[SI-LIST] Re: [SI-LIST]About Impedance

He,

If reflections at the far end are problematic then something
must be done to mitigate them. A far end termination impedance
that matches the etch impedance is a possible solution. In this
case the goal is to get a reflection coeffecient near 0 by solving:

p =3D (Zt - Zo) / (Zt + Zo)

where Zt =3D far end load, termination impedance, and Zo =3D TLine imp.

One can also mitigate far end reflections by launching a half-amplitude
wave by inserting a series resistor at the near end, and then employing
a +1 reflection coeffecient at the far end to reach full amplitude.

The near-end series resistor is sized such that:

Rseries + Rds =3D Zo

where Rseries is what we are solving for, i.e. the near end series R.
where Rds is the on resistance of the output FET.
where Zo is the etch, TLine impedance.

I have left out a great bit of detail and assumptions, and if you want
to continue this thread you can email me directly so as to reduce the
BW on the list in general.

Also, most of the si books and many on-line app notes cover this in
great detail....=20

Jim

-
Jim Antonellis      jim.antonellis@xxxxxxxxxxxxx
Sandburst Corp   www.sandburst.com
Office: 978.689.1669=20
Cell: 978.618.4745

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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of He Hao
Sent: Tuesday, October 26, 2004 9:46 AM
To: si-list
Subject: [SI-LIST] [SI-LIST]About Impedance


If have implemented 50ohm trace impedance in board, then no need to add =
50ohm match resistor?
Or I add match resistor in circuit, then no need to implement approciate =
trace impedance?
Perhaps silly question,welcome your response.

        He Hao
        hehao@xxxxxxxx
          2004-10-26


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