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