Re: ibis VT curves

Todd Westerhoff wrote:
>
> Simon,
>
> The answer to your original question, as I understand it:
>
> SPECCTRAQuest (a.k.a SQ) doesn't use the dV/dT data when rise/fall curves
> are present. As Betty said, the V-I curves and V-T data are used instead.
> SQ only uses the dV/dT data when the rise/fall curves are NOT present (i.e.
> the so-called "linear" ramp model).
>
> Some experimentation yesterday revealed that when V-I curves are present
> (but NOT V-T), SQ uses the dT information, but NOT the dV data. That makes
> sense (the high and low operating points are determined by the output's V-I
> characteristics and the power supplies). The dT information is used to
> define the length of time it takes the output to change state.
>
> There are some interesting implications to this, worthy of note:
>
> If you have an output with a 2V swing that takes 1 nS to switch, you have a
> ramp rate of 2V/nS. Theoretically, you could specify that as 2V/1nS, or
> 1V/0.5nS, or 10V/5nS - all of which would give you the same ramp rate.
> BUT - only the 2V/1nS value would give you the switch time you expect,
> because SQ is only using the dT value.

As far as I know, the ibis spec says, that the ramp rate has to be from
20% to 80%.
In the case of an output with 2V swing within 2 nS, this has to be:

dV/dt = 1.2V / 0.6 nS

Unfortuately, I often get ibismodels where this is not the case. This
can be
checked by looking at the crossing of the VI-curves with the
"50Ohm-load-line"
(if the load is 50Ohm).

Perhaps others have similar experiences.

Katja

>
> Morals of the story:
>
> 1) Ramp data is not used by SQ when V-T data is present.
> 2) When using Ramp Data, tell SQ the actual dV and dT values for the whole
> output transition.
>
> That's the story as I understand it. If anyone has seen different, please
> let me know!
>
> Todd.
>
> > -----Original Message-----
> > From: Simon Schaeffer [mailto:simon.schaeffer@cern.ch]
> > Sent: Thursday, November 16, 2000 8:13 AM
> > To: ibis
> > Subject: ibis VT curves
> >
> >
> > Hi,
> >
> > I'm working on ibis models and I met the same problem as
> > Betty Luk, it's
> > to say that in playing around with my IBIS model (changing
> > the ramp data
> > manually), I didn't see any changes with Sigxplorer (Specctraquest
> > signal explorer expert). However, in making the same changes
> > in the ibis
> > model used in pspice I saw differences.
> > I have done this with an ibis model with and without the VT curves.
> > I have used "ibis2spice" from Intusoft but this program
> > doesn't convert the VT curves, it just uses the [Ramp] parameters.
> > So, I am trying to build a new ibis2spice program using the VT curves.
> >
> > My questions are:
> >
> > 1) Why doesn't the [Ramp] modifications influence the results (Betty
> > Luk's question)
> >
> > 2) Why use the VT curves instead of the [Ramp] parameters?
> > Is it "only"
> > to get a more accurate curve?
> >
> > 3) I am puzzled by the apparent contradiction that might arise if both
> > the
> > VT and VI curves are available. For example, a VT curve has been
> > specified with the following load:
> >
> >
> > out -----/\/\/\/\/-----|
> > 50 |
> > + ___|___ 1.25V
> > ---
> > |
> > |
> > |
> > GND
> >
> > Going from low to high, the VT curve switches from 400mV to 2V.
> >
> > How can I use this information for other loads?
> > For example:
> >
> > - 50 Ohm load only
> > (i.e. the dc offset has been removed. My simulated output
> > will now want
> > to be something defined by the VI curve. How do I sensibly use the VT
> > curve in this case?)
> >
> > - 100 Ohm load only
> >
> > 4) Do I have to take into account the Rfixture and Vfixture
> > values used
> > while specifying the VT curves?
> >
> >
> >
> > Thanks for your feedback
> > Simon Schaeffer
> >

-- 
Katja Koller ICN M TC 6 
Siemens AG Hofmannstr. 51 81359 Muenchen
Katja.Koller@icn.siemens.de
Tel: 0049 89 722 45719 Fax: 0049 89 722 44692