U12 - Analog input reading not stable

4 posts / 0 new

Thu, 02/23/2017 - 11:51

rghuggins

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

Thu, 02/23/2017 - 12:01

LabJack Support

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.

Thu, 02/23/2017 - 12:39

rghuggins

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.