**From:** Ingraham, Andrew (*Andrew.Ingraham@compaq.com*)

**Date:** Mon Jun 26 2000 - 06:35:57 PDT

**Next message:**Joan Vicent Castell Balaguer: "Re: [SI-LIST] : guard traces"**Previous message:**ARiazi: "Re: [SI-LIST] : About Impedance."**Maybe in reply to:**Song In-myung: "[SI-LIST] : About Impedance."

*>Is there any method of getting the input/output impedance value from IBIS
*

model?

It is fairly easy to get the low frequency impedance from the I-V curves by

inspection. But be aware that the impedance of most buffers is a nonlinear

function of voltage or current. There is no standard. You could pick the

small-signal output impedance (negligible load current, voltage at the

rails), or the impedance at Vdd/2, or at Voh(min) and Vol(max), or Vih(min)

and Vil(max), or anything else. And it generally differs between driving

low and driving high.

For output impedance, look at the [Pulldown] and [Pullup] curves; the clamp

curves are usually (but not necessarily!) negligible between Vdd and Gnd.

For driving low, look up the voltage you pick on the [Pulldown] curve, find

the Typ/Min/Max currents, and calculate Vout/Iout for each. For driving

high, it's the same except you use the [Pullup] curve and you would use

Vdd-Vout, where Vout is the output voltage you pick.

Or you may prefer to find and average the output impedance over a range of

output voltages.

For input impedance, use the [GND Clamp] and [Power Clamp] curves and

combine them. The [Power Clamp] is Vdd-relative so you need to adjust the

voltage axis before adding it to the [GND Clamp] curve.

These are large signal impedances. If you prefer to know the small-signal

impedance around some operating point (say, around Vdd/2), look up a few

points next to that operating point and calculate deltaV/deltaI. Which one

you use depends in part on the IC and its application.

The complex package impedance can be added to the low frequency impedance

from the I-V curves. The simple package model is not very representative at

higher frequencies.

Depending on your simulator, you may also find the impedance vs. frequency,

voltage, or whatever, by simulating a small test "circuit". Connect a

voltage source to the input or output pin. Measure current.

*>I think that these values are very important to match the transmission line
*

impedance.

*>Is my thought correct?
*

Maybe. However, impedance control might not very good on-chip, so to get

better accuracy, off-chip resistors might be used. In digital circuits,

most inputs don't match transmission line impedances at all, so external

resistors would be used, if you require a matched impedance there.

Andy

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**Next message:**Joan Vicent Castell Balaguer: "Re: [SI-LIST] : guard traces"**Previous message:**ARiazi: "Re: [SI-LIST] : About Impedance."**Maybe in reply to:**Song In-myung: "[SI-LIST] : About Impedance."

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