DP83822IF: LVDS fiber interface

Tom Schultz

Part Number: DP83822IF
Other Parts Discussed in Thread: DP83822EVM

Team,

Posting on behalf of my customer.

"The optical fiber interface that we are using is not a standard LED driver/detector type, but has LVDS interfaces. We have had problems interfacing the LVDS fiber transceiver to a 10/100 PHY from Broadcom in the past as it had PECL interfaces causing a signal level mismatch. What type of interface is used on the DP83822IF for the TD/RD pins so that I can make sure we are successful interfacing the PHY to the LVDS signals? Based on our experience, we do not believe that connecting the transceiver as per the TI datasheet recommendations will give us proper operation."

Please advise.

Thanks,

Tom

over 2 years ago

0 Lucas Wolter over 2 years ago

TI__Genius 13885 points

Hi Tom,

Here is the user guide for the DP83822EVM: https://www.ti.com/lit/ug/snlu179/snlu179.pdf?ts=1625178039265&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FDP83822EVM

Check the LVPECL termination in Figure 19 for the termination scheme we have used on the EVM board to translate between the fiber transceiver and PHY signaling.

Please let me know if you have any other questions, otherwise I'll be closing this thread.

Thanks,

Lucas

All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and it is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

0 Tom Schultz over 2 years ago in reply to Lucas Wolter

TI__Expert 3665 points

Thanks Lucas.

"Just to clarify, are you saying LVPECL is the interface technology used on the DP83822IF for the TD/RD pins? If so, then that that should be sufficient to find a solution.

-->My only question at this point is whether or not a DC blocking capacitor is required, and I think that the answer is “no” as long as I have a valid LVDS to LVPECL circuit.

Looking at page 23, Figure 19 of the snlu179 document, it appears to be nothing more than an 82 ohm resistor over a 130 ohm resistor. At 3.3V, that gives you a 2.02V termination level, which is in the middle of the threshold as per TI SLLA120. "

Thanks,

-Tom

0 Lucas Wolter over 2 years ago in reply to Tom Schultz

TI__Genius 13885 points

Hi Tom,

We're looking into your question and will get back to you by Wednesday.

Thanks,

Lucas

0 Lucas Wolter over 2 years ago in reply to Lucas Wolter

TI__Genius 13885 points

Hi Tom,

My apologies for the confusion, the DP83822 driver is actually CML. The DC blocking capacitor is required, however some fiber transceivers have it built in. You can refer to your fiber transceiver datasheet to see if the capacitor is included or not.

Does this sufficiently answer your questions?

Thanks,

Lucas

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DP83822IF: LVDS fiber interface