RE: ibis VT curves

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.

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
>

 
Received on Fri Nov 17 04:45:21 2000