You should include the effects of input bias current - the maximum
should be specified, and likely in the pA range at room temperature.
Just put a very high resistance from input to common, and read the
voltage to calculate the current at zero input. Likewise, you can
connect the resistor to various voltages to see how it changes. This
can only be done in ranges that don't have attenuation, where the
input goes directly to the DC amplifier.
If you do your original experiment again, but with the DC source at
opposite polarity, I think you'll find the readings will be quite
different - the voltage may even read higher than what you apply. The
bias current isn't necessarily constant - it can change with input
level and especially with temperature, and it can change polarity,
depending on design.
Ed
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