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ONET4291VA: Clarification on datasheet

Mitchell
Expert 4915 points
Part Number: ONET4291VA


Hi Team,

My customer and I have some questions on the ONET4291VA 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__Guru* 93570 points
    Hi Mitchell,
    First of all, ONET4291VA converts modulation current into voltage into a differential load(close to 100 ohm). Figure 5 shows bias current register setting versus actual bias current monitor read back. Or figure 11 shows linearity of modulation current setting versus actual modulation current.

    2). Figure 13 shows 5mA modulation current into 100-ohm differential scope module. At 5mA and 100ohm, we have about 500mV. Figure 13 shows single ended eye diagram and this is why you see about 250mV peak to peak. By the same token, figure 14 shows 10mA into 100-ohm load (i.e scope) and there we have 100X10 or 1V differential or 500mV peak to peak single ended(figure 14).

    3). Agreed. In figuring out differential output mVpp figure 5 does not matter.

    Regards,,nasser