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

From: Tom Dagostino <tom@teraspeed.com>
Date: Wed Sep 18 2013 - 14:18:34 PDT
Andrey

I've tried this kind of extraction also. The curves looked much more CMOS
current source like than you are showing for the pulldown.  It still looks
like your feedback loop is in control of the driver and it is not happy with
Vcm.

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 

Teraspeed Consulting Group LLC
16 Stormy Brook Road 
Falmouth, ME 04105
401-284-1827


-----Original Message-----
From: Babintsev Andrey-B13285 [mailto:B13285@freescale.com] 
Sent: Wednesday, September 18, 2013 12:57 PM
To: tom@teraspeed.com; 'Muranyi, Arpad'; ibis-users@eda.org
Subject: RE: [IBIS-Users] {Spam?} {Filename?} LVDS IBIS model - simulation
issue

Arpad, Tom, 

Thank you for your responses. 
I used 50 Ohm because it is MLB interface. I called it LVDS to make it clear
that I was speaking about low swing differential signal. 
I inverted the curve polarity on the plot and made both of them ground
relative to make it convenient to compare source and sink currents. In the
IBIS models they were put with correct polarity, pullUp - Vdd relative, in
accordance with IBIS standard and IBIS cookbook. 
While DC sweep on the non-inverted output was done for IV curves extraction,
the inverted output was controlled by the voltage dependent voltage source
v=2*Vos-v(padp), Vos ~ 1.25V (depending on conditions).
Actually the issue was observed for slow and typ cases while for fast case
the models are in good agreement with spice models (Vmax, Vmin, Trise,
Tfall, Duty cycle matched  reference waveforms).

Regards,
Andrey
________________________________________
From: owner-ibis-users@eda.org [owner-ibis-users@eda.org] on behalf of Tom
Dagostino [tom@teraspeed.com]
Sent: Wednesday, September 18, 2013 10:39 PM
To: 'Muranyi, Arpad'; ibis-users@eda.org
Subject: RE: [IBIS-Users] {Spam?} {Filename?} LVDS IBIS model - simulation
issue

Andrey

I've modeled a lot of LVDS buffers in the past and this is definitely the
strangest looking pulldown IV curve I've ever seen.  And as Arpad says it
does not at all look like a reasonable result.

First, why are you loading your LVDS buffer with a 50 Ohm differential
termination?  Most I have seen are looking for 100Ohms.

Have you inverted the pulldown's polarity?  I would also expect to see the
pulldown curve to have positive current between ground and Vdd.

LVDS buffers have a feedback loop in them so stabilize the Vcm of the
buffer.  Usually this only affect the pullup IV curve but sometimes only the
pulldown.  This makes extracting the IV characteristics of the controlled
device difficult.  The feedback is fighting the curve tracer that is
measuring the IV characteristics.  While you are doing the IV sweep you are
by definition changing Vcm and the feedback is trying to correct for this.

BTW, Ip = -In.  One is sourcing current and the other sinking.  The sourcing
should have negative currents and the sinking positive.

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<mailto: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 Muranyi, Arpad
Sent: Wednesday, September 18, 2013 8:31 AM
To: ibis-users@eda.org
Subject: RE: [IBIS-Users] {Spam?} {Filename?} LVDS IBIS model - simulation
issue

Andrey,

I am not sure that your PD I-V curve is correct.

First of all, it should have a positive sign, but its shape doesn't look
right either.

Can you describe how you generated these I-V curves?

Thanks,

Arpad
========================================================


From: owner-ibis-users@eda.org<mailto:owner-ibis-users@eda.org>
[mailto:owner-ibis-users@eda.org] On Behalf Of Andrey Babintsev
Sent: Wednesday, September 18, 2013 9:38 AM
To: ibis-users@eda.org<mailto:ibis-users@eda.org>
Subject: 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.
[cid:image001.png@01CEB462.5402D6F0]

Problem:
In the image below the YELLOW waveform is what I saw and the PURPLE waveform
is what I expected to see.
[cid:image002.png@01CEB462.5402D6F0]
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):
[cid:image003.png@01CEB462.5402D6F0]

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.
[cid:part1.07090501.06090707@freescale.com]

Problem:
In the image below the YELLOW waveform is what I saw and the PURPLE waveform
is what I expected to see.
[cid:part2.00080404.04020708@freescale.com]
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):
[cid:part3.05090108.02030307@freescale.com]

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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Received on Wed Sep 18 14:20:46 2013

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