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2612 incorrectly measuring current under 1mA range

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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edbordin
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Joined: December 12th, 2012, 3:48 pm
Country: Australia

2612 incorrectly measuring current under 1mA range

Post by edbordin » December 12th, 2012, 4:13 pm

I'm developing a testing platform using a 2612 SMU to perform various characteristic measurements on transistors. I've started by attempting to verify the Drain current-vs-Gate voltage curve for a commercially available n-MOS using LabTracer.

As you can see in the attached image, there is major discontinuity in the graph which appears to happen when the instrument switches measurement range.

Relevant test parameters:
NPLC = 10
Auto zero = auto
Measurement range = auto

After seeing this behaviour running firmware 1.2.0, I upgraded to 1.3.4 and saw no improvement. It is worth noting that the instrument has not been calibrated since 2007 (probably at the factory), but the discontinuity in the data seems too extreme to be simply explained by poor calibration. On that note, what are Keithley's guidelines for calibration? The calibration due date would suggest that it is ideally performed annually, and I would like to advise if this instrument should not be trusted (I know for a fact that is has been relied upon very recently to take measurements - worrying given it would seem to be partially faulty).
Attachments
LabTracer_output.PNG
Two forward voltage sweeps exhibiting the same behaviour
LabTracer_output.PNG (42.56 KiB) Viewed 24668 times

Dale C
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Re: 2612 incorrectly measuring current under 1mA range

Post by Dale C » December 13th, 2012, 8:24 am

I would recommend running the same test with a resistor.
This way you could verify if the Model 2612 is operating properly.
Use the same parameters and choose the resistor to give about the same current measurement values.

edbordin
Posts: 3
Joined: December 12th, 2012, 3:48 pm
Country: Australia

Re: 2612 incorrectly measuring current under 1mA range

Post by edbordin » December 13th, 2012, 4:26 pm

I used a 100 Ohm resistor to measure an I-V curve with 1000 points and as you can see in the attached plot, the line does not pass through the origin so it appears that it probably is a calibration issue. Would you agree? I'm waiting on a quote to get the instrument calibrated.
Attachments
100 Ohm I-V.PNG
100 Ohm I-V.PNG (15.56 KiB) Viewed 24639 times

Jane S
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Re: 2612 incorrectly measuring current under 1mA range

Post by Jane S » December 14th, 2012, 12:24 pm

Hello Ed,
It will be great if you can post the result after calibration of the same devices tested.

Jane

Andrea C
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Re: 2612 incorrectly measuring current under 1mA range

Post by Andrea C » December 17th, 2012, 4:49 am

Hello,

One other aspect you might check would be the steady state performance of the instrument for these lower current ranges rather than having it sweep. You could put the 100 ohms back and just manually operate the the instrument such that the lower current measure ranges are in use.

If the ranges are working well under steady state, then you may be facing a transient response type issue that calibration alone will not resolve.

Andrea

edbordin
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Joined: December 12th, 2012, 3:48 pm
Country: Australia

Re: 2612 incorrectly measuring current under 1mA range

Post by edbordin » December 17th, 2012, 8:44 pm

Thanks for your suggestion Andrea. I was intending to be making steady-state measurements but perhaps slightly misunderstood what the Voltage Sweep function does. Either way, I was originally using a custom LabView program to apply continuous voltages to the transistor, stepping the gate voltage periocally and waiting 1s to allow for settling. I still got this behaviour. I've cross-checked the transistor current measurements in the region where the graph "drops off" and found a discrepancy of ~3-4mA when compared to an old Keithley 2400 unit, which would suggest that the 10 mA measurement range is returning values which are much too high. However, I also took some resistor current measurements with autorange disabled and got straight lines without any indication of such an offset. So I now suspect the unit is faulty. We are probably going to send it to the Keithley Singapore office for a calibration and if necessary, a repair.

Jane S
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Re: 2612 incorrectly measuring current under 1mA range

Post by Jane S » April 16th, 2013, 8:48 pm

Check the response after the unit is back from repair if the range change during the 1000 pts sweep has some effect assuming you use autorange with reasonable delay between points.

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