RE: [IBIS-Users] load line error

When this happens it means the IV curves do not match the VT curves.  The IV
curves imply the output impedance of the driver in either the high or low
state for an output.  Think of a circuit that has a voltage source with zero
output impedance.  In series with that source is an impedance described by
the IV curve.  If you place a load on the driver when the output is high and
the load is tied to ground there will be a current that flows.  But not all
of the voltage of the source will be dropped across the resistor to ground,
some of it will be dropped in the output impedance of the voltage source.
There is a voltage divider formed by the output impedance and the load.  And
I'm sure you remember EE101 and can solve for the output voltage.  

 

In your case the first and last voltage points on your VT curve sound
reasonable.  But the parser predicted an output voltage of -0.59 and 0.63V
when a 50 Ohm load is attached to the output and the other end is tied to a
2.5V termination.  

 

What you have to do is figure out why the IV curves are not correct for this
buffer.  Step number one is to plot the pullup, pulldown, powerclamp and
groundclamp curves.  They should look like the examples you see in the IBIS
standard or the IBIS cookbook.  If not you need to understand why and
regenerate the 4 tables.  Remember when the parser computes the output
voltage in the low state it sums the pulldown, powerclamp and groundclamp
together.  Likewise for the high state it sums the pullup, groundclamp and
powerclamp together.

 

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 

 

From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Babid A
Sent: Wednesday, July 09, 2008 11:13 AM
To: ibis-users@server.eda.org
Subject: [IBIS-Users] load line error

 


Hi All,

 

I am getting following error when I run IBIS parser,

 

ERROR - Model drv_out: The [Falling Waveform] 
      with [R_fixture]=50 Ohms and [V_fixture]=2.5V
      has TYP column DC endpoints of  0.50V and  2.50v, but
      an equivalent load applied to the model's I-V tables yields
      different voltages (-0.59V and  0.63V),
      a difference of 89.46% and 153.09%, respectively.

 

I appreciate suggestions or documents to find the solution for this problem.

 

Thanks,

-Babid.



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