Subject: RE: [IBIS-Users] IBIS model Question.
From: Ingraham, Andrew (Andrew.Ingraham@hp.com)
Date: Thu Jul 11 2002 - 10:59:50 PDT
> I am running some simulations on a interface whose
> swing is 800mv. when I simulate the wave forms that I
> get have a swing from -400 mv to 400mv which obviously
> is not correct.
Why is it not correct? It sounds like the swing is what it's supposed to be.
I'll guess that what you find wrong about it, is that the signal is symmetrical around 0V.
> I was wondering as to what might be
> the source of the problem in the IBIS models.
Is there an AC coupling capacitor anywhere in your circuit? All SPICE-like simulators give misleading (but correct) initial results when doing TRANsient analysis of digital circuits with AC coupling capacitors. What the simulator simulated, is that the signal was inactive (at its t=0 state) for all negative time, then it suddenly starts switching. Thus, the charge across the capacitor might be different after a very long time of normal switching.
If that is the case here, there is probably nothing wrong with your IBIS models. You can correct this by controlling the initial conditions, or by running the simulation for a very long time until the bias voltage across the capacitor can re-establish itself.
> 2) The other question is pertaining to the IBIS model
> convention. The signs for the current values in power
> and gnd clamps. I see that the current values for the
> Pwr clamp is positive and negative for gnd clamp. does
> it mean that pwr clamp sources current and sinking
> current for gnd clamp.
IBIS current is positive when the direction is into the component. This is the standard in electrical engineering.
When the applied voltage goes below Vss, the GND Clamp conducts and current flows out of the component, so the polarity of this current is negative. The GND Clamp sources current to the external circuit.
When the applied voltage goes above Vdd, the POWER Clamp conducts and current flows into the component, so the polarity of this current is positive. The POWER Clamp sinks current from the external circuit.
Regards,
Andy
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