[volt-nuts] HP 3456A Input Impedance Check

[ed breya]

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