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ONET4211LD: BIAS current source in open loop is oscillating

Jean-Luc Bolli
Prodigy 160 points
Part Number: ONET4211LD

Dear support team,

I would like to use the BIAS current source as a fixed DC current source, i.e whithout using the regulation loop. The resistor connected to the pin IBMAX should set the value of  the current, according to fig.15 from data sheet.

To have this functionnality, I have done the following steps, according to data sheet:

  • connected APCSET pin to VCC with a 100 kOhm resistor
  • let PD floatting.
  • connected a 220n capacitor between pin CAPC and GND
  • set i.e a 22kOhm resistor between pin IBIAS and VCC to get about 15 mA on BIAS

Unfortunately, I experience an oscillation on the BIAS current at a few MHz with this set-up. Removing the capacitor on CPAC do not change things.

Can you recommend an other set-up to operate the BIAS current source in open loop?

  • 0
    TI__Guru* 91790 points

    Hi Jean-Luc,

    Apologize for the delay.

    In this case, what value are you using for RMODSET? Can you please try a value between 3.1K to 6.3K ohm and let us know the result.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 160 points

    Hello Nasser,

    We do not use a resistor for pin MODSET, but instead we sink a current ranging from 0 to 380 uA, controlled by a DAC. This work very well. It seems to be not related to the oscillation of the BIAS current loop. Do you mean that the value set for IMOD has an influence on the IBIAS current source in open loop? We also do not use a laser diode at all. The ONET4211LD is used as a pulse generator, whose output current and thus amplitude is controlled by the current of pin MODSET.

    Thus, can you explain in details what could lead the BIAS current source to oscillations?  I am an analog IC circuit designer, so do not afraid to speak transistor level langage!

    Regards, Jean-Luc

  • 0 in reply to Jean-Luc Bolli
    TI__Guru* 91790 points

    Hi Jean-Luc,

     For open-loop operation we need RMODSET but i suppose DAC would do the same. 

    Based on the earlier evaluation or schematic, it seems there could be some instability in the APC loop under certain conditions. Given this, we are recommending a Cap between laser diode anode and bias pin. Perhaps this is happening in the open-loop mode as well. However, i don't know the root cause of this instability . Please note attached file: 

    Regards,,NasserONET4201LD_BiasFilter_0v1.pdf

  • 0 in reply to Nasser Mohammadi
    Prodigy 160 points

    Hello Nasser,

    Thank you for this document, which give a path to improve stability. We have already try to add a 220nF capacitor CBIAS-FIL exactly as indicated in the document. Its effectively stopped the oscillation, but unfortunately distorted also the shape of the pulse waveform on the MOD+ signal, due to the loading of the RF bead in series with CBIAS-FIL.

    I suppose also that this problem is corrected on the chip version we are working on, buyed recently! It would certainly help a lot if you could provide a more detailled bloc diagram of the APC system, naturally without going into confidential details Wink.

    Regards, Jean-Luc

  • 0 in reply to Jean-Luc Bolli
    TI__Guru* 91790 points

    Hi Jean-Luc,

    This part was developed long time back and this is the only document describing this issue. 

    Regards,, Nasser