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Why Keithley 2400 Ohm measurement changes with source voltage

Models 2400, 2401, 2410, 2420, 2425, 2430, 2440, 6430
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gyhe123
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Why Keithley 2400 Ohm measurement changes with source voltage

Post by gyhe123 » August 24th, 2016, 12:37 pm

Hello,
I'm using Keithley 2400's Manual ohms measurements to measure a constant resistor of 2700 ohms, but the result decreases as the source voltage I set goes below 0.5 volts. When I use 0.1V its reads only 1900 ohms which is not right. I set the current compliance high enough so there is no clamping.
I also tried to use source current to measure the resistance, it's not right either. However when I use another multimeter to measure the voltage drop of the resistor and divide it by the source current of Keithley, I get the correct resistance value.

brian.d.smith
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Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by brian.d.smith » August 25th, 2016, 5:56 am

Are you using remote sense (4-wire)?
Are you using auto-ranging for the current measure?
When was your 2400 last calibrated?

gyhe123
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Country: United States

Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by gyhe123 » August 25th, 2016, 2:53 pm

brian.d.smith wrote:Are you using remote sense (4-wire)?
Are you using auto-ranging for the current measure?
When was your 2400 last calibrated?
I tried both 2-wire and 4-wire, no big difference.
I'm using manual range since I want to set different source value to measure resistance.
I just tried to do Front panel calibration, but end up with "ERROR: source offset data invalid". I'm not sure what to do next.

Andrea C
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Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by Andrea C » August 26th, 2016, 4:05 am

You may have some gross errors in the voltage sourcing/measuring.
When having the 2400 source voltage, can you use your DMM to validate the sourced value? How well is the 2400 outputting the 100mV?
Try turning on the source readback.
Which current measure range are you selecting (100uA)?

gyhe123
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Joined: August 24th, 2016, 12:03 pm
Country: United States

Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by gyhe123 » August 26th, 2016, 7:05 am

Andrea C wrote:You may have some gross errors in the voltage sourcing/measuring.
When having the 2400 source voltage, can you use your DMM to validate the sourced value? How well is the 2400 outputting the 100mV?
Try turning on the source readback.
Which current measure range are you selecting (100uA)?
At present, if I use the 200mv,100uA range, source voltage 100mv, I actually get 138mv in my multimeter. For source voltage 200mv I get 238mv. Do I need to set an offset value or do some kind of calibration?

Andrea C
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Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by Andrea C » August 29th, 2016, 5:16 am

Is there any possibility that the REL feature is enabled and is applying the 38mV offset? The front panel would have the REL annunciator illuminated if yes.

You could try putting the instrument back into Factory Default (pg 1-16 in 2400 User Manual)
Menu
SAVESETUP, press enter
Select GLOBAL SETUP Menu, select RESET, press enter.
Select BENCH, press enter.

But in general, 38mV of error in the source value is a huge error and makes me think there is possibly a hardware malfunction. I do not think that calibration can correct for such a large offset error.

gyhe123
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Country: United States

Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by gyhe123 » August 29th, 2016, 1:08 pm

Andrea C wrote:Is there any possibility that the REL feature is enabled and is applying the 38mV offset? The front panel would have the REL annunciator illuminated if yes.

You could try putting the instrument back into Factory Default (pg 1-16 in 2400 User Manual)
Menu
SAVESETUP, press enter
Select GLOBAL SETUP Menu, select RESET, press enter.
Select BENCH, press enter.

But in general, 38mV of error in the source value is a huge error and makes me think there is possibly a hardware malfunction. I do not think that calibration can correct for such a large offset error.
No, the "REL" annunciator is off.
I set it back into default, but the error remains.
I managed to get another Keithley 2400 device here and this new one gives no such errors. What if it's really a hardware error?

Andrea C
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Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by Andrea C » August 31st, 2016, 10:38 am

I'd try a calibration first, but the amount of offset is probably too large to be adjusted away. Repair is most likely needed.
You can request a quote: http://www.tek.com/repair

gyhe123
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Re: Why Keithley 2400 Ohm measurement changes with source voltage

Post by gyhe123 » September 1st, 2016, 3:12 pm

Calibration operation cannot fix this. It only shows "ERROR: source offset data invalid".
I'm requiring a quote for repair. Thank you for the suggestions.

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