On Wed, Mar 12, 2025 at 02:59 PM, DerekK wrote:
I am trying to work out some old schematic and circuit issues and am looking for a model for the old LM121 opamp. Anyone have such a thing?
Remember that LM121, LM221, and LM321 are the same, just with different worst-case ratings.
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There is something that claims to be an LM321 SPICE model, here:
It is a text file.? You can rename it if you feel like it.
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There is a test schematic that uses it, here:
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All you need is LTspice's built-in "opamp2" symbol, with the name changed to match that of the .SUBCKT model, and add a ".lib" or ".inc" command to include the model itself.? It's easy.
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CAUTION:? That model is actually for Maxim's LMX321, which is a low-voltage version of the LM321.? I don't have a LM121/LM321 datasheet handy, but I suspect the original was not a low-voltage op-amp, like the LMX321 is.? So, caution is called for.
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Turning now to the PSpice model at the previously-referenced T.I. webpage for their LM321LV - it is not an encrypted model, so chances are good-to-excellent that it works in LTspice.? Most PSpice models are SPICE, and LTspice understands SPICE and most PSpice models quite well.? Forget about all that Orcad stuff.? The .lib file is the SPICE model so it is the only one needed.? Once again, use the "opamp2" schematic symbol.
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The TINA model there is not SPICE, so don't try that in LTspice.
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Now I wonder whether an LM321LV is a suitable replacement for the LM121/LM321.? The "LV" in the part number suggests that it is not.? It is indeed a low-voltage op-amp, so it might not work on your old schematics.
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Can you tell us, what made the LM121 unique?? It's been so long....
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Andy
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