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OPA657 Problem

Other Parts Discussed in Thread: OPA657, OPA656, OPA355, TINA-TI

I'm posting in this forum because the OPA657 problem affects anyone who attempts to simulate a high speed amplifier with an OPA657. The problem is with the macromodel but if this is posted there, it will not be noticed by anyone who is doing circuit design.

The OPA657 macromodel is missing its input capacitance! This gives very misleading results when simulating a high speed transimpedance amplifier, for example. The differential + common-mode capacitance is specified in the OPA657 data sheet as 0.7pF + 4.5pF but measuring it in a TINA simulation results in about 0.24pF.

I haven't checked but the OPA656 may have the same problems. Perhaps others...

There are consistent problems with most of TI's macromodels-- ditto for other vendors, too. Many macromodels omit important parameters such as voltage & current noise (and even if they do, the 1/f spectrum is missing), input capacitance, settling time, supply current, and on and on.

The only TI macromodels that I have confidence in were written by Bill (W.K.) Sands; these models are very complete and are as accurate as practical. Most models were written by a variety of engineers that were not modelling experts so the performance of simulations with these models vary widely. Without consistently well-written macromodels, a designer cannot have confidence that his circuit simulation is truly representative of the device's actual performance. Macromodels hurriedly written by amateurs to satisfy managers who pressure the engineer to "finish the project so that it can be introduced" is not the way to approach such an important part of a new device's introduction.

I contend that having a comprehensive, accurate macromodel for a new device is as important as having a clear, accurate data sheet.

  • This was originally poster in the High Speed Amplifier Forum for the reason explained in that post:

    "I'm posting in this forum because the OPA657 problem affects anyone who attempts to simulate a high speed amplifier with an OPA657. The problem is with the macromodel but if this is posted there, it will not be noticed by anyone who is doing circuit design."

  • Neil,

    I am a modeling engineer in Analog eLab. 

    Thank you for the feedback on the OPA657 Spice model.
    Just so you know, your inquiry was moved to the eLab Simulation & Models forum since your inquiry was about the model and not the device itself.

    You are, of course, correct in your comments about the model's input capacitance.
    This is a critical parameter for many high-speed amp applications, especially a transimpedance amp.

    All of us in Analog eLab also agree with your observation that an accurate, high quality macromodel is as important as a well-written data sheet.

    We are constantly improving our methods. Revising and updating our models and tools is part of that process.

    I can't make a steadfast promise as to when the OPA657 model will be revised.  I will check with the product team to get their feedback and we will proceed from there.

    Thanks again for your comments.

    Regards,
    John

     

  • Thank you for your reply, John. The OPA657 desparately needs correcting and upgrading.

    I'd suggest that you study the OPA355 macromodel and use it as a guide to what TI's models should be.

    The fact that TINA-TI macromodels vary from excellent to pitiful reveals that no one is managing this effort.

    If Rich were an engineer instead of a salesman, he might take things like this seriously.

  • This missing OPA657 macromodel input capacitance problem still has not been corrected.