I've used this circuit as an input stage for a digital oscilloscope. From left to right, the first op-amp provides high input impedance, the second op-amp is used to add DC offset to the signal (which should have no offset), the third op-amp is used to amplify/attenuate the signal.
The problem is when I put a voltage source of 1V on the bottom input of the summer op-amp (the second one) to add a DC offset of 1V to the signal, a DC offset of 2V is added instead. Why does this happen and how can I fix it?
You can view and edit the circuit simulated here.