Note that this is used in Ethernet routing to provide a sink for common-mode noise that may transfer from the PHY side to the connector side.īob Smith termination uses four 75 Ohm resistors (2 for Rx, 2 for Tx) and a capacitor to provide an impedance-matched path back to a ground point in the system. When routing between an Ethernet PHY and discrete magnetics circuitry, this termination scheme is used to ground the center taps of the transformers used in the magnetics circuits. Originally patented by Bob Smith, this refers to a certain set of resistors that provides termination for the center tap of a common-mode choke that is used in Ethernet routing. Let’s look at this issue around Bob Smith termination in more detail. But as most senior designers have stated on this blog, application notes are not to be trusted with impunity. I’ve never seen an application note that doesn’t advocate Bob Smith. Unfortunately, application notes do an outstandingly horrible job of explaining Bob Smith termination, if they try to explain it at all. So who is correct? Is this one of those cases where application notes push bad design advice and everyone follows along, or is this legitimate design guidance that is taken out of context? Smith is flat-out wrong, while others always follow Mr. I was surprised to see that this was so controversial many designers claim Mr. Some of these objections were purely conceptual, while others were backed up with data. As I started to dig on the internet, I started to find some objections to Bob Smith termination. It wasn’t until I was asked to write an article on Ethernet grounding for Signal Integrity Journal that I ever gave Bob Smith termination a second thought. From the time my company received its first job involving Ethernet, we always “Bob Smith” with gusto.
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