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U12 - Analog input reading not stable

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rghuggins
rghuggins's picture
U12 - Analog input reading not stable

Hello,

I assume that someone has already asked this question, but I can't find an answer in the forum. I am trying to measure a 0-5V single channel input using a Labjack U12. When measure either using LJlogger or Labview the analog input voltage fluctuates widely, often between -1V to 3V.

When I connect a multimeter to AI0 and ground the multimeter reads a stable number that scales with the instrument.

What am I doing wrong here?

Thank you, R

LabJack Support
LabJack Support's picture
When you have LJlogger

When you have LJlogger showing readings from "0 SE", the U12 is taking readings of AI0 versus GND.  So if you have a DMM taking readings from AIN0 versus GND at the same time it is looking at the same thing.  About the only possible difference I can think of is that the U12 takes its reading over perhaps 1 ms whereas the DMM might be averaging over 500-1000 ms, so it could be the signal is actually varying but the DMM is averaging it out.

If you have a scope try putting that on AI0 to GND.

Confirm the LJlogger readings are varying all over when a DMM is connected to AI0/GND at the same time and showing a stable value.

Try some other signals.  You can jumper AO0, +5V, or GND to AI0 to confirm it reads as expected.

rghuggins
rghuggins's picture
Thank you. That was a simple

Thank you. That was a simple fix.

FYI: the instrument was a paddle-wheel flowmeter from McMillan company. I previously tried running the signal through a VI to calculate a moving average but it didn't seem to work. I didn't think of increasing the size of the array used to calculate the moving average while reducing the sampling frequency to the millisecond range.

So a sample frequency of 1 mS combined with a moving average of 60,000 data points seems to do the trick.

LabJack Support
LabJack Support's picture
Burst or Stream mode might

Burst or Stream mode might work good for that technique.

https://labjack.com/support/datasheets/u12/programming-reference/aiburst

https://labjack.com/support/datasheets/u12/programming-reference/aistrea...

Typically with a sensor like this you would use the Counter on the U12 to count pulses over time to determine frequency.

https://labjack.com/support/datasheets/u12/hardware-description/cnt

https://forums.labjack.com/index.php?showtopic=1105