RE: [IBIS-Users] full differential buffer IBIS modeling ---clarification

From: Tom Dagostino <tom@teraspeed.com>
Date: Wed Jun 11 2014 - 09:35:44 PDT
Let’s take an LVDS buffer as an example.  Most LVDS buffers are designed to
work into a 100 Ohm differential load and the Vcm of most LVDS buffers is
1.2V but there are exceptions to this value.  So the equivalent load seen by
each side of the differential driver is 50 Ohms to 1.2V.  When setting up
the loads on the buffer typically the R_fixture is 50 Ohms and the V_fixture
is 1.2V.  Rref, Vref and Vmeas would be 50 Ohms, 1.2V and 1.2V respectfully.


 

With a LVDS buffer you cannot load it with a 50 Ohm resistor to ground, you
have biased the driver way outside its normal operating range.  You will not
get the correct characteristics.    ECL will have entirely different loads,
the 50 Ohm resistor is connected to Vcc – 2V so the Vref and V_fixture will
be different for each corner.  CML works with 50 Ohm resistors to Vdd, etc.
These are all differential drivers but the loads are different. You need to
look at the datasheet for the part to figure out the correct loading
conditions.  Very few, but there are some, differential drivers will
function properly with a 50 Ohm load to ground.  

 

Regards,

 

Tom Dagostino

 

Teraspeed Labs

9999 SW Wilshire St.

Suite 102

Portland, OR 97225

USA

 

971-279-5325  Office

971-279-5326   FAX

503-430-1065  Cell

 

tom@teraspeed.com 

www.teraspeed.com <http://www.teraspeed.com/>  

 

Teraspeed Consulting Group LLC

16 Stormy Brook Road 

Falmouth, ME 04105

401-284-1827

 

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Saravanan Ganesh
Sent: Tuesday, June 10, 2014 11:39 PM
To: Lynne Green; ibis-users@eda.org
Subject: RE: [IBIS-Users] full differential buffer IBIS modeling
---clarification

 

Hi

Using ½ of R_diff to ground and simulating the V-T char dos not settle the
outputs at required levels.

actually according to the termination as shown  in attachment the rl =50ohm
connected from each differential output  to common cap and cap is connected
to Vcm_rx w .r.t  gnd.

How to divide this in to half? And get ibis model correct?

What values  to be used in the Rfixture , vfixture,  Rref ,Vref, Vmeas ,etc
in .s2i file 

Regards

Saravanan.G

 

 

From: Lynne Green [mailto:lgreen22@mindspring.com] 
Sent: Friday, May 09, 2014 10:39 PM
To: Saravanan Ganesh; ibis-users@eda.org
Subject: Re: [IBIS-Users] full differential buffer IBIS modeling
---clarification

 

Dear Saravanan,

A differential buffer in IBIS 3.2 requires two buffer models, one for each
output.

To use s2ibis3:
-Use the SAME load to ground for EACH output.  
- Do not use a differential resistor between the two outputs.  Use 1/2 of
R_diff to ground.
- Run s2ibis3 once, using the inverting output pin, and create BUFFER1.  Run
it again for the non-inverting output pin, and create BUFFER2.  Note that no
other changes are made between the two runs.
- In the models, be sure to label the output type to indicate BUFFER1 is
inverting and BUFFER2 is non-inverting.  

For simulation, assign BUFFER1 to the inverting pin and BUFFER2 to the
non-inverting pin.  Under [Diff Pin], list the buffers with ZERO time offset
(the time offset is captured in the V-t tables).  At least one simulation,
with R=Rload, should be used to test the model before releasing it for use,
another with a different R is recommended..

Hope this helps.

PS: if you plan do this often, you can modify the s2ibis3 source to capture
both outputs in just one run of s3ibis3.  Not sure why no-one has shared
their modification with the IBIS Open Forum.

Cheers,
Lynne Green
retired


On 5/9/2014 8:20 AM, Lance Wang wrote:

Saravanan,

The first, s2ibis3 can be used for simple single-end buffer only. It might
not be satisfied for differential buffers.

 

The second, R_load in s2i is the same as R_fixture in the IBIS file. 

 

The third, full/true differential pair buffer IBIS extraction will not be
handled by s2ibis3 correctly. Another word, you will never be able to get
correct IBIS models thru s2ibis3 for your full differential buffers.

 

Hope this helps.

 

Lance Wang

IO Methodology Inc.

978-266-8981

 

SimDE MODEL, the best IBIS modeling tool in the market.

http://www.iometh.com

 

 

 

 

 

From: Saravanan Ganesh <Saravanan.Ganesh@onsemi.com>
Date: Friday, May 9, 2014 at 9:55 AM
To: "ibis-users@eda.org" <ibis-users@eda.org>
Subject: [IBIS-Users] full differential buffer IBIS modeling
---clarification

 

Hi 

I want to  brief on the R_load  I am using

 

I am creating  IBIS model (Version 3.2) of a full differential buffer .

I have also  connected the termination shown in “diffbuff_termination.png”
in differential buffer schematic. 

 

Even though I have connected the required termination in schematic itself ,
the .s2i file need R_load value to be supplied.

Therefore I have used a High value resistor (R_load=100G ohm) in .s2i file.

 

Hyperlinx “Check file for IBIS spec conformance”  gives following  error …

 

ERROR: Model OUTPUT_DIF: The [Rising Waveform] 

      with [R_fixture]=1e+011 Ohms and [V_fixture_max]=0.22V

      has MAX column DC endpoints of  0.08V and  0.31v, but

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

      different voltages ( 0.08V and  0.35V),

      a difference of  0.01% and 16.09%, respectively. 

 

What could be wrong? What can I do to get rid of these errors and warnings?

 

Please mail me if any more clarification required ,Waiting for your replies.


Thanks and regards

Saravanan.G

 

 

From: Saravanan Ganesh 
Sent: Thursday, May 08, 2014 8:04 PM
To: 'ibis-users@eda.org <mailto:%27ibis-users@eda.org> '
Subject: full differential buffer IBIS modeling

 

Hi 

I am creating  IBIS model (Version 3.2) of a full differential buffer .

I have also  connected the termination shown in “diffbuff_termination.png”
in differential buffer schematic. Therefore  I took R_load = 100G ohm in
.s2i file, so that only load in schematic are effective.

Hyperlinx “Check file for IBIS spec conformance”  gives following  error …

 

ERROR: Model OUTPUT_DIF: The [Rising Waveform] 

      with [R_fixture]=1e+011 Ohms and [V_fixture_max]=0.22V

      has MAX column DC endpoints of  0.08V and  0.31v, but

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

      different voltages ( 0.08V and  0.35V),

      a difference of  0.01% and 16.09%, respectively. 

 

What could be wrong? What can I do to get rid of these errors and warnings?

 

Thanks and regards

Saravanan.G


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Received on Wed Jun 11 09:38:08 2014

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