Timeline for LTSpice divergence between AC analysis and transient analysis for low frequency gain
Current License: CC BY-SA 3.0
15 events
| when toggle format | what | by | license | comment | |
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| May 16, 2016 at 20:16 | history | edited | Adam Haun | CC BY-SA 3.0 |
Added Mike's follow-up message.
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| May 16, 2016 at 16:53 | vote | accept | Sergei Gorbikov | ||
| May 16, 2016 at 16:46 | comment | added | Sergei Gorbikov | @Adam Haun Thanks. Below is a second reply from Mike Engelhardt (LTSpice), 7 pm Moscow time, 16 May 2016: "BTW, you can add that I'll look to see if I can improve the issue with level 3 in your case, and I do appreciate your test vector, but you should realized that absolutely every time I see a level 3 question like this, there is never any hardware involved. LTspice is about current circuit design, not digging through obsolete model files." | |
| May 16, 2016 at 16:10 | comment | added | Adam Haun | @SergeiGorbikov Neat! I've added Mike's response to the answer. | |
| May 16, 2016 at 16:10 | history | edited | Adam Haun | CC BY-SA 3.0 |
Added a response from Mike Engelhardt from Sergei's comment.
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| May 16, 2016 at 15:40 | comment | added | Sergei Gorbikov | So, as I see it, the practical conclusion from the situation is: do not use Spice Level 3 models in AC analysis (at least in LTSpice). Use newer SPICE models for AC. | |
| May 16, 2016 at 15:36 | comment | added | Sergei Gorbikov | Official response from LTSpice (Mike Engelhardt) I received several minutes ago : "You'll find most SPICE programs have trouble with level 3 AC linearization(which is what AC is reported on). I've fixed most of the problems but some remain. It's one of the reasons that level 3 was obsoleted 25 years ago. Level 3 is no longer used in IC design." | |
| May 16, 2016 at 15:03 | history | edited | Adam Haun | CC BY-SA 3.0 |
Added new plots based on comments.
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| May 16, 2016 at 14:59 | comment | added | Adam Haun | @placeholder I still see the same discrepancy (AC gain of ~10, transient gain of ~50) even with a 10nV magnitude. I'll add the plots to my answer. The DC analysis suggests the gain should be reasonably constant within a ~30mV range, but I don't know how reliable that is. | |
| May 16, 2016 at 13:42 | comment | added | Sergei Gorbikov | Adam, I’ll mark your answer as a solution to the question by tomorrow unless no one responds further. Dear Placeholder, thank you very very much for your hints re operating point. PS: I was very frustrated yesterday, when noticed that my favorite simulator produced crap. However, after you’ve helped me with this, I feel well better (: | |
| May 16, 2016 at 13:42 | comment | added | Sergei Gorbikov | 3) Intuitively, I feel that the key is the wrong operating point used for AC analysis. Note that near the midpoint the derivative of gain with respect to DC voltage is high (that is, the gain is very sensitive to OP). If we take an OP shifted from the 2.5 V midpoint (e.g. 2.3 V for DC input), then the discrepancy is smaller (1.5x/1.4x for AC and transient respectively). Unfortunately I don’t have a tool to unload the OP point used in AC, as placeholder suggested (I also tried to google it w/o success). Cont'd. | |
| May 16, 2016 at 13:42 | comment | added | Sergei Gorbikov | Dear Adam. Thank you so much for the modelling you’ve done and the well-structured answer with links. You reconfirmed the discrepancy! In summary: 1) Results of AC and transient analyses should correspond to one another. So, we have detected the inaccuracy in LTSpice numerical algorithms that lead to such a large discrepancy. From practical standpoint, such a large difference is unacceptable, imho. 2) I’ll email the bug to Linear Technology and will respond here if they reply. Cont'd. | |
| May 16, 2016 at 4:27 | comment | added | placeholder | Continuation. IF you then use a transient analysis with very small excitation signals, you may not actually shift the OP enough to cause this re-compute cycle. If that is the case, then the AC results will be the very similar to the Transient results. So I conclude by saying that it is possible that is it just the difference between AC and .Tran one cannot rule out that it may just be due to differing OP. I say this because the excitation is very small here. Of course the Posting author can get the tool to dump out the calculated operating points for comparison. | |
| May 16, 2016 at 4:22 | comment | added | placeholder | You're missing the the part about how the transient analyst actually accomplishes the modelling of nonlinear effects etc. The sparse matrix formed at the start of a transient analysis will be very similar to the one formed as part of the AC analysis. During transient analysis the small signal analysis proceeds until such time as it is determined that the OP has shifted significantly (which is parameter of simulation) as which point a new OP is calculated, a new small signal analysis is performed and new matrix is determined. This is how SPICE can time step through and model properly. Cont'd | |
| May 16, 2016 at 1:34 | history | answered | Adam Haun | CC BY-SA 3.0 |