# [SI-LIST] Re: Impedance calcuation

• From: James_R_Jones@xxxxxxxx
• To: joeyoung10@xxxxxxxxx, si-list@xxxxxxxxxxxxx
• Date: Mon, 15 Apr 2002 17:31:26 -0500
```Joe,

I might answer this question rhetorically:  At what trace length, frequency,
and trace impedance?

Zi = Z0 (ZL + Z0 tanh (w l))/(Z0 + ZL tanh (w l))

where
Zi= input impedance to a loaded transmission line
Z0= trace impedance
w= propagation constant
l= length of the trace

Two simplifications:
l is very short -> Zi= ZL
l is very long -> Zi= Z0

I would try to use the simplifications to compute a short trace scenario and
recognize that input impedance will be Z0 for long lines.

BTW, I find the Intro to Electromagnetics books from college to be somewhat
helpful.  The only catch is that many of these books are written with
sinusoidal steady state in mind.  When working with digital pulse trains,
you would have to compute these impedances at every frequency that exists in
a psuedo random pulse train.

The easiest and best answer probably is to simulate using some spice like
simulator and then looking at the current that enters the trace (T-line
segment).   You will notice that this current may not look like what you
might expect from the theory, depending on your termination scheme and the
size of your parasitics.  This current will give you the best indication of
what kind of current drive capability that your drivers will need to have.

Hope this helps,
James J.

-----Original Message-----
From: Joe Young [mailto:joeyoung10@xxxxxxxxx]
Sent: Monday, April 15, 2002 4:53 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Impedance calcuation

Hi Experts,
I have a very basic (silly?) question in my mind. I need to
calculate the total load or impedance faced by the Driver.I know the input
capacitance of the receivers. And also i know/decide the trace length
information.
Device1 --- 8.8 nH , 1.0PF
Device2 --- 6 nH, 2 pF
Device3 --- 8 nH, 7 pF
Device4 -- 3.3nH 0.7 pF
Device5 -- 3.3nH 0.7 pF and assume device 2 is 1 inch from device 1
and device 3 is 2 inch from Device 1(1 inch from Device 2) and Device 4 and
5 are 4 inch from device 1 (2 inch from device 3)

Device 1 drives Device2 to Device 5.
I have to come up with a best topology. Device 2 operates at 83
Mhz and Device3 to 5 operates at 33 Mhz.
Now i want to know the impedance seen by the Device 1 when it is
Ofcourse, i can refer the books. But i would like to know is there any
easier formula for come to quickest conclusion to decide th topology? Any
thumb rule?.Reference to any formula?.
Am i missing any parameter that has to be included?

This question may be a very basic and can be found in books but i would
like to hear from the Experts to know  how they would decide.

Expecting the inputs from the experts to add up my experience.

If anybody could give me a detailed paperwork, really it would be helpful
for me to do further experiments.

rgds,
Joe

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