Jon, Thanks for your questions about IBIS! The Cookbook is intended to be general-purpose, so the recommendations are crafted to be appropriate for a variety of designs. Though your drawing got a little mangled in transit, I believe I can make a few recommendations on how your eventual IBIS model should look. 1) You are correct that an I/V curve should look like a 50 ohm resistor referenced to VCC. However, only one I-V table should look this way. Based on the drawing and an assumption that one resistor is internal and one is external, I believe your I-V tables should resemble the following: - [Pullup]: omitted; no pullup devices drive, so this should not be needed - [POWER Clamp]: should show the effects of the resistor and any diode structures connected to VCC but not VSS; should pass through "0" when plotted VCC-relative, per the specification - [GND Clamp]: should NOT show the effects of the resistor, to avoid double counting; should only show the effects of any diode structures connected to VSS. Will therefore also pass through "0" axis when plotted VSS-relative - [Pulldown]: tricky. This should NOT show the resistor's effects or the diode effects. In the operating region, I would expect the transistor + current source I-V table will be fairly horizontal, at a current corresponding to the output drive current target. The less ideal the source, the less perfectly horizontal the I-V curve. The Ron of the transistor will have some effect, of course. 2) Each "half" of the driver can be modeled independently, so that two IBIS models (each consisting of the resistor and transistor/source combination) can be "tied together" using the [Diff Pin] keyword. The model could therefore be of type Open_sink, IO_open_sink or similar variants. V-t tables could include, at a minimum, 1 [Rising Waveform] and 1 [Falling Waveform] into a VCC-connected load, to characterize the pulldown section turning off and on, respectively. Using the external 50 ohm load for your V-t tables should be fine. This should be enough to get you started and help resolve any issues you may have been encountering so far. Good luck! - Michael Mirmak Intel Corp. Chair, EIA IBIS Open Forum -----Original Message----- From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of Jon Read Sent: Thursday, August 09, 2007 12:17 PM To: ibis-users@server.eda.org Subject: [IBIS-Users] IBIS model for differential bipolar current mode logic output IBIS users, (See schematic below). I am a new creator of IBIS models. The circuit that I need to model are bipolar. The first one I did was for s single ended AB driver with 50 ohm output impedance. I have now been chalanged to create/extract a model for a bipolar differential current mode logic output buffer, similar in principle as I have attempted to draw below. So the differential extraction processed outlined in the IBIS cook book seems rather complex. 1) I know there are no paths for differential currents to flow. 2) The I/V curve should look like a 50 Ohm resistor referenced to VCC. The problem I had extracting the IV curves is that the IV curve should go to zero I at some point and this is where the VT curves are tested to match. However, the actual circuit is driven into a region it was not intended to operate in and the curves don't match. Another way of saying this is the circuit simply will not operate unless driven into the 50 ohm pullup. Does anyone have an idea how to get around this??? Jon Read Inside die/pkg | Outside die/pkg | VCC | VCC | | | ____|___ | ____|____ / \ | / \ 50 Ohms \ / 50 Ohms | 50 Ohms \ / 50 Ohms / \ | / \ \ / | \ / QB |--------|-----------|-----=========================------| | | |-----------|-----=========================----------------| | | Q | 50 Ohm Transmission lines | / \ | | |/ \| | D ___ | |____DB | | | | |< >| | | \ /| | \ / | |_________| | | | | | Current Source -- 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 majordomo@eda-stds.org |with 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 e-mail a request to ibis-request@eda-stds.org. | |IBIS reflector archives exist under: | | http://www.eda-stds.org/pub/ibis/email_archive/ Recent | http://www.eda-stds.org/pub/ibis/users_archive/ Recent | http://www.eda-stds.org/pub/ibis/email/ E-mail since 1993Received on Thu Aug 9 15:17:41 2007
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