[SI-LIST] Re: 100 ohm VS 85 ohm

Hello Chris,

There is actually quite a bit of work going on at 85 Ohms.  

As for testing non-standard impedance devices, we at Samtec did a joint paper 
with Agilent a couple years ago that addresses the issues.  Bottom line, 50 Ohm 
instruments can work fine.

http://samtec.com/Documents/WebFiles/Technical_Library/Reference/Articles/DesignCon2009_Non-Standard_Impedance.pdf



We've also developed several 85 Ohm connectors and cables recently, and 
re-characterized many of our interconnect products to help designers evaluate 
their performance in an 85 Ohm environment.

You can find some of that info here:

http://www.samtec.com/signalintegrity/85ohm.aspx


Julian Ferry

High Speed Engineering Manager
Samtec, Inc


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Chris Padilla (cpad)
Sent: Friday, June 17, 2011 1:08 PM
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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