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ONET8501V Bias Setting

Joe_B
Prodigy 145 points
Other Parts Discussed in Thread: ONET8501V

I am confused about how to correctly set the bias current on the ONET8501V. I see in the datasheet that the calculation for open loop is "PDRNG = 01 (see above); IBIAS-VCSEL" ": IBIAS-VCSEL = 86 μA × BIASC". So, If I solve this calculation, I get IBIAS-VCSEL = 86uA x 20 = 1.72mA. How would I enter this number into the registers? I guess I confused about how to convert this calculation into a binary format.

  • 0
    TI__Expert 8915 points
    Joe,

    The 86uA step size for the bias current is just an approximation based on the typical bias current when the register is at full scale (0xFF). From testing, the typical bias current is around 22mA so the approximate step size is 22mA/256 = 86uA.

    For open lop operation, bit 4 of register zero (OLENA) needs to be set to 1 and the setting for PDRNG doesn't matter. In your particular example, the register setting is 20 decimal or 0x14 (hex) or 00010100.

    As an another example, if you wanted 6mA bias current, the approximate setting is 6mA/86uA = 70dec or 0x46 (01000110).

    For a more accurate measurement of what the bias current is, the bias current monitor (MONB) should be used.

    Regards,
    Alex.
  • 0 in reply to Alex Davidson
    Prodigy 145 points
    Alex,

    Thank you. I will actually be using the device in its closed loop setting I was just starting the question off using open loop as an example because it seemed a bit more simple to start with. Could you explain to me how the PDRNG works? If I wanted 6mA of drive current in a closed loop application, what would be the register setting for this? I don't know the CR of the TOSA at this point so I understand there will be some testing required to get the results accurate. Why would I choose PDRNG 00 vs 01 vs 1x?
  • 0 in reply to Joe_B
    TI__Expert 8915 points
    Joe,

    Typically the TOSA coupling ratio is unknown and even if it is known, there is usually part-to-part variation. Therefore, in closed loop the only accurate way to know the bias current is to use the bias current monitor. For the PD range, my recommendation is to use setting 01 or 1x, not the lowest setting of 00. This is because the loop gain is high in setting 00 which may cause bias current overshoot and false triggering of the fault detection circuit. This however, depends on the TOSA that is being used.

    Alex.
  • 0 in reply to Alex Davidson
    Prodigy 145 points
    Alex,

    Thank you. I see my mistake now. I was getting confused about how the registers settings for Bias Current and Modulation current were supposed to be set because of the PDRNG and MODRNG bits, but I just noticed that those bits are stored in seperate registers. /facepalm/
    What would be your recommendation for the initial MODRNG bit settings? Thanks.
  • 0 in reply to Joe_B
    TI__Expert 8915 points
    Joe,

    I recommend leaving the MODRNG bits in their default setting of 00.

    Alex.