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LTspice convergence problems with MCP6N11

Discussion in 'Circuit Simulation & PCB Design' started by tomizett, May 17, 2018 at 1:44 PM.

  1. tomizett

    tomizett Active Member

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    Hi All,
    I admit this is a bit of a lazy question, but it may be easy for someone.
    I'd like to simulate the Mircochip MCP6N11 instrumentation amplifier, but I'm having no luck. Originally I was getting no convergence at all, with the "Timestep too small" error; I've added the .OPTIONS you'll see in the file (recommended as a good all-round fix on the LTwiki site) and the simulation now converges, but proceeds extremely slowly - only ever gets to a few hundred picoseconds.

    The file doesn't say it was written for LTspice, so it may not be completely compatible. To be honest, I'm not desperate to use this model - but I'd like to use this part, and if I can use the model then I'll be able to simulate the entire circuit... which would be quite nice.

    So if there are any LTspice experts out there who'd care to take a look, I'd be interested to hear from you.
    Thanks!
     

    Attached Files:

  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    That model doesn't work well for me either :(. Locks up after ~80pS.
     
  3. tomizett

    tomizett Active Member

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    In that case I'm beating you :p... I think mine got past 200ps before I gave up on it.
     
  4. dave miyares

    Dave New Member

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  5. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Try "Skip Initial operating point solution" (.uic) in the Transient Simulation window and see if that makes a difference.
     
  6. tomizett

    tomizett Active Member

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    Thanks, but I've tried that. It does get into the .tran simulation and starts plotting data, but it's incredibly slow - something is clearly not right.
    Like I said, it's no big problem I don't think. I'll try to build a functional model with something like the right CMRR.

    ...or just pony up for an LT part, which will simulate fine...
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Usually a slow simulation means something is oscillating at a high frequency.
     
  8. dave miyares

    Dave New Member

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  9. tomizett

    tomizett Active Member

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    There's no sign of it on the output - just a monotonic drift downwards, about 60pV in 450ps (and then I stopped the sim - it was managing abut 1ps/s simulation speed).
    If it's oscillating, then it's doing so on one of the internal nodes, and I don't have the skill or inclination to work out why!
     
  10. eTech

    eTech Active Member

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    Hi

    Microchip models are known to simulate poorly in LTspice. I wouldn't waste my time trying to make it work. Use an equivalent LT part.

    eT
     
    • Informative Informative x 1
  11. alec_t

    alec_t Well-Known Member Most Helpful Member

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    ... or you could assume the instrumentation amp is perfect and model it as a simple BV source.
     
  12. tomizett

    tomizett Active Member

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    Useful to know - it's not just me then.
    It's not critical to the project to model the instrumentation amps - I know that the majority of the errors will be coming from elsewhere. I can model the common and differential mode gains with voltage-controlled-voltage-sources and add a realistic output impedance. That will be good enough.

    Thanks for the input.
     

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