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OPA858: Device model in PSpice for TI does not model the disable function

James Rapka
Prodigy 30 points
Part Number: OPA858
Other Parts Discussed in Thread: OPA855, LMH5401, SN74LVC2G66-Q1, LMH34400

I wouild like to use PSpice for TI to simulate a circuit using the OPA858 and OPA855, but the models are missing the disable function which we need to simulate. Are there models available with the disable pin modeled?

  • 0
    TI__Genius 17671 points

    Hello James,

    There are not alternative models which have disable functionality modelled.  The shutdown/disable/power down functionality of amplifiers is not typically modelled.  The information in the datasheet regarding input & output impedance, as well as voltage thresholds and timings for disable, provide enough information to understand the use of a disable action.  Please use the SPICE model to simulate the characteristics and behavior of the amplifier when 'enabled'; for disable, use the datasheet information.  There is not much performance or information to simulate when an amplifier is disabled, so the ability to simulate a high impedance node is not provided.  

    If you have questions about understanding disable functionality, I would be happy to answer your additional questions.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 30 points

    Hi Alec,

    Thanks for the quick response. Actually there are amplifier models that model the disable function. The PSpice for TI models for the LMH34400 and the LMH5401 model the disable function. In our situation the disable feature is something that we need to simulate to assess the behavior of the system. I'll probably use an analog switch such as the TI SN74LVC2G66-Q1 to simulate the disable feature of the OPA858 and OPA855 amplifiers.

  • 0 in reply to James Rapka
    Prodigy 30 points

    I have a related question. The OPA858 datasheet says that when the /PD pin is low that the output is in the high impedance state. In the power down state, is the output pin still connected to the feedback pin of the device?

  • 0 in reply to James Rapka
    TI__Genius 17671 points

    Hello James,

    My apologies; I declined to include the LMH34400 in my answer as the ambient light cancellation circuit 'disable' did not come to mind.  I am looking into how the disable/power down function is provided in the LMH5401 model.  I am also taking a cursory look at the analog switch you mentioned; I do believe you could reasonably implement a circuit to 'model' the impedance characteristics of the 'disabled' amplifier.  I have not done this myself, so it will be an adventure to see what we come up with.  Please keep me updated.

    As for your second question, I will look into the specifics to confirm.  Please allow for a bit of time to check with members of the team.

    Thank you.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 17671 points

    Hello James,

    I am still working on what my recommendation for modeling power down on the OPA858 would be; however, I have an answer to your second question:

    The connection between OUT and FB within the OPA858 is a metal-layer connection; it is physical and simple, without switches or methods of breaking the connection during PD.  Please consider and anticipate the amplifier to perform in ways which align with the understanding OUT and FB are physically shorted within the device, with a selected implementation on the metal layer of the device.

    Best,

    Alec