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This is my first ethernet circuit, and I am a noob in differential signal lines. That is why I want to learn with a "simple" circuit.

I use the reference schematics from the LAN9512 usb to ethernet chip (100mb/s).

My question is with the Bob Smith terminator. The reference schematic uses a rj45 with a transformer and the terminator included. But I want to test an external transformer (next project will be a "simple" ethernet switch).

So, I checked other reference designs, and I found different variations for the Bob Smith circuit. Some uses 51ohm resistors, others 75ohm... I am quite confused here.

This is the termiantor that I am using. It is the only one that I found that uses resistors on both sides of the transformer (i guess that to match impedance in both sides of the transformer)

enter image description here

This is my schematic:

enter image description here

What do you think?

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  • \$\begingroup\$ on the pin 7 of the transformer, there is a wreid gnd symbol. That is because I had it on my cursor when I took the screen shoot. Sorry for this. \$\endgroup\$ – Javier Loureiro Apr 20 '16 at 11:02
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The resistors on the cable side are supposed to terminate each pair being treated as one "wire". The purpose is to reduce EMI and they do not functionally affect the actual signals. The impedance is not so well defined. With 20 years of experience, the magnetics nowadays with built-in common mode chokes are effective such that those terminations do not make much difference.

The resistors on the driver side of the transformer are for terminating the differential signals. Those terminations are critical.

On your schematic, the R7 feeding the two center taps (pin 2 and 7) let open an unnecessary coupling path between the transmit and receive. I don't see why you would want that.

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  • \$\begingroup\$ you mean the 10r resistor? you would let the pins 2 and 7 wired independently with a 10r resistor? I followed the reference datasheet from the lan9512 chip (and in the raspberry pi rev 2.1 datasheet)... I dont know why they connect it \$\endgroup\$ – Javier Loureiro Apr 20 '16 at 10:57
  • \$\begingroup\$ Yes. I would wired them independently if a 10 ohm resistor is wanted. Although the node is heavily shunted and the common mode rejection of the device probably eliminates any real issue. \$\endgroup\$ – rioraxe Apr 21 '16 at 0:13

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