[SI-LIST] Question about common mode termination of Ethernet transformers
- From: Joel Brown <joel@xxxxxxxxxx>
- To: SI-List <si-list@xxxxxxxxxxxxx>
- Date: Tue, 16 Sep 2014 15:24:32 -0700
I am working on a design that has gigibit Ethernet running between two
cards connected by a backplane using 100 ohm differential traces
approximately 12 inches in length. My understanding is that actual data
rate per pair is 125 Mb/S. My first thought was just to use AC coupling
capacitors between the two parts (Ethernet controller and Ethernet switch).
Since Ethernet parts are usually connected through transformers to provide
isolation and filtering I decided I should include the option for either
the AC coupling capacitors or a transformer.
Most of the time the cable side of Ethernet transformer center taps have a
common mode termination of 75 ohms connected to ground through a 1000pF
capacitor (aka as the Bob Smith termination). I have read some things that
claim 75 ohms isn't the optimum value and that the common mode termination
may not be needed at all (see people.eecs.ku.edu/~callen/713/Bohnert.pptx)
I am sure that 100 differential traces are more like 25 ohms common mode
impedance, so I am wondering if I should use 25 ohms instead of 75 ohms or
just not have any common mode termination.
Thanks - Joel
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