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LTSpice: Slow on noise sims with foreign opamps

Discussion in 'Electronic Design' started by Joerg, Jan 4, 2013.

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  1. Joerg

    Joerg Guest

    Folks,

    Yahoo has made the sign-in procedure so onerous (can't get in anymore)
    that I'll ask here instead of the LTSpice group:

    I want to do some noise simulations, for example with a TI opamp in
    there. Loading the complete model it spends forever in the stepping
    phase, then pseudo-damped analysis and whatnot.

    Is there any way this can be sped up? All I need to see is ballpark
    where the 1/f knees will be with various circuit constellations, input
    current noise, input voltage noise, stuff like that.
     
  2. Joerg

    Joerg Guest

    I did ID myself. But now they slapped on a stupid "captcha". Which
    promptly resulted in "Can't identify the machine bla-bla-bla, followed
    by a challenge question to which they cannot have the answer.

    IOW, they are on the path of oversophisticating the Yahoo groups to death.

    But can you probably can't ask questions via that.

    I don't do noise analyses much, that's usually done on the bench here
    but in this case it should be sim'd.

    OPA4140.

    [...]
     
  3. Joerg

    Joerg Guest

    Tried numerous times, always the captcha. Those things are nasty but
    after four tries it let me past the virtual bouncer. Wot a pain.

    I know they do. But so far I haven't figured out a way inside without
    that dreaded Yahoo GUI. So where's the back door and how many knocks?
    Essentially I've plopped the whole Texas model in there. Right now
    supplied +/-5V, for the test stripped back to a follower with just 10k
    OUT to IN-, IN+ to 0V (ground).

    With any LTC models it zips right through even with complicated stuff
    around them, as usual.
     
  4. Joerg

    Joerg Guest

    I've got the circuit working, it's just that it doesn't seem to like
    Texas models. No, no, not those :)
     
  5. Joerg

    Joerg Guest

    Kind of my hunch, but I hadn't thought it was this bad :-(

    Burr-Brown used to be a pretty good bunch of engineers, real hands-on guys.

    Looking at the bright side it did manage to heat my office by one degree
    Fahrenheit over lunch.
     
  6. Joerg

    Joerg Guest

    I'll have to include it in a measurement set-up and play around with
    component values to find a good compromise. Possibly they have included
    something that LTSpice does not like. I doubt cockpit error because
    various other opamps run blink-eye speed here as well. Plus I did not
    have a sip of the Fin du Monde ale yet, that's for later tonight.

    From a noise point of view I think we can do a tad better than the 4140
    anyhow. It's mostly up to a coupole hundred keelohoitzes in this case,
    no RF stuff.

    Thanks for testing it. In case you want to try: When you hang a 10k in
    the FB and feed a current into IN-, then runs .NOISE, does it run on
    your sim? If yes then it's highly likely that the model doesn't fit LTSpice.
     
  7. Joerg

    Joerg Guest

    Then I get this:

    Circuit: * E:\Simulations\PGL\PosSense_AC_1.asc

    Expanding empty subcircuit: opa4140
    WARNING: Less than two connections to node V+. This node is used by V1.
    WARNING: Less than two connections to node V-. This node is used by V2.
    Direct Newton iteration for .op point succeeded.

    Date: Fri Jan 04 17:14:43 2013
    Total elapsed time: 0.047 seconds.

    tnom = 27
    temp = 27
    method = trap
    totiter = 6
    traniter = 0
    tranpoints = 0
    accept = 0
    rejected = 0
    matrix size = 17
    fillins = 0
    solver = Normal
    Matrix Compiler1: 32 opcodes
    Matrix Compiler2: 967 bytes object code size
     
  8. Joerg

    Joerg Guest

    Nope. This was the error message LTSpice let off.
     
  9. Joerg

    Joerg Guest


    Maybe because I leave myself logged in until it throws me off. Usually
    lasts a few weeks. I really don't like log-in hassles. Why can't they
    just set a cookie and be done with it?
     
  10. Joerg

    Joerg Guest

    I always turn cookie on when I look onto the, it's sitchable.
     
  11. Jasen Betts

    Jasen Betts Guest

    If you want to play with TI psrts perhaps try the free TI spice program
    "TINA"
     
  12. miso

    miso Guest

    Usually if spice is choking, there is something wrong with the circuit.
    Now you can't float single nodes in spice, but you can have nets that
    should be tied to something that might be floating.

    I would look carefully at the connectivity of the subcircuit.
     
  13. Joerg

    Joerg Guest

    Nothing in this case because LTSpice stopped. Plus it normally never
    opens such message windows unless it is unhappy with something. The
    usual waveform window never opened and that's a clear sign that it ain't
    happy.
     
  14. Joerg

    Joerg Guest

    Well, I don't want to learn a new CAD everytime a vendor thinks they
    absolutely have to boil their own proprietary broth. The stuff I do with
    it often gets to be complicated and that would take a long learning
    curve every time.
     
  15. Joerg

    Joerg Guest

    But no workie.


    So how come other opamp model work fine with exactly the same setup?
     
  16. Joerg

    Joerg Guest

  17. SoothSayer

    SoothSayer Guest


    Parasitics not properly included in the faulty engine?
     
  18. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Hey Joerg, are you the Bear of spice ?-)
     

  19. I am not sure many know just how funny that is.
     
  20. Joerg

    Joerg Guest

    Ursus simulatorus? :)
     
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