*Arpad*, In this case DC sweep is a series of independent DC solutions. *Tom,* You are right, Vcm is changing unexpectedly, I will investigate further why it is changing and how to control it. Can you share how you generate LVDS models, how you control the inverted pin? Is it appropriate way: http://www.eetimes.com/document.asp?doc_id=1225499 (c) http://www.eetimes.com/document.asp?doc_id=1225499 Regards, Andrey On 09/19/2013 02:11 AM, Muranyi, Arpad wrote: > Just another question about generating the I-V data: > > How fast are you sweeping the voltage when you measure the > current? Is it in the ns, us, ms range? The reason I am > asking is because if you sweep it too fast, you might actually > be measuring parasitic capacitance on the die (I=C*dV/dt)... > > Thanks, > > Arpad > =============================================================== > > -----Original Message----- > From: Tom Dagostino [mailto:tom@teraspeed.com] > Sent: Wednesday, September 18, 2013 4:19 PM > To: 'Babintsev Andrey-B13285'; Muranyi, Arpad; ibis-users@eda.org > Subject: RE: [IBIS-Users] {Spam?} {Filename?} LVDS IBIS model - simulation issue > > 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 > > -- > This message has been scanned for viruses and dangerous content by > MailScanner<http://www.mailscanner.info/>, and is believed to be clean. > > -- > This message has been scanned for viruses and dangerous content by > MailScanner<http://www.mailscanner.info/>, and is believed to be clean. > > -- > This message has been scanned for viruses and dangerous content by > MailScanner<http://www.mailscanner.info/>, and is believed to be clean. > > -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. -------------------------------------------------------------------- |For help or to subscribe/unsubscribe, e-mail mikelabonte@eda-stds.org |or ibis-request@eda-stds.org | |IBIS reflector archives exist under: | | http://www.eda-stds.org/ibis/email_archive/ Recent | http://www.eda-stds.org/ibis/users_archive/ Recent | http://www.eda-stds.org/ibis/email/ E-mail since 1993Received on Thu Sep 19 07:07:42 2013
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