Re: Differential pins in IBIS

DC, Stephen,

What if you created additional "virtual" Vref pins, and referenced each
differential input to a different pin? I don't think this would be a problem, as
long as they share the same POWER and GND busses.

Regards,
Christian Klein

"D. C. Sessions" wrote:

> Weston Beal wrote:
> >
> > Stephen,
> >
> > Although this receiver is implemented as a differential receiver, from the
> > outside it looks more like a single ended receiver because the system really
> > only feeds in one data signal. Vref is held constant at 1.25V. You can
> > simply use the parameters Vinl and Vinh to specify the thresholds Vref -
> > 350mV (0.9V) and Vref + 350mv (1.6V).
> >
> > Now, since Vref should follow Vcc, it changes for min and max cases. For
> > this we use Vinl and Vinh under the [Model_Spec] keyword. They work the
> > same but now have typ, min, and max values.
>
> Not good enough. Vref and Vtt are related to Vddq, but there are both
> tolerances and noise terms involved which don't track the simplistic
> typ/min/max axis. In particular, for the devices Mr. Nolan is discussing,
> the common-mode rejection of the receivers is an issue. Differential is
> a must.
>
> For instance, right now there is a JC-42.3 task group dealing with the
> impact of SSI noise on Vref. Crosstalk analysis where is can make or
> break standard technology.
>
> > The IBIS committee is still working on the AC thresholds vs. DC thresholds
> > problem.
>
> One of the reasons that Mr. Nolan now represents TI on the IBIS committee.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com