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Original question:

ONET4291VA: modulation current vs bias current

ONET4291VA: design questions

Nir Flysher
Expert 3367 points
Part Number: ONET4291VA


Dear Team,

My customer is looking to design in the ONET4291VA.

Would anyone on the team be able to support us with the questions below ?

  1. what is the different between bias current to modulation current?
    Bias current determinate the laser current level, what is the purpose of monitor current register?
  2. In which cases is better to use open or close loop (OLE) ?
  3. According to the schema which polarity I need to set (PDP) ?
  4. We would like to review our scheme  uplink and downlink - would anyone be able to help ?

 

Questions about the 2 – Wire Digital Interface :

1.What is the minimum and maximum frequency of SCK pin ?

2.According to the data sheet we assume that the device includes 100K Ohm Pull up resistors. Can you approve it ?

3.Can you give up timing diagram for example with real data clk & data ?

4.In our side we need to implement "Master " side ,Do you have an example of software code to see ? Can we base on I2C Master code ?

Regards,

Nir.

  • 0
    TI__Guru* 93570 points

    Hi Nir,

    Please note comments below. 

    what is the different between bias current to modulation current?
    [Nasser]: Bias current provides the current to enable the laser to modulate(transmit 0 or 1).
    It sets up the bias point.
    On the other hand, modulation current it uses input voltage for either 1 or 0 level to drive current through the laser diode.


    Bias current determinate the laser current level, what is the purpose of monitor current register?
    [Nasser]: It is desired to use a constant bias current. Given temperature and laser aging bias current
    consumed by the laser may change. So it is desirable to maintain this current at the same level.

    In which cases is better to use open or close loop (OLE) ?
    [Nasser]: This is based on laser diode parameters. Please contact laser diode vendor to check on
    degradation to the again and temperature. As long as this is maintaned, then open loop could suffice.

    According to the schema which polarity I need to set (PDP) ?
    [Nasser]: This depends on the laser diode used. Please discuss with your laser diode vendor.

    We would like to review our scheme uplink and downlink - would anyone be able to help ?
    [Nasser]: Please send us a clear schematic and we can follow up.

    Questions about the 2 – Wire Digital Interface :

    1.What is the minimum and maximum frequency of SCK pin ?
    [Nasser]: Minimum data high time is 100nS. Maximum clock rate is 10MHz.Typically we use 100KHz for the I2C interface and this should give us good margin.

    2.According to the data sheet we assume that the device includes 100K Ohm Pull up resistors. Can you approve it ?
    [Nasser]: Yes indeed this is the case.

    3.Can you give up timing diagram for example with real data clk & data ?
    [Nasser]: Interface to the device is 11-bits shift register.
    1). First bit shifted out is the start bit
    2). Followed with 3-address bits(MSB first)
    3). 8-data bits with MSB first
    4). Last bit going out is the stop bit

    4.In our side we need to implement "Master " side ,Do you have an example of software code to see ? Can we base on I2C Master code ?
    [Nasser]: This is not available. This is based on a shift register structure. There are 11-bits + start, and stop shift register

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 30 points

    Hi Nasser,

    thank you for the answer.

    i can't sed you the entire schema, i did my best to send you good quality of picture.

    i have more question regarding to your answer.

    1. How the schema connection change between open and close loop in ONET4291VA?

    2. what about the connection of ONET4201VA?

  • 0 in reply to Amir Moses
    TI__Guru* 93570 points

    Hi Amir,

    By forcing OLE pin low, closed loop operation is enabled. Please note data sheet BIASC7 through BIASC0 closed loop APC coupling ratio factor as well.

    Regards, Nasser