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Original question:

ONET1151L: Photodiode Current Measurement Formula

ONET1151L: Digital MONB and MONP resolution

Philipp Mittendorfer
Prodigy 65 points
Part Number: ONET1151L


Hi,

could you please provide me the formula for the digital BMF and PMF fault register threshold settings?

In my setup I am e.g. testing the following for BMF in open loop:

BIASPOL=1 => 1/70 => Ibias=0.12mA*ADC=17.87mA

BIASC=200, MODC=0, OLENA=1

When I lower BMF I get the laser turned off when BMF reaches 46 and can be turned on again at 47 by toggling ENA.

How can I relate the 8bit BMF=46 to the measured Ibias by the 10 bit ADC?

And is the difference between the BIASC=200 set Ibias=200*102uA=20.4mA and the measured 17.87mA normal?

In my setup I am also testing the following for PMF in closed loop:

BIASPOL=1,PDRNG=01, MODC=0,OLENA=0

APC enabled and set to 1mW laser power by tuning BIASC

Ipd=2.6uA*ADC=410.7uA

When I lower PMF, I get a laser power fault at 61 and am able to turn the laser on again at 93.

Can you provide me the formula to compare both and an explanation for the large difference between turn off and turn on values?

Is the PMF threshold e.g. triggered by an overshoot of the laser power when turning on again?

Cheers

Philipp

  • 0
    TI__Guru* 93570 points

    Hi Philipp,

    I am assuming you have DMONP and DMONB enabled when reading PMF and BMF and external resistor on MONB is removed. Further, MONPFLT is set to 0.

    Concentrating on the open-loop case, can you please let us know ENA and DIS pin settings when reading ADC? To get accurate reading we need ENA to be set while DIS is reset. It is my understanding that BMF and PMF should reflect programmed fault currents. 

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 65 points

    Hi Mohammadi,

    you are right DMONP and DMONB bits are enabled and no external resistors are utilized.

    MONPFLT is set to 0 to use the digital version ...

    ENA is set when reading the ADC ... as I am also doing this during normal operation for DDM readout.

    DIS pin is pulled low by hardware like on an SFP module socket - to reactivate I toggle ENA.

    Yes, BMF and PMF should be the programmed fault levels for the bias current monitor and laser power monitor diode current.

    I would like to use the hardware to implement a fast extra layer for eye safety ...

    As far as I understood BMF and PMF are independent of the ADC operation?!

    So I do not need to read the ADC to trigger those faults?! 

    At least I could trigger both errors when lowering each value independent of the ADC setting.

    I guess you have some kind of comparator built in the chip fabric that triggers once the threshold is reached.

    However the formula for the calculation of the threshold is not given like for the ADC ...

    For BMF this also must be dependent on the photodiode current range PDRNG[1:0] setting.

    Cheers

    Philipp

  • 0 in reply to Philipp Mittendorfer
    TI__Guru* 93570 points

    Hi Philipp,

    I am on a business trip at the moment. Your understanding is correct ADC is just a read out and not needed to trigger the fault. There is internal comparator that checks and monitors bias and mod current and then sets the fault condition. So even if you don't read the ADC fault gets set.  I had checked and could not find a formula. 

    Regards , Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 65 points

    Hi Nasser,

    do you have any additional documentation on the chip besides the datasheet and reference schematic?

    Cheers

    Philipp

  • 0 in reply to Philipp Mittendorfer
    TI__Guru* 93570 points

    We have optical module EVM and we can share this document with you. Please send me a friend request and we can send you this document.

    Regards,Nasser