RE: [IBIS-Users] Vmeas For Open Drain IBIS model

From: Tom Dagostino <tom_at_.....>
Date: Fri Mar 18 2005 - 08:34:50 PST
Let's think about this output buffer and its specified test circuit as it
presently is in your model.  There is a pulldown FET connected to ground.
There is also a 1M resistor tied to ground.  Now, when the pulldown FET is
on the junction of the resistor and FET is pulled down to ground, the same
potential that the other end of the resistor is tied to.  When the pulldown
device is off there is no current flowing in the resistor so the junction of
the FET and resistor is floating but the other end is tied to ground.  Thus
the junction of the FET and resistor is at 0 Volts.

So, when you have a open sink or open drain buffer the Rref must be
terminated to a voltage other than ground for any current to flow in it.
Usually Vref in this kind of buffer is Vdd but obviously Vref needs to be
larger than Vmeas.

Tom Dagostino
Teraspeed Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
http://www.teraspeed.com
tom@teraspeed.com

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

Teraspeed is the registered service mark of
Teraspeed Consulting Group LLC

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
Behalf Of lee yang
Sent: Friday, March 18, 2005 3:19 AM
To: ibis-users@eda.org
Subject: [IBIS-Users] Vmeas For Open Drain IBIS model


Hi,

Do we need to specify the Vmeas, Rref, Cref and Vref values in an open drain
model?

Why do i get the flowing warnings?

WARNING - Model 'test': TYP VI curves cannot drive through Vmeas=1.25V
          given load Rref=1e+006 Ohms to Vref=0V
WARNING - Model 'test': MIN VI curves cannot drive through Vmeas=1.25V
          given load Rref=1e+006 Ohms to Vref=0V
WARNING - Model 'test': MAX VI curves cannot drive through Vmeas=1.25V
          given load Rref=1e+006 Ohms to Vref=0V

The test conditions:
Vmeas=1.25V
Rref =1M
Cref =0pF
Vref =0V

Could someone help?
Thanks.
-Lee

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Received on Fri Mar 18 08:37:29 2005

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