AD848 Op-Amp offset

Question

I am using 3 similar op-amps (AD848JN) from Analog devices, tha name of the op-amps for clarification are Op-Amp #A,#B and #C.

The configuration used to check the output offset is that one shown in the schematics. I have also attached the Voltages at every pin of the Op-Amps.

The point is that I am having an output voltage different of 0 V. According to the datasheet, making use of a potentiometer between the PIN1 & PIN8 with the wiper in PIN4 (Vs-) I can reach barely the 0.0 V, however when I unplug the potentiometer the values come again to the previous ones.

Does anyone has a solution for that?

Thanks in advance.

^{simulate this circuit – Schematic created using CircuitLab}

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Answer 1

In general you should not use the offset adjust to trim out significant offsets that originate from other than the Vos of the op-amp.

If you do try it you'll most likely get excessive temperature drift of offset voltage (TCVos) as there is no temperature-stable reference voltage within the op-amp chip.

Answer 2

Your offset range on the first is pretty good -- if you need better, you need a different circuit, not an offset adjustment, which is usually used to correct smallish biases. Try a summer

For the other two, the op amp is saturated at the negative rail, and it likely has little to do with your offset circuit. Something is likely wrong with your connections.

I can't tell more without a real circuit diagram.

Answer 3

use the opamp like this

^{simulate this circuit – Schematic created using CircuitLab}