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I have a doubt regarding an opamp non inverting amplifier. I am using a dual opamp AZ4580MTR. One opamp is configured as a non inverting amplifier having gain 2 and the other is a voltage follower. The input of the opamp is connected from a DAC AD5675. When I reduced the input to 0v, then the Output is about 11v. If a any other input voltage like 1 or 2v applied to the input, it gives the correct output(as per the gain). How this this circuit is behaving like this, if I changed the opamp to any other part will it works fine or work like this? Waiting for a reply. Regards Sebastian

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    \$\begingroup\$ What is the opamp's power supply? Please link a datasheet for the opamp, and while you're at it, find the opamp's input common mode range. \$\endgroup\$ – replete Jun 28 '17 at 12:34
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    \$\begingroup\$ 'Waiting for a reply'? If you think that's a more polite way to say 'hurry up you lazy so-and-sos, I NEED an answer NOW!', then it isn't, really, downvoting for that alone. \$\endgroup\$ – Neil_UK Jun 28 '17 at 13:19
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  1. Google datasheet.

  2. Look at page 6, find "Input Common Mode Voltage Range ±12V (with ±15V supplies)." This means input common mode must be at least 3V away from power rails.

Opamp is not rail to rail.

  1. If you supply this opamp with a single supply (ie, 0V/+15V) and not with ±15V then obviously its input common mode range will +3 ... +12V and not ±12V .

  2. Your 0V signal is not inside the interval +3 ... +12V therefore the opamp does not work as intended.

Use the right rail to rail opamp.

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  • \$\begingroup\$ Even then, the phrase "rail-to-rail" and "single-supply" are not magical ("It's a trap!"). Always look carefully at the datasheet for both input and output CMVR for your supply range for every type of amp you use. \$\endgroup\$ – Chris Knudsen Jun 28 '17 at 14:18
  • \$\begingroup\$ Yes, Rail to rail can be output only, or input and output, plus the current that can be handled by the output transistors as they are squeezed against the rails is often not that much. \$\endgroup\$ – peufeu Jun 28 '17 at 17:30
  • \$\begingroup\$ Even worse, "Single-supply" just infers that the input or output (or both) signal range includes one of the rails. "Which one?" Look in the datasheet. ...and even then, not all the way. \$\endgroup\$ – Chris Knudsen Jun 28 '17 at 21:07
  • \$\begingroup\$ Yes. I replaced "use a rail to rail opamp" with "the right rail to rail opamp!" (choosing the right one is usually a bit of a headache as you say!) \$\endgroup\$ – peufeu Jun 29 '17 at 7:18

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