[SI-LIST] Re: Driver Impdance from IBIS
- From: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
- To: "Traa, Boris" <boris.traa@xxxxxxxxxxx>, "Jiang, George Q" <george.q.jiang@xxxxxxxxxx>, "Nijagunamurthy, Hithesh (GE Intelligent Platforms)" <hithesh@xxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 14 Dec 2010 08:53:55 -0500
What capacitive load (remember, t-lines look resistive)...:)
I believe the most effective way to see a driver's output-Z (Zs) is to
drive a "long" transmission line of known impedance (in simulation).
Probe at the output of the driver. Note the "porch" (Vp). Note the
final voltage (Vf). You can then solve for Zs, with this equation :
Zs = Zo * ((Vf/Vp)-1)
...It would be interesting to see if this method agrees with the
"Terminating Wizard" you mention below.
Best regards,
Jim Peterson
Honeywell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Traa, Boris
Sent: Tuesday, December 14, 2010 8:27 AM
To: Jiang, George Q; Nijagunamurthy, Hithesh (GE Intelligent Platforms);
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Driver Impdance from IBIS
Dear All,
It is very likely that the driver dynamic output impedance (as far as it
can be calculated or simulated) is also depending on the capacitive
load. So how deal with this?
Kind regards
Boris Traa
System design engineer EMC
It's the currents that make circuits work or fail.
Philips Applied Technologies/EMC center
Room 2.020
High Tech Campus 26
5656AE Eindhoven, The Netherlands
Tel: ++ 31 40 27 43766
Fax: ++ 31 40 27 42224
E-mail: boris.traa@xxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Jiang, George Q
Sent: 2010 Dec 14 2:07 PM
To: Nijagunamurthy, Hithesh (GE Intelligent Platforms);
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Driver Impdance from IBIS
MentorGraphics has a tech note MG23434 regarding how HyperLynx
calculates the drive impedance. Hope it helps.
George Jiang
Delphi Electronics & Safety
The following is what is in MG 23434
___________________________________________________
The Terminator wizard is calculating the impedance of the driver by
checking the delta V / delta I in 1/10 volt increments along both the
pull-up and pull-down VI tables. It keeps the lowest impedance it sees
for each table and then averages the two together to come up with the
driver impedance.
If you look at the pull-up and pull down table data graphically in the
IBIS editor, you will see that both the pull -up and pull-down tables
are quite non-linear, so there is a wide range of impedances present
along each line.
The pull-up table shows impedances ranging from about 32 ohms to an
average of 88 ohms (when you draw straight line from 0 volts to 3.3
volts).
The pull down table shows impedances ranging from about 28 ohms to an
average of 96 ohms (when you draw a straight line from 0 volts to 3.3
volts
The help text in the IBIS editor defines the output impedance the
following way:
*****************************************************************
Driver output impedances are not always specified in the IC datasheet.
Lacking a manufacturer's specification for impedance, there are several
ways to obtain it, described in the following sections.
Measure from Published V-I Curves
Many manufacturers are now publishing (or have readily available) V-I
curves for their output-buffer stages (high and low). If you can obtain
such curves, output impedance is easy to calculate.
To measure impedance from a V-I curve:
1. Draw a straight line along the linear portion of the V-I
curve, before the current "flattens out" or saturates.
2. Take two points on the line. Measure delta(v) and delta(i)
between the points.
3. Then calculate the output impedance as Zout = delta(v) /
delta(i)
************************************************************************
*
When this method is used and a line is drawn along the most linear
portion of the graphs from 0.0 volts to 1.1 volts, the impedances come
out to be 40.2 ohms for the pull-down and 44.8 ohms for the pull-up.
Xilinx must be calculating the average driver impedance differently then
Terminator Wizard. If you draw a line from 0 volts to 1.6 volts on each
graph, then you get about 50 ohms. This must be close to what Xilinx is
doing.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Nijagunamurthy, Hithesh (GE Intelligent Platforms)
Sent: Tuesday, December 14, 2010 7:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Driver Impdance from IBIS
Hi All,
How do you find the driver impedance information from the IBIS file.
In the V/I table, is the listed voltage just V or Vcc- V
The way I find impedance is
For pull up -
(Vcc-V in table)/I
For pull down -
(V in table)/(I in table)
What voltage do you consider? 1.00 or 1.5?
Some IBIS files mention if V in table is Vcc-V others don't.
For PCI devices like power pc, Intel 21555 bridge, the impedance values
I get is around 11 to 18 ohms.
Thanks,
-Hithesh
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