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ONET1151L: Termination of laser driver output

Part Number: ONET1151L

I have the ONET1151L implemented in a PCB design. When doing the layout, it was overlooked that the trace impedance of the output lines should be 25 Ohms. I am seeing functionality however the eye diagram looks fuzzier than I would like. 

I DO have an optional component placed between the two output lines(C33 which is currently not populated). There are also serial 0 ohm resistors (R14 and R36) on each of the output lines that lead to the TOSA device.

Is there a component or combination of components that could be added to improve the termination of the ONET1151L outputs given that the traces are not impedance controlled? I am just trying to improve the performance of the current hardware, as best I can, without having to do a complete board re-spin.

Thank you,

Michael Nycz

  • Hi Michael,

    Part is back terminated with 25-ohm and as soon as we see 50-ohm there would be reflections. One thought crossed my mind and thought it might be worth a try. I am wondering if it is possible to put your scope in real time mode to observe over/under shoot due to 25-ohm back termination and 50-ohm trace. Then use small Cap to gnd or change peaking inductor values to dampen these over/under shoot. Unfortunately, to really see device performance we need 25-ohm trace however this requires a new layout. The idea above is just to see what is the best you can get with the board you now have.

    Regards,Nasser

  • Hi Nasser,

          I was wondering what value of cap would be appropriate to place on the trace? I tried a 75pF value and the signal completely went away. Can you  suggest what range I should start in? Also, if you look at my schematic, there is a cap between MOD- and MOD+. Is there a cap value that I could try there? Again, I tried a 75pF cap there and it did not seem to have an effect. 

    Thank you,

    Mike Nycz

  • Hi Michael,

    75pF is too high. I am thinking perhaps 0.1pF or 0.2pF could be worth a try - you can fine tune as needed. You can put it on top of your preset Cap at MOD+/-.

    Regards,Nasser