Subject: RE: [SI-LIST] : About Impedance.

From: "Ingraham, Andrew" <[email-masked]>
Date: Mon, 26 Jun 2000 09:35:57 -0400

>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