[SI-LIST] Re: 100 ohm VS 85 ohm
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Jun 2011 11:25:31 -0700
I believe the choice for differential pcb traces boils down to a kind of
loading-and-power tradeoff. A lower impedance line helps overcome input
capacitance at the receiver, and other shunt capacitances along the way, in
exchange for teh dissipation of greater power.
A detailed discussion of many reasons for and against the use of 50-ohm
transmission lines appears here:
"Why 50 Ohms?", www.sigcon.com/Pubs/edn/why50.htm , and
"Why 50 Ohms (mailbag)?", www.sigcon.com/Pubs/edn/why50mail.htm .
If you use both-ends termination on every link (combining source and end
terminations on every link) you may find it easier to interconnect your
sysetm with test equipment, because, at least to within a first-order
approximation, no impedance adapters are necessary.
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505, howie03@xxxxxxxxxx
www.sigcon.com -- High-Speed Digital Design seminars, publications and films
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chris Padilla (cpad)
Sent: Friday, June 17, 2011 10:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 100 ohm VS 85 ohm
Folks,
I'm wondering if some of your higher speed designs are considering moving to
a < 100 ohm differential Zo?
We know that a 50 ohm via is difficult to make and the connector vendors
have equal trouble trying to reach 100 ohm differential on their high speed
connectors. Going to < 100 should make it easier to have lower crosstalk
and matched impedance to improve return loss, possibly better signal to
noise ratio, and wider traces could yield slightly lower loss (depends on
how you adjust the PCB geometries to reach 85 ohm, of course).
A negative is the 50 ohm test equipment environment. One will have 42.5 ohm
on their board. Can this be easily dealt with? Of course, most chips are
design with 100 ohm in mind so finding chips designed at something else
could be difficult.
I just wonder if the headache of moving off-standard is worth it or not.
I'm curious what the experience of folks here have witnessed.
Thanks,
Chris Padilla
Cisco Systems
San Jose, CA
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