• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 1831 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LASER diode series resistance and the ONET4201LD

Michael Nycz
Intellectual 835 points
Other Parts Discussed in Thread: ONET4201LD

I was looking at the ONET4201LD for a new project. While looking at the datasheet I found the following....

LASER DIODE INTERFACE
The output stage of the ONET4201LD is optimized for driving a 20-Ω load. The combination of a damping
resistor, RD, along with the resistance of the laser diode must be 20 Ω for impedance matching. The suggested
typical value for RD is 6 Ω to 15 Ω. A bypass capacitor of 10 nF placed close to the laser anode also helps to
optimize performance.

In my case, the laser diode that I am planning to use is a FINISAR HFE4192-581 which has a nominal series resistance of 35 ohms.

My question is, will the ONET4201LD have an issue driving this device? Or, is there something that can be done to improve the compatibility?

  • 0
    Intellectual 835 points
    One additional note: typically, when I had driven TOSA lasers in the past, I had used 50 ohm impedance routing layers. For the ONET4201LD, does this mean that my routing layer needs to be 20 ohms impedance?
  • 0
    TI__Expert 8915 points

    Michael,

    The ONET4201LD does not have internal back termination so the transmission line impedance connecting the 4201LD to the laser does not have to be 20ohms. When the IC was designed, a 20ohm transmission line was used and the return loss was optimized to this. Most customers that I have worked with do not use 20ohm TLs because they are too wide. If the laser load resistance is 35ohms then a 35ohm TL would be good but I am sure it would not affect the performance much if it were higher. I mainly suggest to use a drive circuit with an external back termination resistor as shown in figure 3 of the attached file. You can optimize the resistance for the performance. R3 and C3 may not be required and I suggest putting the series combination in parallel with the laser diode, that is across the anode and cathode.

    2843.ONET4201LD-4211LD Output Drive.pdf

    Regards,

    Alex.

  • 0 in reply to Alex Davidson
    Prodigy 100 points

    Alex,

    Good inputs.

    As per figure 3, If we drive the Laser with 50 Ohms TL, does it effect the performance in 1GBPS ?

    Thanks

    Regards

    Sudarshan

  • 0 in reply to sudarshan ns1
    TI__Expert 8915 points
    Sudarshan,

    If the TL is long, then it could result in more reflection but at 1Gbps the impact will not be as much as at 4Gbps. I suggest trying to keep the TL as short as possible.

    Alex.