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We have run into a layout issue where the size/package of the 75R resistor used for Bob Smith termination on unused Ethernet pins is in question. The current package of this resistor is 1206 and we want to reduce it to 0603 size. Can this be done?

As far as I know or can think of, it can be done without any problems. But just want a confirmation. Some brief justification would also be helpful.

Thanks.

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  • \$\begingroup\$ You might be interested in reading electronics.stackexchange.com/questions/161436 \$\endgroup\$ – PlasmaHH Jun 19 '15 at 8:24
  • \$\begingroup\$ Although I am unfamiliar with Bob and his termination ;-) from a transmission line/cable termination point of view I would say: the cable obviously needs to be terminated with 75 ohms, this can be done with any 75 ohms resistor, 1206 or 0603. I would have a look at the required power dissipation of the resistor though. If there are signals on the line these must be dissipated in the resistor. Since an 0603 is smaller it can handle less power. If the DC component on the line is causing most dissipation, a series capacitor might solve that. \$\endgroup\$ – Bimpelrekkie Jun 19 '15 at 8:36
  • \$\begingroup\$ @IC_designer_Rimpelbekkie - the Bob Smith termination is for common mode signals on ethernet lines and not for direct termination. Direct terminations are still needed. \$\endgroup\$ – Andy aka Jun 19 '15 at 8:39
  • \$\begingroup\$ OK, now I see that indeed, didn't look at the schematic in the article in enough detail. But then dissipation caused by the signal should be even less of an issue (assuming the signals are differential and the common mode is just "noise") meaning you could safely use an 0603. \$\endgroup\$ – Bimpelrekkie Jun 19 '15 at 8:44
  • \$\begingroup\$ Thanks for the replies. I know about power dissipation, but since on Bob Smith termination I don't expect to see any signals flowing, I wasn't sure if the power dissipation is relevant or not. Therefore, I want to know if anyone has a general idea about the package of this resistor. If the power dissipation could be calculated somehow, I would have understood properly which package to select. \$\endgroup\$ – LoveEnigma Jun 19 '15 at 8:47
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Micrel uses 0603 on its demo boards, for example, see KSZ8463ML/RL Eval Board documentation.

In mass dense products, such as mainboards, 0402 is normal way.

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  • \$\begingroup\$ Thanks so much for the example. Gives more weight to the decision of going with 0603. \$\endgroup\$ – LoveEnigma Jun 20 '15 at 4:09

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