DP83640: TD+/- 50 ohm Terminations

Zachary Morris

Part Number: DP83640

Hello,

In the design notes for the DP83640 it shows the terminations near the PHY as opposed to the transformer. Is there a reason it is recommended that the terminations are on the load end and not the source end?

Thanks

over 7 years ago

0 Zachary Morris over 7 years ago

Prodigy 40 points

Correction, I meant to ask why the terminations are on the source end and not the load end

0 Rob Rodrigues over 7 years ago in reply to Zachary Morris

TI__Mastermind 19780 points

Hi Zachary,

The DP83640 needs a CML pull-up to VCC on the TD pins, and a 100 ohm termination on the RD pins. The 49.9 ohm resistors provide both those functions. The requirement to pull-up both pairs to VCC is because auto-MDIX swaps the pairs randomly until a link partner is detected.

Best Regards,

0 Zachary Morris over 7 years ago in reply to Rob Rodrigues

Prodigy 40 points

Hey Rob,

I understand the purpose of the terminations, but when laying out the terminations, shouldn't the terminations on the TD pair be closer to the ethernet transformer than the PHY?

0 Rob Rodrigues over 7 years ago in reply to Zachary Morris

TI__Mastermind 19780 points

Ah sorry I see what you mean. The resistors should be close to the PHY as they are part of the CML driver and we should minimize the impedance between the CML pull-ups and the transmitter.

Best,

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DP83640: TD+/- 50 ohm Terminations