Istvan Nagy
>Concurrent Technologies plc, UK.
>
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