Re: about IBIS model's ramp

jxz wrote:
>
> Hi,
> I am a engineer of Huawei company in the city of ShenZhen in China.
> Now I am engaged in PCB signal analysis . I have met many IBIS
> models .
> May I ask a question now? Thank you .
> In IBIS model, there are definition of pullup ,pulldown and other
> V/I curves . Other parameters such as C_comp and PKG parameters are also
>
> listed. So in my idea, if a driver have these V/I curves and
> parameters, when a square wave is input to the driver( it has an a
> standard 50 ohm load ), the output wave should be a trapeziform wave.
> >From this trapeziform wave , we can measure the tr and tf .
> Now I want to ask that though we can measure the tr and tf from the
> output waveform, why should we define the ramp parameter in IBIS model?

This would be true if the output waveforms were dominated by current
into a capacitive load. In many signal-integrity applications this
assumption is not valid.

In the simplest case, the output is into a transmission line, which
acts as a real (nonreactive) load at high frequencies. In that case,
the output waveform is determined by the I/T properties of the driver
which in turn are dictated by the design of the predriver stage. The
V/I curves don't provide this information.

In more complex cases, the driver is connected to the PWB interconnect
through small inductors such as bond wires. In that case, the output
waveform is even dependent on the dI/dT of the driver, which is even
farther removed from the V/I curves in an IBIS 1.x model.

-- 
D. C. Sessions
dc.sessions@vlsi.com