Subject: RE: [IBIS-Users] IBIS model driver impedance
From: herbert_lage@agilent.com
Date: Fri Jan 10 2003 - 01:52:47 PST
Hi Shee Kian,
As a variation on Scott's explanation, I usually determine the impedance differentially which allows me to calculate it as a function of the buffer voltage.
To determine the impedance Z at a voltage V you can do the following:
* Choose a voltage V1 in the tables where V1 < V, current is I1
* Choose a voltage V2 in the tables where V2 > V, current is I2
* Then calculate Z(V) = (V2 - V1) / (I2 - I1)
This can be easily done if you import the IBIS file into a spreadsheet. If the voltages you are interested in are below Gnd or higher than Vcc, you may have to add the Power/Gnd clamp tables to the Pullup/Pulldown tables.
Best Regards,
Herbert.
-----Original Message-----
From: Scott McMorrow [mailto:scott@teraspeed.com]
Sent: 10 January 2003 09:52
To: Shee Kian Wong; ibis-users@server.eda.org
Subject: Re: [IBIS-Users] IBIS model driver impedance
From ohms law
V=IR
therefore
R =V/I
In the ibis model is a V/I table.
The impedance is real, thus at any point in the table the impedance is:
Z = V/I
As you will see, the impedance will change depending upon the output voltage. If you want to find a good average impedance, then find the current at approximately half of the voltage swing of the driver. You can compute this for the pullup and the pulldown curves at all three corners.
best regards,
scott
--Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com <http://www.teraspeed.com>
Shee Kian Wong wrote:
Hi all,
Can anyone suggest a way to measure the driver impedance of an IBIS buffer? Do the look-up tables in the IBIS file carry this information?
Regards,
SK
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