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OPA855: Gain Stage

Mark Philip Felipe
Genius 9705 points
Part Number: OPA855
Other Parts Discussed in Thread: TLV3604, OPA695

Hi Team. Would you also recommend the OPA855 as gain stage following the initial TIA OPA855, or is there an alternate part that you would propose?

Thank you in advance.

-Mark

  • 0
    Guru 140200 points

    Hi Mark,

    can you show a schematic of your circuit?

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 15555 points

    Hello Mark,

    Can you also help provide your input characteristics and what overall gain you are trying to achieve?

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 9705 points

    Hi Hasan, I apologize for the delay in response. Please refer below for the details

    Input is a "digital" signal, as this is for a (research) communication application. The photodiodes themselves have roughly 10fF of capacitance, the responsivity of approximately 0.7A/W, will produce photocurrents on the order of 1-4uA, and the shape of the input current to the initial TIA is simply a binary pulse train. Not to concerned with gain, as my initial idea was to design a TIA with sufficient BW to adequately capture the fundamental of this input signal (OPA855, possibly two), then square its output using a high-speed comparator (similar to the example application in your TLV3604 datasheet). 

    Thank you very much.

    Regards,

    Mark

  • 0 in reply to Mark Philip Felipe
    TI__Genius 15555 points

    Hello Mark,

    I'm confused about the details here, if gain is not a concern then why add a following OPA855 stage after the TIA? Also what is the bandwidth or rise time that is needed & what is the desired output range? Can you provide a schematic of the OPA855 TIA stage?

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 9705 points

    Hi Hasan, I apologize for the delay of response, according to the information. 

    The second OPA855 would be there to provide input of sufficient amplitude to the following comparator if necesarry; They would design the OPA855 to achieve ~650MHz of BW, which will limit the max transimpedance they can get to a couple kOhm, so output will be a few mV at these input currents. In terms of output, preferably CML or LVDS. Schematically it is very similar to the TLV3064 example optical receiver application.

    Regards,

    -Mark

  • 0 in reply to Mark Philip Felipe
    Guru 140200 points

    Hi Mark,

    to prevent me from having to guess please give us a complete schematic :-)

    Kai

  • 0 in reply to Mark Philip Felipe
    TI__Genius 15555 points

    Hello Mark,

    The OPA855 can be used as a following stage, although with a needed bandwidth of 650MHz your gain will be limited to about 12 V/V. If more gain is needed and DC precision is not a main concern, a current feedback device such as the OPA695 can be used. These devices allow for large gains without compromising the bandwidth.

    Best,

    Hasan Babiker