[SI-LIST] Re: ac drive strength

Jim, All,

The initial question was very general in nature.  The frequency range could 
be from tens of MHz to tens of GHz; the risetimes varying accordingly, the 
physical size (thus roundtrip delay) unspecified, as is the signaling method.

There are many different ways the ac strength of a buffer can be 
specified.  Various suggestions were presented on the reflector, some are 
based on different assumptions for the loading.  Your application should 
determine the choice of ac drive strength specification as well as modeling 
and simulation method.  So you have to understand how your system works.

The problem of choosing among the suggestions remains.  This choice should 
be based on the physical characteristics of the device, what it does in a 
real system.  If nothing is known beforehand, then experiments with 
different loading conditions similar to the intended use on a real physical 
device yield insight to the suitability of a particular modeling method and 
ac drive specification.  Comparisons with similar devices and applications 
can assist the selection.  The model is based on how the device behaves, 
and the test conditions need to identify (quantify) the model 
parameters.  Choose test loads that mimic your real system and how it operates.

Every model we use is an approximation to the real world.  It is important 
to understand the limitations of these models and methods.  A salesman 
tries to convince you his product can do most everything.  A scientist 
tries to figure out how well a method is suited to a problem.


Chuck Hill, consultant



At 12:56 PM 7/26/01, Muranyi, Arpad wrote:

>Chuck,
>
>I disagree, testing the characteristics of a buffer is
>best done with a resistive load (or a long transmission
>line).  Measuring the voltage swing tells you how
>strong the buffer is, and measuring the time it took
>to do that swing is telling you how fast the buffer
>is.  The 20-80% of these are the dV and dt numbers
>IBIS uses also.
>
>If you load the driver with a capacitor you mix the
>strength and speed into one number, and you will not
>be able to distinguish between strength and speed.
>The same goes for dividing dV/dt into a slope.
>
>Arpad Muranyi
>Intel Corporation
>========================================================
>
>-----Original Message-----
>From: Chuck Hill [mailto:chuckh@xxxxxxxxxxx]
>Sent: Monday, July 23, 2001 10:49 AM
>To: si-list@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: ac drive strength
>
>
>
>You could try measuring risetime into a capacitive load.  This is
>essentially dv/dt into C and measures the current drive of the
>buffer.  Measure this at several different capacitive loads.  Plot the
>points and see if the current source is a good model for your buffer.  If
>the risetime doesn't change much over a range of capacitive loads, then
>your buffer is better modeled by a voltage source.
>
>Watch out for stray inductance in your test setup.
>
>
>Chuck Hill, consultant
>
>
>
>At 11:30 AM 7/23/01, Peterson, James F (FL51) wrote:
>
> >Hello,
> >
> >What's the best way to specify the ac drive strength of a buffer? The
> >requirement should be testable.
> >
> >I was thinking about defining it with a source impedance at typical
> >voltage/temp's....any suggestions on how to test this?
> >
> >thanks,
> >Jim
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