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WEBENCH® Tools/OPA657: Transimpedance simulation error on Webench

Stani Bell Ngoufack Tchoufack89
Intellectual 1970 points
Part Number: OPA657
Other Parts Discussed in Thread: OPA656, TINA-TI

Tool/software: WEBENCH® Design Tools

Hello,

i have been try to perform a transimpedance simulation in the WEBENCH® Amplifier Designer tool but this hastn't been successful. I get the notification that i should relax my contraints. This happens only when i take my current from 2uA up to 2mA. The values ae enter in the tool are my actual systems contraints. 

I would like to know if there is anything i can do, to be able to simulate with currents in the mA range. Also i would like to know if there is the possibility to do some noise simulation on the OpAmp in this WEBENCH Designer.

Thank you.

  

  • 0
    TI__Expert 7875 points
    Hi,

    I think the WEBENCH Designer is giving you the error with 2mA input current because working solution at the required gain and peaking values that you chose is likely resulting in a feedback loop that causes the amplifier to become unstable. The OPA657 is a decompensated amplifier and need to be in a high frequency gain of about 7 V/V to maintain stability. In transimpedance configurations, the high frequency gain naturally occurs from the combination of the feedback capacitor, feedback resistor, and input capacitance. However, sometimes the desired parameters will limit the feedback network to the point where it cannot find a stable solution.

    For your circuit, I would suggest trying the OPA656 instead. It is a unity gain stable amplifier similar to the OPA657 but at slightly less bandwidth. However, that shouldn't be an issue for your target circuit bandwidth of 50 kHz.

    For noise simulation, I would suggest building your design in our TINA-TI simulator using the corresponding device model file.

    Regards,
  • 0 in reply to Jacob Freet
    TI__Intellectual 1970 points
    Hello Jacob,

    thank you for the reply. For noise simulation i will rely on TINA-TI.
    The Webench simulation i made was not directly related to the OPA657. What i did was to select "Transimpedance" as a topology, then clicked on "start Design", once in the Webench designer i selected the "positive reverse bias" then clicked on "start amplifier design".
    The page i shared in first post opened and i modified the the Vbias = 6V and IpdMax = 2mA, then clicked on create design and got the answer "No solution found. Please relax constraints".
    Since it is not directly related to a particular device, is there a way i can still run my simulation with the 2mA?

    thank you.

    Best regards,
    Stani
  • 0 in reply to Stani Bell Ngoufack Tchoufack89
    TI__Expert 7875 points
    Hi Stani,

    I'm going to go ahead an move this over to support from our WEBENCH tools team to answer your question. I'm not familiar with exactly the algorithms WEBENCH uses to to calculate what solutions exist. If you have a particular amplifier design in mind, please feel free to create another thread and we can assist with any questions.

    Regards,
  • 0 in reply to Jacob Freet
    TI__Expert 3482 points

    Stani and Jacob,

    We will look into this and get back to you ASAP.

    Regards,

    Alfred

  • 0 in reply to Alfred Chong
    TI__Expert 3482 points

    Stani and Jacob,

    After looking into the selection criteria for this topology, here is the explanation why no part is returned for the given design spec.

    Majority of the part is excluded because they do not meet the minimum gain where amplifier is stable.

    The calculated AC Noise gain = (1+Cin/Cf) is 1.01. This value is then compare to minimum gain where amplifier is stable (Avcl).

    Where, AC_NoiseGain >= Avcl. Amplifier Designer is very conservative and it adds margin to the requirement.

    The equation with margin is (AC_NoiseGain/1.5) >= Avcl.

    Since majority of the amplifiers have Avcl to be 1 and the requirement is 0.6733. Hence, no part meets this selection criteria.

    I'm regret to inform you that If no part meets the design spec, we can't proceed to the simulation page.

    Regards,

    Alfred