The schematics: enter image description here

It should convert 0-3.3V input from the DAC to -4.1 + 4.1V (roughly)

The positive part works great, the negative output voltage saturates at -2.95V.

Is it ok? The simulation looks ok (and it is the expected behavior): enter image description here

The part: https://www.st.com/content/st_com/en/products/amplifiers-and-comparators/operational-amplifiers-op-amps/standard-op-amps/tl084c.html#design-scroll


the negative output voltage saturates at -2.95V.

Yup. If you look at Figure 2 of the data sheet, it shows a nominal maximum for +/- 5 volt supplies to be 6 volts pk-pk. In other words, about +/- 3 volts, and you're getting -3. Just about what you'd expect.

The positive side is doing better than expected, but that's just an unexpected bonus.

TL;DR - You should never count on getting +/- 4.1 with your power supply and this op amp.

  • \$\begingroup\$ Yes . Thanks. The positive part is actually described in the DS as almost RR. So the output will be -2.5 to 2.5V \$\endgroup\$ – P__J__ Jun 23 '18 at 23:26

You should not only rely on simulations only1. Check the datasheet.

That shows:
On a +/-15V supply the output swing is between +/-10 and +/-12.

For running on +/5V select a low-voltage rail-to-rail opamp.

1In LT spice I recently found an opamp swinging between +/-220V on a +/-20 V supply

  • \$\begingroup\$ I took the model from the STM website :) \$\endgroup\$ – P__J__ Jun 23 '18 at 21:03
  • 1
    \$\begingroup\$ @PeterJ_01 They usually still don’t behave correctly at clipping or even close to the rails. \$\endgroup\$ – winny Jun 23 '18 at 21:51
  • \$\begingroup\$ There are lies, damn lies, and SPICE models... -- Disgruntled Engineer \$\endgroup\$ – Richard the Spacecat Jun 23 '18 at 22:27
  • 1
    \$\begingroup\$ Spice is reasonably good at telling you something will NOT work, but take anything it says works with a pinch of salt. In particular spice opamps are all too often really three terminal models (and real ones have at least 5 pins, which should hint at the problem), any model that references spice node 0 should get the stink eye. \$\endgroup\$ – Dan Mills Jun 23 '18 at 22:51

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