[SI-LIST] Re: SSO pushout, ground bounce definition

From: "Kai, Francis" <francis.kai@xxxxxxxxx>
To: "'mark.gailus@xxxxxxxxxxxx'" <mark.gailus@xxxxxxxxxxxx>,chen@xxxxxxxxxxx
Date: Fri, 19 Apr 2002 15:20:20 -0700
Dear Gurus,

     I do not agree with Mark on certain concepts listed below, however, any

comments are welcome.

(1) "Fluctuation" has a quite different meaning if you agree with the
concept
in Statistical Mechanics or Thermodynamics. The "Fluctuation theory"
phenomena
do differ from the "ground bounce" effect occurred in signal integrity.
Therefore, if we want
to be consistent with physics (or physical concepts), the term "Power/ground
fluctuation" will 
not be as nice as "ground bounce" in signal integrity.

(2) I do not see the term "voltages" is not "uniquely defined". All those
theorems,
Ohms Law, Tellegen theorem, Maxwell Equations, do represent that voltages
are well-defined uniquely in an Electrical System. These theorems satisfy
the ODE (ordinary differential equations) and PDE (partial differential
equations)
and are well-posed. Therefore, "uniqueness" is guaranteed. There is no
theorem
to prevent voltages to be measurable, like Heisenberg's Uncertainty
Principle in 
quantum mechanics. However, whether Mark or any other engineers/technicians
can 
accurately measure those voltages is a different story.

Best regards,

Francis Kai

-----Original Message-----
From: mark.gailus@xxxxxxxxxxxx [mailto:mark.gailus@xxxxxxxxxxxx]
Sent: Friday, April 19, 2002 2:06 PM
To: chen@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SSO pushout, ground bounce definition




Raymond,

Thanks for a very clear presentation.  I agree with you on all points,
especially the following:

(1) "Power/ground fluctuation" is a much clearer term than "ground boun=
ce",
which is always misleading and should not be used.

(2) In the real world and in EM theory, "voltages" (i.e., differences o=
f
electric potential) are frequently not uniquely defined or measurable,
particularly between "distant" points or where radiation can take place=
.

(3) With respect to multiconductor transmission lines: "Voltage drop al=
ong
the ground conductor (except DC) is not well defined based on EM theory=
."

I will add a couple of generic observations:

Much confusion and mischief results when inappropriate oversimplified
circuit-theory models are applied to electromagnetic problems.

There is wisdom in the microwave engineers' dictum that "there is no su=
ch
thing as ground".

Best regards,

Mark Gailus

Teradyne







"Raymond Y. Chen" <chen@xxxxxxxxxxx>@freelists.org on 04/15/2002 01:16:=
11
PM

Please respond to chen@xxxxxxxxxxx

Sent by:  si-list-bounce@xxxxxxxxxxxxx


To:   <khui@xxxxxxxx>
cc:   <si-list@xxxxxxxxxxxxx>

Subject:  [SI-LIST] Re: SSO pushout, ground bounce definition



Kevin,

You raised a good question, and I've been asked many times on this issu=
e,
so
please see my comments after your post:

> Raymond,
> Thanks for the link to the doc. I read
> "http://www.sigrity.com/papers/ECTC2001/ECTC_LI1.pdf";.
> In there, the authors show the effect of on-die decoupling on SSO
> power/gnd
> noise. The tables and conclusion say
> on-die decoupling cap can reduce power/ground noise. I can see how it=

> reduces the power line noise.
> However, I can not understand how the on-die decoupling cap which
> is placed
> between power and
> gound (on die, vss or substrait) can reduce the gound bounce. None of=
 the
> waveforms in this paper shows
> the ground line. Can you please comment on that? Thanks.
> Regards,
> Kevin Hui
> LSI Logic

First of all, voltage is defined between 2 nodes. So power bounce is no=
t
something happening on power rail alone, it is the voltage fluctuation
between power and ground.

So where is the ground bounce voltage? It must be defined between 2 nod=
es.
And usually people will show such circuit diagram to define ground boun=
ce
(I
borrowed the drawing from ADEEL AHMAD's 4/10 post on this topic):

               -------PAD
               |
               |
                NMOS
               |
               |<<internal GND
               INDUCTOR
               |
              GROUND PIN

Here the ground bounce is defined as the voltage drop crossed the induc=
tor,
which models the interconnect parasitics (ground via, trace, plane)

However, this definition derived from circuit-theory point of view ofte=
n is
not valid and causes misleading concepts, especially in the high-speed
design arena where Electromagnetic (EM) phenomena is the basics for all=

circuit or SI issues. Because:

1.   measuring voltage across a big distance (compare to wavelength) is=
 not
well defined. For example, defining the voltage between die-pad and the=

package-pin; or defining the voltage between a point in the middle of a=
 PCB
and a point at the edge of the PCB, are not good if you are working in =
the
hundreds of MHz range and beyond.
2.   measuring AC voltage drop along the conductor itself is not define=
d.
For
example, we often measure the voltage at the ends of a transmission lin=
e
(the 2 ports); we don't measure the voltage drop across the individual
transmission line conductor. Voltage drop along the ground conductor
(except
DC) is not well defined based on EM theory. Think about this, in the ab=
ove
drawing, the inductor can not associate with just the conductor, the
inductor has to associate with a loop, where is the loop?

After all, the terminology of ground bounce itself can be misleading,
because lot of people think that ground bounce can be viewed ON the gro=
und
conductor, whereas actually ground bounce happens BETWEEN power and gro=
und.
The only time you may and you can well define the voltage between 2 gro=
und
points is if these 2 ground points are very close (local port). For
example,
between 2 ground C4 bumps. At that time, most flux in this loop is well=

captured between these 2 points.

Therefore it may be better to use the term Power/ground fluctuation ins=
tead
of ground bounce.

Lastly, please take a look at the commonly used one-dimension power
delivery
model (illustrative):

          L     R         L     R         L     R         1
     |---ooo---^^^---|---ooo---^^^---|---ooo---^^^---|----|
     |               |               |               |    |
VRM ___              |               |               |    |
     -               |               |               |    |
     |        decap =3D=3D=3D       decap =3D=3D=3D        Cdie =3D=3D=3D=
  |> buffer
     |               |               |               |    |
     |     PCB       |    Pkg        |    chip       |    |
     |               |               |               |    |
     |---ooo---^^^---|---ooo---^^^---|---ooo---^^^---|----|
     2                                                    0

To a driver, what is important is the local supply voltage between node=
 1
and 0 (power and ground). And only V(1,0) can be well defined in the
correct
EM sense. The so-called "ground bounce" between node 2 and 0 will be il=
l
defined and meaningless to the driver.

Regards,

Raymond Y. Chen
Sigrity, Inc.




=


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[SI-LIST] Re: SSO pushout, ground bounce definition

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