Keithley 220 behavior

[jrmymllr]

I have a trash picked 220 that is in great condition and seems to work fine, other than a damaged triax connector that I replaced very carefully following another thread on this forum. Just playing with it, I noticed something that I can't explain. If I connect the output (center pin and inner shield) directly to my ammeter, the output is nearly right on. From 1uA to several 10s of mA it's very close. Down to 100nA also seems ok, and maybe 10nA too but that stretches the capability of my AM-140A.

I thought I'd try a 1M resistor on the output, and at 1uA output that should measure 1V out, right? Well I know Ohm's law and 1V is what it should be. But the 220 isn't mustering any more than about 0.7V, and at 10uA it's only getting about 3V out. I've measured the 1M resistor, and measured the current using the meter, and it's simply not generating enough voltage to reach the current output. V-LIMIT is set to 5V, but increasing to even 20V makes no difference.

At the other extreme, using a 100 ohm, I see around 10V for 100mA and 0.01V for 100uA, which seems right.

I don't believe I'm exceeding any specifications. What am I doing wrong, or is there something wrong with this thing?

[gamalot]

How do you measure the voltage drop across the 1MOhm resistor?

[jrmymllr]

How do you measure the voltage drop across the 1MOhm resistor?

Just with the DMM....but now I realize my error with regard to voltage measurement! It's so easy for me to forget the meter draws something from the DUT. I have resorted to using the DMM to measure current through the 1M instead and it's better, but still not as accurate as if the DMM is directly across the 220 output. For example, at 5uA output, directly across the meter is something like 5.02uA, but through the 1M is 4.6uA. That doesn't change even when raising V-LIMIT.

[The Soulman]

Not sure how you are connecting things but you must not forget your multimeter has an input impedance of (roughly) 10 mega ohms, if you're measuring voltage across
a 1 mega ohms the combined resistance is now 0,90909 mega ohms. Run your calculations again en see what that tells you.

[The Soulman]

Or measure the output "voltage" directly with your dmm.
Keep in mind your dmm already has a 10 mega ohm "shunt", also note that that 10 mega ohms isn't really precise.

[jrmymllr]

Or measure the output "voltage" directly with your dmm.
Keep in mind your dmm already has a 10 mega ohm "shunt", also note that that 10 mega ohms isn't really precise.

Someone reminded me of that above and I somehow seem to forget this fact every once in awhile. Now to determine why adding a load resistance makes my ammeter measure a different value.

[Zucca]

Also for very low currents it is normal to see the voltage creeping up in a resistor, because:
Ohm warm resistor > Ohm cold resistor