Re: [IBIS-Users] {Spam?} {Filename?} LVDS IBIS model - simulation issue

Re-sending with updated attachment

Hi,

I am making IBIS models for LVDS (low voltage differential signal) IO 
buffer.
To model an LVDS buffer I created the following test circuit, assigned 
the generated IBIS model to both buffers, set one buffer as inverted output.


*Problem:*
In the image below the YELLOW waveform is what I saw and the PURPLE 
waveform is what I expected to see.

YELLOW = IBIS model
PURPLE = Spice ( reference ) model

Here is combined pullUp (yellow) and pullDown (green) IV curves 
(actually it is spice curves from which IV tables were derived):


Lets assume that simulator tries to find operational point for LVDS 
buffer driving high. Then for the non-inverted buffer a point (Vp, Ip) 
will be taken from pullUp table, for the inverted buffer a point (Vn, 
In) from pullDown table, the following conditions should be satisfied: 
Ip = In, Vp - Vn = Ip*50. On the image above it is seen that there are 2 
points satisfying the condition: Vp =~  1.45V and Vp =~ 2.2V. Seems that 
something like that happened during simulation and simulator got to the 
second point. (BTW, there are 60 points for the region 0.9 - 1.6V and 10 
points for the region 1.6-2.5V in the IBIS model IV tables).

*Question:*
What can be done to resolve this situation? Should IV tables or another 
IBIS structures be modified and how? I am making *ibis ver. 4.2* models.

Thanks,
Andrey



On 09/18/2013 06:33 PM, Andrey Babintsev wrote:
>
> *Warning: This message has had one or more attachments removed 
> (fail.min.png). Please read the "EDA-Attachment-Warning.txt" 
> attachment(s) for more information.*
>
> Hi,
>
> I am making IBIS models for LVDS (low voltage differential signal) IO 
> buffer.
> To model an LVDS buffer I created the following test circuit, assigned 
> the generated IBIS model to both buffers, set one buffer as inverted 
> output.
>
>
> *Problem:*
> In the image below the YELLOW waveform is what I saw and the PURPLE 
> waveform is what I expected to see.
>
> YELLOW = IBIS model
> PURPLE = Spice ( reference ) model
>
> Here is combined pullUp (yellow) and pullDown (green) IV curves 
> (actually it is spice curves from which IV tables were derived):
>
>
> Lets assume that simulator tries to find operational point for LVDS 
> buffer driving high. Then for the non-inverted buffer a point (Vp, Ip) 
> will be taken from pullUp table, for the inverted buffer a point (Vn, 
> In) from pullDown table, the following conditions should be satisfied: 
> Ip = In, Vp - Vn = Ip*50. On the image above it is seen that there are 
> 2 points satisfying the condition: Vp =~  1.45V and Vp =~ 2.2V. Seems 
> that something like that happened during simulation and simulator got 
> to the second point. (BTW, there are 60 points for the region 0.9 - 
> 1.6V and 10 points for the region 1.6-2.5V in the IBIS model IV tables).
>
> *Question:*
> What can be done to resolve this situation? Should IV tables or 
> another IBIS structures be modified and how? I am making *ibis ver. 
> 4.2* models.
>
> Thanks,
> Andrey

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