Let's think about this output buffer and its specified test circuit as it presently is in your model. There is a pulldown FET connected to ground. There is also a 1M resistor tied to ground. Now, when the pulldown FET is on the junction of the resistor and FET is pulled down to ground, the same potential that the other end of the resistor is tied to. When the pulldown device is off there is no current flowing in the resistor so the junction of the FET and resistor is floating but the other end is tied to ground. Thus the junction of the FET and resistor is at 0 Volts. So, when you have a open sink or open drain buffer the Rref must be terminated to a voltage other than ground for any current to flow in it. Usually Vref in this kind of buffer is Vdd but obviously Vref needs to be larger than Vmeas. Tom Dagostino Teraspeed Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 http://www.teraspeed.com tom@teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 Teraspeed is the registered service mark of Teraspeed Consulting Group LLC -----Original Message----- From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On Behalf Of lee yang Sent: Friday, March 18, 2005 3:19 AM To: ibis-users@eda.org Subject: [IBIS-Users] Vmeas For Open Drain IBIS model Hi, Do we need to specify the Vmeas, Rref, Cref and Vref values in an open drain model? Why do i get the flowing warnings? WARNING - Model 'test': TYP VI curves cannot drive through Vmeas=1.25V given load Rref=1e+006 Ohms to Vref=0V WARNING - Model 'test': MIN VI curves cannot drive through Vmeas=1.25V given load Rref=1e+006 Ohms to Vref=0V WARNING - Model 'test': MAX VI curves cannot drive through Vmeas=1.25V given load Rref=1e+006 Ohms to Vref=0V The test conditions: Vmeas=1.25V Rref =1M Cref =0pF Vref =0V Could someone help? Thanks. -Lee _________________________________________________________________ Find just what you are after with the more precise, more powerful new MSN Search. http://search.msn.com.my/ Try it now. |------------------------------------------------------------------ |For help or to subscribe/unsubscribe, email majordomo@eda.org |with just the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or email a written request to ibis-request@eda.org. | |IBIS reflector archives exist under: | | http://www.eda.org/pub/ibis/email_archive/ Recent | http://www.eda.org/pub/ibis/users_archive/ Recent | http://www.eda.org/pub/ibis/email/ E-mail since 1993 |------------------------------------------------------------------ |For help or to subscribe/unsubscribe, email majordomo@eda.org |with just the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or email a written request to ibis-request@eda.org. | |IBIS reflector archives exist under: | | http://www.eda.org/pub/ibis/email_archive/ Recent | http://www.eda.org/pub/ibis/users_archive/ Recent | http://www.eda.org/pub/ibis/email/ E-mail since 1993Received on Fri Mar 18 08:37:29 2005
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