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While building an opamp based non-inverting amplifier for my project I noticed that it always has some considerable output voltage despite having the input at zero. To verify that problem I built the following simple test circuit:

enter image description here

I would imagine that the output voltage of this circuit would be very close to zero, however it's about 5 mV. According to the datasheet of OPA189 the input offset is only 3 uV, so not sure why am I getting 5mV output considering so low gain of only 2? In fact based on my tests, the output offset voltage doesn't even depend on the gain value. I tested another op amp ADA4522, and this one has similar problem with more than 7 mV output offset. Could anyone give me pointers on why this could be happening and what can be done to lower the output offset to 1 mV or less? Thanks!

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    \$\begingroup\$ Could you consider the idea of using a negative rail voltage for the opamp rather than ground? Just to see what happens? (Not because I'm telling you this is the way you have to use it. I just want you to experiment for a moment and see what does happen when you try that. To be clear, I want you to still tie the (+) input to actual ground, not the new negative rail.) \$\endgroup\$
    – jonk
    Aug 18, 2020 at 20:31
  • \$\begingroup\$ Thanks for the hint, looks like I will need to use negative supply voltage to overcome this issue \$\endgroup\$
    – Isso
    Aug 19, 2020 at 0:53
  • \$\begingroup\$ Yes. As the answer you did select discusses in more detail, the problem was simply that the output cannot quite reach the negative rail. Even though it is a rail-to-rail output, it's not really 100%. Very good. But not exact. This was the first thing I checked on the datasheet -- (1) was it rail to rail? and (2) how close can it get? But before I looked I already knew the problem because no opamp is that good -- enough to meet your mental expectations of perfection. I wanted you to think about it, though. Because when you get the answer yourself it sticks better and this one wasn't hard. \$\endgroup\$
    – jonk
    Aug 19, 2020 at 0:58

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If you check the datasheet, there is a section called "Voltage Output Swing from Rail" This is because although it is advertised as rail to rail, it can't actually hit the bottom rail. So there is a slight offset. This datasheet says 5mV offset without a load, which is the exact offset you are getting.. enter image description here

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  • \$\begingroup\$ Thanks a lot, appreciate your quick help! Does anyone by any chance know an op amp that has less than 5mV of Voltage Output Swing from negative Rail? \$\endgroup\$
    – Isso
    Aug 19, 2020 at 0:23
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    \$\begingroup\$ You can search online for "Single Supply" op amps. Some have a "trim option" to help eliminate any voltage output swing \$\endgroup\$
    – JohnnyMac
    Aug 19, 2020 at 1:46

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