RE: [IBIS-Users] IBIS DIFF input buffers

Tim

There problem with including the ODT in a power or ground clamp if that
differential termination is meant to float. Once you put that resistive
characteristic in one or both of the clamp table the resistor is now tied to
a voltage - it won't float anymore. Many if not all LVDS IBIS drivers
behave totally different when driving a floating differential termination
vs. a termination to a fixed voltage. A clamp based termination works well
for CML receivers that typically have the 50 Ohm termination tied to Vdd but
many technologies such as LVDS use floating terminations.

Regards,

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@teraspeed.com
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Timothy Coyle
Sent: Friday, May 28, 2010 9:45 AM
To: 'Eric Monteiro'; ibis-users@eda.org
Subject: RE: [IBIS-Users] IBIS DIFF input buffers

Hi Eric,
If I understand you correctly you are using single-ended buffers to model
the inverting and non-inverting pins of a differential input buffer and you
are using the Series R keyword in IBIS to represent the on-die termination.

If so, then in general, your approach will be valid. Basically you will want
to generate the ground and power clamp IV curve on one differential input
pin. Depending on the differential buffer design and technology
(LVDS,CML,etc) you can leave the other differential input pin floating or
you may have to put some type of load on it as a feedback loop. Other
assumptions are that there is not differential current flowing as in a true
differential buffer and that you can essentially treat your differential
design as pseudo differential. If your differential design is unbalanced
then you should create separate buffer models for each differential input
pin.

Unfortunately there are still a lot of EDA tool issues with using Series pin
mapping and keywords so be aware of your end customers target environment.
You could also always try and include the on-die termination current in the
clamp IV curves to avoid using the Series keywords.

Hope this helps.

Best,

Tim

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Eric Monteiro
Sent: Thursday, May 27, 2010 4:45 PM
To: ibis-users@eda.org
Subject: [IBIS-Users] IBIS DIFF input buffers

Hello Everyone

I need a little validation/clarification.

I have a differential input IBIS model, however my IBIS simulator is so out
of date that it does not recognize the Series Keywords. So instead what I
did was remove the series maping, and placed 2 single ended buffers with a
resitor whos value equalled that in the Series R definition: i.e.

------------|BUFFER|
        |
        Z
        Z
        |
------------|BUFFER|

My understanding is that the Diff buffer in IBIS DC tables should be the
COMMON mode current into ONE of the diffential pins (i.e. I assume its not
the sum of the current into the 2 pins, or 2 times the current I think it
is). Based on that, the above should be valid. Can anyone confirm?

Cheers,
Eric

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