RE: [SI-LIST] : Transient impedance--IBIS models

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From: Chuck Hill ([email protected])
Date: Sun Nov 21 1999 - 20:23:52 PST

At 09:45 AM 11/18/99 -0800, Muranyi, Arpad wrote:
>I will attempt to answer your question on feedback and Cdg quickly.
>The effects of the parasitic capacitance of the output transistor is
>included in
>the Vt curves (when the buffer is driving) and in C_comp (when the buffer is
>A prime example for this is the small bump in the Vt curve that precedes the
>edge going in the opposite direction. This bump is a result of the gate
>bleeding onto the drain while the gate voltage is less than the threshold
>of the output transistor (Vth). The output transistor has a fairly high
>when its gate voltage is below Vth, therefore the Cdg capacitance can couple
>waveform from of the gate to drain easily. When the transistor starts
>turning on,
>its impedance goes low, and the drain voltage starts going the right
>The coupling effects of the Cdg capacitance are still there, but only in
>down the edge somewhat. All of this is true for P-channel pullups and
>For N-channel pullups you will not see this, because the gate to drain
>is not inverting. In fact, you might observe a small initial "drive" before
>real edge starts, again, doe to the capacitive coupling of the gate waveform
>drain. In this case the gate voltage helps the edge, and may speed it up
>When the buffer is 3-stated, the transistor channels are high impedance, the
>gate voltages are steady. So the Cdg capacitance will look like an
>Cds, which is (or supposed to be) included in the IBIS C_comp value.
>I am just wondering, why do you ask "how does one measure the effect of
>within the output"? Do you want to "measure" this effect separately because
>want to know the value of the Cdg capacitor for some reason? Or do you just
>to know whether this effect is modeled correctly in the IBIS model?

My concern is the performance of IBIS models. There are two issues here:
1. How many effects can IBIS represent. 2. Creating models in IBIS. It
seems, as you say, that the Cdg capacitance effect and higher impedance
effect can be represented by IBIS. The remaining issue is the creation of
the IBIS model.

If one uses the Spice model to create the IBIS model, the results depend on
the quality of the Spice model. In turn, that depends on the methodology
used to create the Spice model--this is kept confidential by silicon
vendors and not open to review. What were the assumptions used in creating
the Spice model? So we should have objective ways of checking the accuracy
of any model.

Creating the IBIS model from measurements requires more resouces: skill,
equipment, time. IV plots measured by curve tracer imply DC parameters
since the curve tracer is intended to measure DC parameters by its design.
If one gets into dynamic effects, the timing scale becomes
important---feedback falls into this area. This implies dynamic testing is
required--then the measurement process becomes more difficult.

After reading messages on the list on the IBIS datasheet discussion,
perhaps a set of written performance tests would help prove the model.
These tests would be done on the IBIS model, and on the bench. That should
help everyone, consumer and supplier, have confidence in the modeling
results. Perhaps the tests that Pat Zabinski mentioned would be a good set
to check the output.

Charles Hill, consultant

>I don't know if you are interested in other types of feedback, but in
>IBIS does not address feedback buffers in a true sense. IBIS can model the
>so-called "bus-hold" buffers which work based on feedback, but it doesn't
>the timing related response completely. This, however, is not because it
>be done. It just was not implemented yet, because no one came forward with
>the need
>and behavioral description methodology yet.
>I hope this will answer your questions.
>Arpad Muranyi
>Intel Corporation
>-----Original Message-----
>From: Chuck Hill [mailto:[email protected]]
>Sent: Wednesday, November 17, 1999 6:59 PM
>To: [email protected]; '[email protected]'
>Subject: RE: [SI-LIST] : Transient impedance--IBIS models
>You're right. I looked at your slides on the IBIS modeling presentation.
>If one does the measurements right, and creates correct V-t tables, the
>output impedance through the switching interval can be higher. The problem
>I encountered was a poor method of creating an IBIS model.
>On a related issue, how does one measure the effect of feedback within the
>output? For example, a gate to drain capacitor. I guess this falls in the
>"slew rate controlled" output arena. So how does IBIS account for these
>effects and how could one measure an output driver with these
>characteristics and create an IBIS model?
>Charles Hill, consultant
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