[SI-LIST] Re: Seperate Sparameter for gnd & supply plane !!
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Rajesh K <rajeshk0212@xxxxxxxxx>
- Date: Thu, 02 Jun 2011 03:44:50 -0700
The problem as: Brad, Larry, and I have all tried to point-out is what
one means by noise voltage on the gnd rail. What is the noise in
relation to? Between different points in the interconnect there will
occur potential differences. What you need to get straight is when and
where those potentials matter. They usually matter when someone has
done something very unfortunate in the way that they handle their signal
returns and/or references.
Take your example of looking at something in the lab. Assume you can
probe without any radiated noise pick-up. If you probe differentially
across some chunk of Vss interconnect, you will indeed see the effects
of DC and AC current drop across that interconnect. Does that matter?
Well it would if both points you are measuring constitute the power
connections for some circuit. That's what happens when you do a poor
job of layout and insert all kinds of series interconnect impedance
between Vss and Vdd. The noise that you see will be across the Vss and
Vdd port. It is a single difference potential. As far as the load is
concerned, if Vss is its reference, then the noise potential all belongs
to Vdd. If you introduce a third signal that comes from somewhere else,
and you don't reference that signal to Vss, then you have swapped
references, and as Larry explained, by doing so inserted a corresponding
noise source.
You will do yourself well to extricate the word ground from your
vocabulary. SPICE node 0 is literally where you define it. If you
take up the practice of using differential voltage probes in SPICE life
might get a lot less confusing.
Steve.
On 6/2/2011 3:15 AM, Rajesh K wrote:
> lets say, one is interested in simulating some x interface and want to
> find noise voltage on power and gnd rail separately.
> s-parameter model (of power/gnd loop) will give only relative noise
> difference between power/gnd. gnd will always be assumed to be quiet.
> all noise will appear on power rail. But in actual system, when you
> take it in lab. one can measure noise voltage independently on power
> and gnd rail. noise on gnd rail is called ground bounce and noise on
> power rail is called power noise.
> How to mimic such scenario in simulation. There are possibilities
> that gnd noise can be higher than power noise. i need to know which is
> worse, power plane routing or gnd plane routing? how to find it..
>
>
--
Steve Weir
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