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I'm reading this datasheet about the LM358P op-amp. I'm trying to understand what is the output voltage swing from rail. I suspect it is the amount of voltage that the op-amp will deviate from the rails if it tries to reach the max. output voltage.

That is, if the output should be 4 V according to the op-amp's ideal equations, but VS is 5 V, the actual output will be 5 V - OVSFR ≈ 3.5 V (having that OVSFR is the output voltage swing from rail, which in the above datasheet is 1.5 V). Is this correct?

According to the above datasheet, the output voltage swing from the negative rail would be of the order of millivolts instead. This means that this op-amp would work better (from this point of view) in an inverting configuration. Am I right?

Note: As you probably have noticed, I'm reading the table corresponding to the LM358, not the LM358P, which is the one I have. I do this because there is no data for the LM358P specifically, so my guess is that the LM358 will be similar.

What I have tried:

According to the above datasheet, part 7.7, the LM358 has a voltage swing from positive rail of 1.5 V @ VS = 5 V RL ≥ 2 kΩ.

I have replicated those conditions in my workbench with a non-inverting op-amp configured for a gain of 2 with an input of 2 V. That would be 4 V, however it stays at ~3 V. That is approx. the 5 - 1.5 = 3.5 V I was expecting. I'm assuming that error is caused by not-very-good-quality equipment. In addition, I have slowly increased VS to 9 V and have seen the output voltage going up as expected, so I guess that matches my initial guess.

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  • \$\begingroup\$ Yes it is how close the output can get to the rails when outputting max or min \$\endgroup\$
    – DKNguyen
    May 12, 2021 at 19:44
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    \$\begingroup\$ Real-world measurement electronics.stackexchange.com/questions/432775/… and some "theory" e2e.ti.com/blogs_/archives/b/thesignal/posts/… \$\endgroup\$
    – G36
    May 12, 2021 at 19:44
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    \$\begingroup\$ But then if the table says 1.5V under conditions xyz, that means that you can get V_S - 1.5V under those conditions, right? \$\endgroup\$
    – Martel
    May 12, 2021 at 19:45
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    \$\begingroup\$ @Martel Note that the conditions are also at the top of the table like Vs, not just in the table. I make it a general rule to not push the limits on the output unless it is rail to rail because swing specifications for non rail-to-rail outputs always seem to be spottier and less well defined (you need to extrapolate what is there a lot of the time). \$\endgroup\$
    – DKNguyen
    May 12, 2021 at 19:53
  • \$\begingroup\$ Driving into a 2k-ohm load matters, too. \$\endgroup\$
    – TimWescott
    May 12, 2021 at 22:13

1 Answer 1

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The comments and this other post explain it very well.

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