R340 is not open, I just triple-checked it.
I took a multimeter with more digits and better precision and I measured the input pins again (in the original configuration):
Pin 3 (non-inverting): +0.0073mV
Pin 4 (inverting): +0.0473mV
Output: -50.2mV
Having reconsidered the effect of the bias current, as some of you guys pointed out, I actually believe the limits for the output should now be +/-84mV. So the output is indeed within limits, but I would have expected to see the effect of the input bias current at the input pins themselves. At such a large output voltage, I would have expected to see some 0.77mV actual difference between the two input pins, which I do not see.
Changed R340 to 180R and R341 to 10k, so as to keep their ratio constant, and measured again:
Pin 3 (non-inverting): +0.0076mV
Pin 4 (inverting): +0.045mV
Output: -2.7mV
In this case the limits are +/-14mV. So well within limits in this case.
If I just reconfigure it to a voltage repeater (R340 removed and R341 short-circuit), I get:
Pin 3 (non-inverting): +0.0085mV
Pin 4 (inverting): +0.05mV
Output: +0.05mV
In this case the limits are +/-0.125mV. So well within limits in this case.
Why is the effect of the input bias current not visible on the input pins, in the case of the original configuration?
Best regards,
Cristian