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Problem with low current measurements.

Models 2601, 2602, 2611, 2612, 2635, 2636
Models 2601A, 2602A, 2611A, 2612A, 2635A, 2636A
Models 2601B, 2602B, 2604B, 2611B, 2612B, 2614B, 2634B, 2635B, 2636B
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Mikael Bjork
Posts: 5
Joined: January 11th, 2017, 6:04 am
Country: Sweden

Problem with low current measurements.

Post by Mikael Bjork » May 3rd, 2021, 6:51 am

Hi, We have a Keithley 2635B that we uses to adjust and calibrate our own instruments with. To be able measure we added a box between the Keithley 2635B and the object under test. I have attached a schematic image of the connection. The problem is that there seems to be a small current that gets added to the measured value for the object. To get away from this current I have added an USB-isolator between the computer and the object. this seems to remove this current, which makes me think there is a ground loop or something like that. But that USB isolator is causing communication problems. so we need to get rid of that.

we use the connection to the chassis ground in the Phoenix connector. when I remove that connection. we do not get any measurements.

In the box between the Keithlay and the object we have put a 5 M ohm resistor. Without that we get very poor measurements on low values (pA). I think that resistor is causing problems. before we got the 2635B we used a Keithley 263 and that worked very well without that resistor and the connection to the chassis.

I do not know much about this so I hope I can get some help.

Best regards
Mikael.
Attachments
Connection to the object under test.pdf
(124.95 KiB) Downloaded 3 times

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