I want to include to my design an 'variable offset adjustment' circuit using an operational amplifier. This circuit would allow me to ajust the offset of my design. In my circuit, I already has a buffer (non-inverting unity-gain, voltage follower) in my input, so, I think maybe I can use this op-amp with a potentiometer to ajust the voltage offset level. The thing is that I have heard about this 'variable offset adjustment' circuits, but I don't have any idea of how they look. If I look it on the internet I just found op-amp in which both inputs are connected to GND and that does not make sense to me (and would be against my idea of using the existing buffer). How can I do this? I don't think if it is better to do it in the voltage followers or in the differential amplifier. I don't know if I have this opportunity in the "offnull" pins, the datasheets does not give any clue (maybe this means, that the package has no offset pins?). Would a better solution be to change the part numbers for zero-drift (zero-offset) amplifiers? Would I need offset adjustment in both parts? I need a correction of milivolts.
EDITED:
Why this design would not work properly according to an answer to this question?
EDITED 2:
In order to understand the discussion and to save a final solution:
Best Answer
The 741 is not a very good op-amp. Also you should not use the offset adjust pins on an op-amp to adjust anything other than the offset of that particular op-amp since you'll otherwise likely be introducing temperature drift.
The first thing is to identify where the zero offset that you want to trim out is coming from. If it's from the amplifiers, you can use amplifiers with lower offset voltage or so-called "zero-drift" types. If it's coming from elsewhere you should estimate the maximum possible offset and figure out how stable that offset adjustment has to be. The more stable it has to be, the more expensive it will be. For example, you might have to create two stable precision references (say +/-2.5V) for the trimpot and buffer the wiper voltage with another op-amp.
In the case shown, if you replaced the three op-amps with a good instrumentation amplifier such as INA849 you could could simplify it a bit. Maximum room-temperature input-referred offset is 35uV so with a gain of 100 you'd have +/-3.5mV at the output with no adjustment. This one is not the "zero-drift" type so it lacks the weirdness at the inputs (transient current spikes).