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I am trying to match an op amp (OP27) to its data sheet while doing some stability analysis tasks in LTSpice. I am following the video provided here (http://www.linear.com/solutions/4449). However, when I follow the details of the video tutorial for the specific op amp I am interested in, I do not find the sorts of results expected based on the datasheet.

For instance, the DC gain is >140 dB and the low-frequency rolloff starts much below 1 Hz, in contrast to the Analog Devices datasheet.

Are op amp models in LTSpice generally not that accurate to real devices?

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    \$\begingroup\$ Looking quickly at an op27 datasheet, I see the DC gain is typically 124dB, and the GBW as 138dB(Hz), which should put the rolloff point at 14dB(Hz) or 5Hz. How about you put in the question what your calculations are, and what your spice observations are, as not the sorts of results expected tells us far, far less than we need to be able to comment properly. Generally, opamp models can be good or bad depending on where they come from. They often use minimums, sometimes use typicals, usually don't model noise, very much caveat emptor. And no, no-one's going to flog through the video. \$\endgroup\$ – Neil_UK Jul 7 '17 at 5:22
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    \$\begingroup\$ There are limitations to all simulators and models that need to be understood. See linear.com/docs/24958 and linear.com/docs/4139 \$\endgroup\$ – Peter Smith Jul 7 '17 at 6:56
  • \$\begingroup\$ In my experience LTSpice doesn't simulate opamps that accurately, but then again it is somewhat expected that a simulator be just a simulator. I tried to simulate an ideal integrator with bias currents using an ideal opamp and I didn't get at all what I was expecting. \$\endgroup\$ – mickkk Jul 7 '17 at 8:08
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That makes sense based on what I see in the data sheet. In the DS there is a graph of open loop gain (figure at top right of page 10) and it shows a corner frequency of about 5 Hz at a gain of about 125 dB. Now if I extended that graph to extend the dc gain a few more dB higher you would see something like this: -

enter image description here

At 140 dB, that corner frequency could be argued to be "much below 1 Hz".

The graph in the DS is typical and you could reasonably argue that there is a case for the open-loop gain being potentially higher. But it's all a moot point because nobody uses op-amps in this region without negative feedback and, in reality, who cares that much if this is a few dB out in this not-really-very-realistic-test?

Are op amp models in LTSpice generally not that accurate to real devices?

I'd argue that the op-amp model you have is reasonable and would give reasonable results in real world applications using negative feedback.

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  • \$\begingroup\$ @FelipeMedLev are you sure this comment is relevant here? \$\endgroup\$ – Andy aka Jul 1 at 15:29

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