RE: [SI-LIST] : 50 or 60 ohm impedance

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From: Moran, Brian P (brian.p.moran@intel.com)
Date: Tue Dec 12 2000 - 10:02:55 PST


Scot,

The 50 ohm controlled impedance board was sort of the original impedance
standard back in the ECL days and it carried over to this day. People have
generally found, however, that there are advantages to using 60 ohms, in
terms of speed and power, and it plays better with most CMOS logic.
Current PCB fabrication technology generally allows either, with the high
impedance end being squeezed in high layer count designs. If you just use a
typical a 4/4/4 dual stripline stack up with 4 mil traces, which is pretty
standard, you get about 56 ohms, give or take. As someone mentioned
previously, some high speed logic and hybrids do have internal terminations
at 50 ohms, which can force the issue. What I have generally done has been
to use 56-60 ohm stack ups and add a little trace width to those signals
requiring 50 ohms. One problem with using 50 ohms across the board is you
often lose some routing density, if you stack up requires say 5 mil traces
to get 50 ohms. The one place where 50 ohms can benefit is that it tends to
have less impedance variation wth capacitive loading which can help if you
have irregularily loaded busses or multidrop interconnects. There are other
pros and cons as far as coupling and very high frequency characteristics,
but this is the basic argument, at least as far as I am concerned.

Brian P. MOran
Signal Integrity Engineering
Intel Corp.

-----Original Message-----
From: Sampson, Scot [mailto:ssampson@sonusnet.com]
Sent: Tuesday, December 12, 2000 7:58 AM
To: Si-Questions (E-mail)
Subject: [SI-LIST] : 50 or 60 ohm impedance

  
   Hello All

    What are some of the criteria that is used when selecting 50 or 60 ohms
for board impedance?

   Thanx Scot

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