[SI-LIST] Re: Ground, the preferred reference plane

  • From: "Abhijit Mahajan" <amahajan@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 11 Dec 2002 10:34:05 -0800

I believe your explanation.  I think noise on power plane might be a 
factor for the preference.

Also, I believe it is usually harder in systems to provide the "correct" 
power reference plane (i.e., the supply from which the IOs draw current) 
for the entire length of the signal. GND however is universal and it is 
easier to make an entire plane ground.  Due to numerous power supplies 
in most designs, providing a solid power plane for the correct power 
supply might be quite difficult.  It is thus better to stick to a gound 
reference (and rely on bypass caps) unless you specifically design the 
power plane to be as nice a return path as ground.

Ideally a line that will be driven both high and low should have both a 
power and a ground return path.  For open drain signaling the return 
current will be only through ground.

Abhijit.



D G wrote:

> Anand,
> 
> As for why ground and not power, I can't answer that.  Possibly power planes 
> are noisier in general than ground planes.  I'm surprised the book doesn't 
> explain its preference.  
> 
> As for decoupling caps, anytime a signal transitions, current will flow 
> between power and ground.  Depending on the speed of the signal, current 
> could be supplied from bypass caps, power-plane capacitance, or all the way 
> back at the power source, but it will eventually flow between power and 
> ground.
> 
> As for current paths on signal traces and reference planes, it is true that a 
> current on a signal trace induces a current on the reference plane.  However, 
> this induced current will eventually have to travel through one of the paths 
> mentioned above.  Remember, currents travel in loops.
> 
> - Daniel
> 
> From: "Kuriakose, Anand" <Anand.Kuriakose@xxxxxxx>
> 
>>Hi All,
>>
>>In "High Speed Digital System Design" by Stephen Hall, it is mentioned  that
>>the ground-referenced signals have cleaner signal integrity when compared to
>>power-referenced signals. 
>>
>>Chipset design guides (not all) also recommend to have the high speed
>>signals like processor signals routed over ground plane rather than over
>>power plane. Also similar statements are made in a few other docs.
>>
>>I'd like to understand how does it improve the signal integrity of the
>>signal when routed over GND plane rather than over power plane. In
>>otherwords, what makes GND plane the preffered reference plane?
>>
>>One other point is that when signals are routed over power planes, the
>>return current can get back to where it started without passing through any
>>decoupling caps, making the return loop smaller (assuming that signals do
>>not cross splits in the plane and no return path discontinuity due to layer
>>changes). However, if the same signal is routed over GND plane, the return
>>current will have to pass through a decoupling cap to complete the loop.
>>Correct me if i am wrong in my above point. 
>>
>>Regards,
>>Anand.
>>
> 
> 
> --
> Daniel
> ZZZ-dgun-ZZZ-@xxxxxxxxxxxxxxx
> (remove the Z-'s to reply--they're what I do when I read spam)
> 
> 



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