LTSPICE runs Transient Analysis Correctly but not AC Analysis with an Imported OPA172,Help!

Discussion in 'Programmer's Corner' started by stetin, Jan 15, 2016.

  1. stetin

    Thread Starter New Member

    Jan 15, 2016
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    I downloaded the OPA172.LIB from TI, attached it to the OPAMP2 symbol. It runs transient analysis correctly but when I run AC analysis, it appears to not work correctly. I checked various places in the circuit, and even the negative terminal of the op-amp is incorrect with the AC analysis.
    From another thread I downloaded an OPA170 model that was said to have simulated correctly and it does the exactly same thing. If I use one of the TI op-amps everything works OK.
    I have not seen this before where the transient analysis works correctly but not the AC analysis. I am tweaking values in a MFB low pass filter and need the AC analysis to work.
    Can anyone shed light on this?
    Thanks
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If you post all your files, I bet someone will try it.
    Please add what it does wrong.
     
  3. stetin

    Thread Starter New Member

    Jan 15, 2016
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    It behaves as if the op amp is not active, just a passive component that does not amplify but the transient analysis works exactly as it should.

    I am uploading the .asc file and the OPA172 file. Since the forum won't allow uploading a .SUB file, I renamed the OPA172.LIB file to OPA172.txt and called that out in the .asc file. It still runs the same with the different file extension.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
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    Here is what I got. Is this what is expected?
    Clip_3.jpg
     
  5. stetin

    Thread Starter New Member

    Jan 15, 2016
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    No, you are getting what I got.

    If you look at the circuit and assume zero frequency, where all the caps essentially are zero pF and drop out of the circuit, the circuit turns into an inverting op-amp circuit with a gain of negative one.

    The sweep should start at zero dB and then hit the -3dB point at about 1KHz.

    This starts at -30dB and then does stuff, but not what it should. If you substitute an LT op amp, I used a LT1217, it works correctly.

    Thanks for running it.
     
  6. stetin

    Thread Starter New Member

    Jan 15, 2016
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    OOPs, I mean the caps go to infinite impedance, not zero pF. At infinite impedance, they essentially drop out of the circuit.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It may have something to do with the pin out. Or maybe the model is no good.
     
  8. stetin

    Thread Starter New Member

    Jan 15, 2016
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    I double checked the pin out and it is valid. I ran an operating point analysis and that came out correctly.

    If you change the SPICE simulation to transient, you will see that it works perfectly. I have verified that it meets what I measure on the bench, even to the little overshoot this particular op amp has with fast rising and falling edge input pulses.

    It may be the model, but my question is, what is wrong with it? What could affect only AC analysis, and nothing else?

    I need to get this resolved as it is important to some documentation I am publishing.

    Thanks for looking at it.
     
  9. ifixit

    Distinguished Member

    Nov 20, 2008
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    I can't get it to work either. Even if I add 30db of gain, it still doesn't have a 0db output. A call to TI may be in order.

    Pin order seems okay and works in trans mode.

    Changing R1 to -15K causes the output to start at 0db, but the rest of the response shape changes a little as well. I don't know what this means other than something is weird with the AC part of the model. The DC part is okay. It amplifies 1MHz okay.

    I'll look at the model a bit more. It is a complex one... perhaps too much so.
     
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