RE: [IBIS-Users] IV-VT Correction

I concur with Arpad. Most of the models we develop are from measurements of
physical devices. It is very easy to make IV measurements with high
accuracy. But when it comes to VT measurements there are lots of issues
that can affect the accuracy of the fit to the IV curves. Scopes are not
nearly as accurate as DC meters, any attenuation between the DUT and the
scope add errors that need to be corrected. Circuit board trace impedance
control adds more uncertainty, etc.

When we make IBIS models from simulations we usually have fewer corrections
to make but we have higher confidence in the DC measurements than the AC so
we also correct the VT curves to the IV.

Tom Dagostino

Teraspeed Labs

13610 SW Harness Lane

Beaverton, OR 97008

503-430-1065

tom@teraspeed.com

www.teraspeed.com

Teraspeed Consulting Group LLC

121 North River Drive
Narragansett, RI 02882

401-284-1827

www.teraspeed.com

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Muranyi, Arpad
Sent: Tuesday, September 28, 2010 8:10 AM
To: ibis-users@eda.org
Subject: RE: [IBIS-Users] IV-VT Correction

Shiju,

My preference is to keep the I-V curves and adjust the V-t curves.

The reason is because I believe that I-V curve measurements can be

made more accurately than V-t measurements. An I-V curve sweep

is (should be) typically a slow sweep in the range of tens or

hundreds of milliseconds. On the other hand the V-t curve measurement

is today's fast edge rates are on the sub nanosecond scale which are

fairly hard to make (consider noise, probe grounding, etc...).

In addition, the I-V measurement is usually done when the chip is in

the steady state, i.e. it is finished with any transitioning. It

is just driving a steady high or low. This means that the I-V curve

is what it should be when the device is fully on. If you toggle

the device to get a V-t curve, and you don't wait long enough for

the transition to be completely finished, it is not the I-V curve's

problem, it is that you didn't let the transition finish, a problem

in the V-t curve measurement.

I hope this helps...

Arpad

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  _____

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of shiju.abraham@wipro.com
Sent: Tuesday, September 28, 2010 4:13 AM
To: ibis-users@eda.org
Subject: [IBIS-Users] IV-VT Correction

Hi,

I have a doubt regarding IV-VT correction.

When the parser complains about IV-VT mismatch, I used to correct I-V
table(s) so that I get rid of the mismatch. But, one of the tools does the
correction by modifying the V-T table.

Which method of correction is recommended? I hope the reason would also be
explained.

Thanks,

Shiju

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