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Voltage Reading Disconinuities


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#1 frustrated technition

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Posted 27 March 2013 - 02:29 PM

I have a heater circuit that consists of a power supply, heater, difference amplifier, current limiter and resistor. I'm using the labjack U6 to measure the voltage across both the heater and the resistor. I use the drop over the resistor to find the current and thus can find the resistance of my heater (and infer its temperature). The current limiter is toggled to slowly ramp the current from 0 to 500 mA. The issue I am facing is that every single test run (excess of 15 tests) there is a discontinuity in both the current and voltage plots whenever the measured voltage crosses from 0.39 V to 0.40 V and when it crosses from 1.04 to 1.05 V. This introduces an error in my resistance calculations that is untenable. My first impulse was that the Labjack AIN gain was switching and giving some slightly different offset. I therefore specified the resolution and range for each input (res = 8, range = 0 or +-10V). However this did not fix the problem. Any ideas what's going on or how to fix this? Thanks in advance Gordon Side notes: The difference amplifier is used to step the voltage reading over the heater to something usable by the labjack (0-2 V) I'm using python to interface with the labjack

#2 LabJack Support

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Posted 27 March 2013 - 03:21 PM

That does not raise a flag as a common problem. Can't think of anything in particular about those voltages. Can you do some sort of test with a different signal to see if anything interesting happens at the same voltages? You could use DAC0, or use a potentiometer.

#3 frustrated technition

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Posted 28 March 2013 - 08:48 AM

That does not raise a flag as a common problem. Can't think of anything in particular about those voltages. Can you do some sort of test with a different signal to see if anything interesting happens at the same voltages? You could use DAC0, or use a potentiometer.


I did the test with both a lab power supply as well as the DAC0 input and noticed the same issues (discontinuities at 0.4 V and 1.05 V). I actually have 2 U6's so I tried all the tests with the second labjack. None of these issues were present. As such, the issue is with the analog inputs of the initial U6. Now the question is, can this be fixed?

#4 LabJack Support

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Posted 28 March 2013 - 08:53 AM

What exactly happens at these voltages? What is the discontinuity? Do you see the problem with our software? Try LJLogUD or the test panel in LJControlPanel. If you see the same thing, email [email protected] as we probably need to look at the device.

#5 frustrated technition

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Posted 28 March 2013 - 01:46 PM

What exactly happens at these voltages? What is the discontinuity?

Do you see the problem with our software? Try LJLogUD or the test panel in LJControlPanel. If you see the same thing, email [email protected] as we probably need to look at the device.


The discontinuity is a sudden 4 mV change from the expected behaviour, and is not a problem with the software. I'm sending an email to the support, thanks for your help


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