Re: gEDA-user: 3 questions on a 4 layer board

[Rick Collins <gnuarm.2007@xxxxxxxxx>]

At 04:33 PM 4/29/2008, you wrote:
>Dan McMahill wrote:
> > If my traces were carrying ground referenced analog signals I wouldn't
> > want the power plane being my "ground" plane.
> >
> > And yes, I've seen a real life case with some rather frustrating
> > behavior that the correct stackup totally fixed.  There was a moment of
> > clarity when a bypass cap way on the other end of the board made a much
> > larger than expected difference.
>Would you mind elaborating on this?  I could imagine a scenario where
>the power plane could be a source of AC noise - especially if components
>aren't bypassed adequately.  The AC noise could then couple to the
>signal traces in its quest to complete a low impedance circuit.  Is this
>what you observed, or am I way off?
>
>-Ethan


I don't want to come off as argumentative, but if you both think 
about what has been said here, you can see the fallacy.  If the 
frequency of the noise is such that it can easily couple into a 
signal trace, then that same noise would be well decoupled between 
the power and ground planes and so would not be present to any 
significant degree.  The decoupling would not be through caps as it 
would be from the very high frequency capacitance of the planes themselves.

Ground or power noise in circuits is nearly always due to 
ground/power bounce and only affects the signals in or out of the 
chip that has the "bounce" problem.  Even if a board has inadequate 
decoupling of the power plane, that noise is not going to 
significantly couple into signals.  Of course, because of the 
ground/power bounce on the chip making the noise, its signals will 
look like they are coupled to the noise because they are... but not 
directly from the power plane to the signal traces, through the power 
pins on the chip.

Rick

Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design      URL http://www.arius.com
4 King Ave                               301-682-7772 Voice
Frederick, MD 21701-3110                 301-682-7666 FAX



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