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

From: "D G" <dgun@xxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Date: Thu, 12 Dec 2002 01:29:43 +0800

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

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