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Original question:

ONET2591TA: ONET2591TA input referred noise

ONET2591TA: ONET2591TA input referred noise

Patrick Lepage
Prodigy 50 points

Part Number: ONET2591TA

The datasheet of the ONET2591 does not mention a maximum input capacitance in the operating conditions.  We may have to use an APD with 2.0pF capacitance.  I have a few questions:

1- Is stability of the TIA guaranteed with a 2.0pF APD ?

2- What bandwidth can be achieved ?

3- Can you provide an estimate of the input referred noise with a 2.0pF input capacitance ?

Thanks,

Patrick Lepage

  • 0
    TI__Mastermind 28785 points

    Hello Patrick,

       Would you be able to share the specific end equipment for this application? I ask because the ONET2591 wasn't designed to be used with APDs; since APDs usually require high amounts of reverse bias voltages, and the cathode must be connected to the FILTER pin or else the internal AGC will not function properly. This device was designed for applications within ethernet and fiber channel data communications. If you can provide the bandwidth and input current range you need, I can suggest transimpedance amplifiers that might fit better for you application. One device to look at would be the LMH32401. 

       The ONET2591 was tested at 0.6pF. The photodiode capacitance won't affect bandwidth in this case, but rather will determine the overall stability. More explanation can be found at this thread.

    Thank you,

    Sima  

  • 0 in reply to Sima Jalaleddine
    Prodigy 50 points

    Hello Sima,

    I plan to use a separate power supply to bias the APD, and connect 200pF to GND at the FILTER pins.  Of course the APD anode will be connected to the IN pin.  I understand from the datasheets that the AGC and RSSI will not function properly, and that the TIA will always be at maximum gain.  But the AGC will not be used for our application anyway.

    Thanks for your answer about bandwidth and stability of the TIA with higher capacitance photodiodes.

    Patrick Lepage

  • 0 in reply to Patrick Lepage
    TI__Mastermind 28785 points

    Hello Patrick,

       Yes, that is correct. The AGC works by lowering the internal transimpedance feedback resistance. So, if unconnected, you are left with the constant max gain. Feel free to reopen the thread by replying for further questions!

    Thank you,

    Sima