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Incorrect Simulation from TINA-TI??

Mike O
Prodigy 20 points

Other Parts Discussed in Thread: TINA-TI, UA741, OPA277

Would someone please review this simple op-amp circuit, which should be a non-inverting Schmitt trigger, but which TINA-TI is showing as an inverting amplifier: http://img42.imageshack.us/img42/4968/nonamerc.jpg

The basic circuit is taken from Wikipedia at http://en.wikipedia.org/wiki/Operational_amplifier_applications#Schmitt_trigger .  Nominal values were selected for R1=R2=20 kOhms, and Vcc = +/- 10V. Input to the circuit is a 5 kHz sine wave with amplitude of +/-4 V.  TINA shows the circuit has an output exactly equal to the negative of the input, which is not expected.

Thanks,

Mike

  • 0
    TI__Expert 5515 points

    As the Wikipedia article notes, the switching thresholds are ±(R1/R2)Vsat, where Vsat is the maximum output of the op amp. With ±10V supplies, the uA741 model sets this just above 8V, and with R1=R2, your threshold is therefore just above 4V - and your signal reaches exactly 4V. Change R1 to 2k instead of 20k, and you'll see something more like what you're expecting.

    I'd note that this isn't a very reliable way to build something with predictable thresholds, since the Vsat will vary from op amp to op amp. But for a quick 'n dirty approach, or if you're only building one, it's acceptable.

    You know, TI makes a whole line of Schmitt trigger parts ;-)

  • 0 in reply to Rick Downs - WEBENCH Design Center
    Prodigy 20 points

    Thank you very much for taking the time to look at this issue.

    However, with R1=R2 and a +/-10V supply, the switching thresholds are around +/- 8V, not 4V.

    So, with an input of only +/- 4V (i.e., an input below the thresholds), the output of the circuit should have remained pegged at either +8V or -8V, depending on startup transients.

    But, as simulated by TINA, the circuit does not show this behavior.  Rather, TINA shows an output like that of an inverting amplifier, which is wrong.

    So, the question is: Is TINA showing an incorrect output (I think it is), and if so, why?

    The question is pertinent since it goes to the trustworthiness of a TINA simulation.  For this simple circuit, TINA seems to have failed.  If TINA fails for a simple circuit, is it rational to trust its output for more complex circuits?

    Your thoughts are very much appreciated.

  • 0 in reply to Mike O
    TI__Expert 5515 points

    Mike,

    You're right, the thresholds would be 8V - which means that the comparator should never switch, because as designed, that circuit is not then a comparator. Put the thresholds where they should be and it behaves like a comparator. As the original article noted, R1 is often not even used, relying only on the source impedance of the input (which might only be a few 10s or 100s of ohms).

    If you did this with an ideal op amp, this would indeed be an inverting amplifier - no current flows into the inputs, and the + and - inputs are the same, so you have a virtual ground and the output would then be -Vin. So your question is, does TINA give the right results? My question is: does the op amp macromodel used give the right results? Many macromodels are very simplistic and would certainly simulate the way we see this behaving.

    Try replacing the ua741 with an OPA277 model, and run a transient analysis:

    Now reduce R1 to 2K, so it actually is a comparator:

     

     

    The concern you have is with the model, not the simulator.