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DP83849 Fiber interface biasing

Tobias Meyer8
Prodigy 10 points

Hi,

the information on fiber interface biasing in the datasheet is a bit to brief.

Are the inputs true PECL inputs?

Should they be biased to 2V (Vcc - 1.3V) as a true 3V3 PECL in, or rather to 1.4V as on the evaluation board?

As both options can be achieved by interchanging the 82Ohm / 130Ohm bias resistor combination this is easily confused.

The bias suggestions for the outputs are also not indicating true PECL operation.

Am I correct in assuming they are not PECL outputs but produce a PECL compatible voltage swing when capacitively coupled to a fiber transceiver.

Could you please clarify, provide electrical specification for the pins in fiber mode or input / output equivalent circuits?

Can the inputs be 50Ohm terminated differentially and biased by larger resistors to reduce bias current?

Thanks!

  • 0
    TI__Mastermind 23675 points
    Hi Tobias,

    The DP83849 interfaces with fiber transceivers through AC coupling. The inputs support PECL signal swings but the biasing is slightly different, which is why we have a reference diagram for interfacing fiber transceivers to the DP83849.

    Please follow the 100Mb/s Fiber Pair Interface on the IC side of the isolation caps. (i.e. 50 ohm termination on FXTDP/TDM, 130 ohm and 80 ohm on FXRDP/RDM).

    Kind regards,
    Ross