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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: 6
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
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Andrea C
Keithley Applications
Keithley Applications
Posts: 1565
Joined: October 15th, 2010, 10:35 am
Country: United States
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Re: Problem with low current measurements.

Post by Andrea C » May 9th, 2021, 9:49 am

Mikael,
For your object under test, what do you mean by guard?

In the case of the 263 calibrator, you were connecting an isolated and floating signal source to your object under test. In the triax, that product made use of center pin and inner shield for HI and LO.

To have the equivalent from the 2635B, you'd remove the jumper on the green Phoenix to float the SMU and then have your converter box would have two triax: for for HI and one for LO. Inside the box, route them to your object under test.

For the resistor part, I'm not sure. If you are forcing I with voltage limit, make sure the I*R drop on the series R is accounted for in the voltage limit. For low pA, the 2636B is likely to be much faster responding product vs. the 263. Turning on some digital filtering and increasing the NPLC might allow you to have more similar outcomes.

Mikael Bjork
Posts: 6
Joined: January 11th, 2017, 6:04 am
Country: Sweden

Re: Problem with low current measurements.

Post by Mikael Bjork » May 11th, 2021, 9:37 pm

Yes, that worked, Thanks. I could remove the ground connection in the phoenix connector. and this made it possible to remove the USB-isolators. :D

I still need to use the resistor. we started using it because the object measured a noisy signal. and for the low signals the noise went below zero, making the objects measurements to be below the true value.


best regars
Mikael.

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