I am going to be using a processor that has a few SGMII interfaces. From what I understand, these interfaces have LVDS logic levels. I am not use to seeing the termination scheme as recommended below.

  1. I thought the 100 ohm resistor should be near the output and not near the input
  2. Not sure what the 50 ohms to GND does after the ac coupling caps.

Any ideas?


recommended termination example


1 Answer 1

  1. Transmission lines can be terminated at both ends. In this case, the source termination also provides DC load bias, as the transmission line is AC coupled.

  2. The 50 ohm resistors to ground are the 100 ohm differential termination of the transmission line with 0V bias.

Note that this diagram is only an example, and the actual circuit may be different depending on what DC bias the other device wants, and if it includes built-in termination.

  • \$\begingroup\$ So is the source termination used since there is AC coupling? Does the 100 ohm termination only affect the transmitter and doesn't depend on the receiver design? Also could I just use a 100 ohm resistor across the p and n and not two 50 ohms? Thanks \$\endgroup\$
    – Matty
    Sep 19, 2020 at 16:57
  • \$\begingroup\$ Source termination is for two reasons, it might be due to DC biasing the transmitter correctly, and it may need double termination to get amplitude right, and if there is no internal termination, and termination also stops any reflections. The two 50 ohms could possibly be changed to single 100 ohm termination, but only if the receiver sets the DC bias operating point internally, otherwise the common mode can be outside the operating range. \$\endgroup\$
    – Justme
    Sep 19, 2020 at 17:07
  • \$\begingroup\$ So with the double termination, does the transmitter see 100 in parallel with 100 which is 50 ohms? And since Zo=50 would we then see 1/2 of the amplitude sent with double termination? \$\endgroup\$
    – Matty
    Sep 21, 2020 at 11:27

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