« Reply #3 on: November 30, 2016, 09:04:38 am »
Good points. I will have to try this with my KE236. It can resolve pA and can source +/- 110V. I do not have an electrometer.
Electrometers are nice when you want the current measure on the LO side. SMUs measure current on the HI side and can create hazardous voltages on the guard.
The input impedance is the voltage divided by the input bias current. The input bias current is most likely fixed. You can characterize the input bias current by plugging a 10M resistor into the input and allowing the instrument to tell you what the bias current is.
This is discussed on other threads in this forum.
In most cases the input current will vary with the input voltage and with temperature. The only way to characterize it properly is to measure it directly with an electrometer over the whole input voltage range. I did it for my two Keithley 2015 meters and the input current was -15pA at 0V, -95pA at +10V and -7pA at -10V for one meter and +12pA at 0V, -120pA at +10V and +100pA at -10V for the second unit (the specification is <200pA).
Cheers
Alex
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working instruments :Keithley 260,261,2750,7708, 2000 (calibrated), 2015, 236, 237, 238, 147, 220, Rigol DG1032 PAR Model 128 Lock-In amplifier, Fluke 332A, Gen Res 4107 KVD, 4107D KVD, Fluke 731B X2 (calibrated), Fluke 5450A (calibrated)