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ONET2591TA: Clarification on figures in datasheet

Mitchell
Expert 4915 points
Part Number: ONET2591TA


Hi Team,

My customer and I have some questions on the ONET2591TA that I'd like to get your clarification on:

  1. We'd like to get a better understanding of the small signal transimpedance vs the transimpedance dependent on the average filtered bias current. How does small signal impedance change with respect to average bias current?

  2. We are also assuming that Figures 10,11,12 and 13 are based on small signal transimpedance. Can you please confirm that this is correct?

    Tying this to example eye diagrams (Figure 12/13) is a bit more confusing since 1mAppk -> 200mVppk suggests transimpedance of 200, 2mAppk -> 200mVppk suggests transimpedance of 100. The datasheet is not clear about how the gain is limited. Is gain limiting working here?

  3. In figuring out the differential output mVpp, does Figure 5 matter here?

    Small signal transimpedance (2000~3200ohm) * modulation current (I_input 1mApp) gives 2~3.2Vpp.This looks like the work of gain limiting, but then we are having trouble understanding why it has such a wide range of 140mVpp~310mVpp as MAX diff voltage.

Thanks,
Mitchell

  • 0
    TI__Expert 8680 points
    Hi Mitchell,

    I think I can best answer your question by explaining the architecture of the ONET2591TA. The device has a small linear range amplification range where the gain is set by the transimpedance value, but it then will limit once the output voltage reaches a certain value. The exact transimpedance and output limiting value vary somewhat based on process and temperature, which is why we have the min and max values in the datasheet. When looking at figures 10 and 11, they show the amplifier in its linear range where the output it not hitting the limit. Figure 10 is using a very small input signal of only 10uApp so the output is also quite small at about 20mVpp. Figures 12 and 13 show the output in the limited state where it doesn't increase in voltage even though the current is increased from 1mA in figure 12 to 2mA in figure 13.

    Please let me know if you have any further questions.

    Regards,
  • 0 in reply to Jacob Freet
    TI__Expert 4915 points
    Hi Jacob,

    Thanks a bunch for the explanation. This definitely helps clear things up.

    Give your feedback, there's some fear about how my customer is setting their bias current and modulation current for their VCSEL TX diode.

    VCSEL bias current (assuming past lasing threshold), should provide DC illumination which would eventually translate to avg_current on the TIA after hitting photodiode. Since figure 5 maps transimpedance vs avg_current, they were hoping to get a snapshot of where they are in terms of signals getting squashed given certain modulation current settings. But since small signal impedance is ranging from 2000ohm to 3200ohm, I guess we are using just the small signal transimpedance for cases not hitting the limit regardless of avg_current.

    If this is the case, what is the response of AGC with huge bias_current (avg_current)? How does it saturate?

    Thanks,
    Mitchell
  • 0 in reply to Mitchell
    TI__Expert 8680 points
    Hi Mitchell,

    They should be able to get the signal monitoring information from the RSSI pin. As far as the actual signals through the amplifier, this part features a DC bias cancellation so it will try and remove some DC signal from the input. And as you've also mentioned it additionally has an AGC which will lower the gain as the signal input increases by just changing the transimpedence gain resistor internally using a MOSFET. They will want to limit the signal input to 2mA because after that point the input will become overloaded.

    Does that help to clarify the operation?

    Regards,