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

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From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Thu Nov 18 1999 - 09:45:30 PST


Charles,

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
receiving).

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
voltage
bleeding onto the drain while the gate voltage is less than the threshold
voltage
of the output transistor (Vth). The output transistor has a fairly high
impedance
when its gate voltage is below Vth, therefore the Cdg capacitance can couple
the
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
direction.
The coupling effects of the Cdg capacitance are still there, but only in
slowing
down the edge somewhat. All of this is true for P-channel pullups and
N-channel
pulldowns.

For N-channel pullups you will not see this, because the gate to drain
relationship
is not inverting. In fact, you might observe a small initial "drive" before
the
real edge starts, again, doe to the capacitive coupling of the gate waveform
to
drain. In this case the gate voltage helps the edge, and may speed it up
somewhat.

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
additional
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
feedback
within the output"? Do you want to "measure" this effect separately because
you
want to know the value of the Cdg capacitor for some reason? Or do you just
want
to know whether this effect is modeled correctly in the IBIS model?

I don't know if you are interested in other types of feedback, but in
general,
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
describe
the timing related response completely. This, however, is not because it
couldn't
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:chuckh@altaeng.com]
Sent: Wednesday, November 17, 1999 6:59 PM
To: si-list@silab.eng.sun.com; 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : Transient impedance--IBIS models

Arpad,

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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