RE: [IBIS-Users] RE: an equivalent load applied to the model's I-V tables yields different voltages.

From: Tom Dagostino <tom_at_.....>
Date: Wed Mar 07 2007 - 17:54:13 PST
It looks like you have multiple problems with this buffer's model.   This
error is very common and means that the IV and VT tables do not agree with
each other.
 
First the IV tables are predicting a swing of 20mV (1.19V - 1.17V) or 40 mV
(1.37V - 1.33V).  I don't think these swings will work in any application.
So I question your IV tables.
 
Second, I'm not sure what you mean by pseudo LVDS but your termination
voltage should be Vcommon_mode, not ground.  The weak 4 mA or so LVDS pullup
driver will never pull a 50 Ohm load to ground to over a Volt.  LVDS is
usually differentially terminated with something like 100 Ohms which is
equivalent to a 50 Ohm resistor to the average output voltage from each
output.
 
Are you terminating both sides of the LVDS driver when you are doing your
extractions, For the VT waveforms make sure you have a 50 Ohm load from both
outputs tied to your termination voltage.  In many cases in IV extraction we
load the non-measured side with 50 Ohms so the buffer is working more like
its typical application.  We have seen funny IV tables when this is not
done.
 
 

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
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@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Marek Gebura
Sent: Wednesday, March 07, 2007 5:09 PM
To: ibis-users@server.eda.org
Subject: [IBIS-Users] RE: an equivalent load applied to the model's I-V
tables yields different voltages.



Hi,

 

I am trying to make a model for a pseudo LVDS pad, but I can't extract the
proper IV tables from it.

To extract the rise/fall waveforms I have to run the simulation at high
frequency for a while (250ns) - before it gets to the proper voltage levels.

But it looks like the IV curves yield different voltages. 

These errors are coming from my Hyperlynx session:

 

 

ERROR - Model LVDSTX: The [Rising Waveform] 

      with [R_fixture]=50 Ohms and [V_fixture]=0V

      has TYP column DC endpoints of  0.01V and  0.38v, but

      an equivalent load applied to the model's I-V tables yields

      different voltages ( 1.17V and  1.19V),

      a difference of 6615.72% and 4640.59%, respectively.

ERROR - Model LVDSTX: The [Rising Waveform] 

      with [R_fixture]=50 Ohms and [V_fixture]=1.29V

      has TYP column DC endpoints of  0.78V and  1.64v, but

      an equivalent load applied to the model's I-V tables yields

      different voltages ( 1.37V and  1.33V),

      a difference of 1712.86% and 916.29%, respectively.

 

 

Has anyone seen such an Error & has an Idea on how should I extract the IV
tables correctly?

I made some simple IBIS models before - but I can't make one for this pad. I
am not an expert in IBIS & any help is appreciated.

 

Thanks 

Marek



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Received on Wed Mar 7 17:59:45 2007

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