[IBIS-Users] Re: Question about differential pair buffer

Ramiro:

Your diagram appears to be in a standard PECL configuration.
IBIS covers that by special rules for high state and low
state I-V sweeps and V-T extractions to Vcc-2 V.  The model
could be docuemnted by Model_type Output_ECL.  Then it
could be configured as a differential buffer by connecting
the two single-ended models with the [Diff Pin] keyword.

Bob

Muranyi, Arpad wrote:
> Ramiro,
> 
> 1)  I think I need to clarify something to you first.
> IBIS has many different buffer types.  It can have push/pull
> types as well as open_XXX types.  So it is perfectly legal
> to have models without pullup, or pulldown IV tables.  However,
> in order to get a signal, something will have to pull up or
> down in the rest of the circuit, just as in real life, but
> this doesn't have to be in the buffer itself.  It could be
> a termination resistor or similar device somewhere in the
> circuit.
> 
> 2)  Looking at your drawings, the first one on the top, it
> seems that the output transistor is in an emitter follower
> configuration, hanging off the top supply rail.  This looks
> like a pullup to me.  The two series transistors on the bottom,
> with "bias" look like a pulldown.  So you do have both pullup
> and pulldown in this buffer.
> 
> 3)  In terms of IBIS, however, what you need to consider is
> what these are doing.  If you have a transistor that is always
> on, acting as a terminator, or something similar, you would
> need to put its IV data in the clamp tables.  In IBIS terminology
> anything that is not switched goes into the clamp tables.  Everything
> else that is turned on or off as the buffer changes states will
> go into the pullup and/or pulldown tables.  So in your case I would
> put those BJT-s on the top into the [Pullup] IV tables, and the
> MOSFET-s on the bottom into the [GND Clamp] tables.
> 
> 4)   The IBIS parser will not give you an error if you don't have
> pullup or pulldown tables just because they are missing.  These
> are optional keywords in IBIS.  However, you will get errors which
> are related.  The Vt curves and IV curves work together, and there
> are checks for which they both need to be there in certain configurations.
> If you do not satisfy some of these requirements, you will get errors.
> 
> 5)  Regarding figure 2, I do not understand your labeling.  What is
> Vos  (or is it Vds?) stand for?  Is it the supply voltage (Vcc or Vdd)?
> However, I have to caution you that you have an extra issue here
> because your output transistors are BJT-s.  They tend to influence
> each other much more than MOSFET-s, so what you do on one output
> will have a relatively strong effect on the other output.  What this
> means is that in reality, IBIS should have an I-V surface instead
> of an IV curve for these [Pullup] tables, output current as a function
> of the voltage of both outputs.  Conventional IBIS cannot do this,
> and you may need to find your own compromises if you want to make
> models with the normal IBIS keywords, or you will have to find ways
> to model this device with the *-AMS extensions of IBIS 4.1.
> 
> 6)  Regarding figure 3, I wonder about that short across the source...
> and also about the labeling I asked you already.
> 
> 7)  I would also recommend that you look at my presentation on how
> to characterize differential buffers at:
> 
> http://www.eda.org/pub/ibis/summits/oct03/muranyi.pdf
> 
> I hope this will help you in your modeling.
> 
> Arpad
> ========================================================================
> 
> -----Original Message-----
> From: RODRIGUEZ_LECONA_RAMIRO [mailto:rlecona@siu.buap.mx] 
> Sent: Monday, June 13, 2005 6:43 AM
> To: Muranyi, Arpad
> Cc: ibis-info@eda.org; ibis-users@eda.org
> Subject: Re: Question about differential pair buffer
> 
> Hi Arpad, 
> 
>       Attached I send you the figures that show the buffer schematic and 
> procedures I am using to extract the IBIS curves for this differential 
> buffer and I have a few questions: 
> 
>       If I don't specify the pullup curve, how does  the IBIS simulator know 
> how to drive a high state?
>       If I don't specify the pullup curve, Will the IBIS parser give error 
> messages? because it can't match the pullup curve with the rising waveform.
>       Are these the suitable procedures to extract the IBIS curves? 
> 
>       Thank you. 
> 
>       Regards, 
> 
>      Ramiro
> 

-- 
Bob Ross
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