U3 and LJLogUD v1.20 | LabJack

U3 and LJLogUD v1.20

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U3 and LJLogUD v1.20

I am confused trying to setup LJLogUD for the U3-HV. I am trying to log a signal with maximum amplitude: +/-1V pk-pk.Most often the signal is +/- 0.1 V pk-pk. Sample interval is 100 ms. I'm not sure what to set Resolution to but I believe with the U3 it doesn't matter.

I connected the signal to AIN0 and GND.

In LJLogUD, I set Channel 0 +Ch to 0 and -Ch to 32 and Ranges to LJ_rgBIP1V.

I used an oscilloscope to confirm the signal has both positive and negative values, but when connected to the U3-HV the scope shows a dc component on the signal approaching 1 V. This dc component is not present on the signal source itself. Also, the plot in LJLogUD shows only positive values and the logged data has only positive values.

What am I missing in the setup?

Thanks for helping,


LabJack Support
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You almost always use

You almost always use Resolution=0 with the U3.  See the description of "Resolution" on the LJLogUD page for more info:


The Range parameter does nothing with the U3.  The high voltage channels on the U3-HV have a normal range of +/-10V, and you can get the special -10 to +20V range by setting the -Ch=32.  In your case the normal +/-10V range is what you want so leave -Ch set to 199 which means a normal single-ended reading.


So why are you not seeing the signal you expect?  First, connect the oscilloscope and U3 to the signal at the same time so the oscilloscope is showing you the voltage across AIN0 & GND.  Second, what is the frequency of your signal, as you want to sample perhaps 20x the frequency or more (might need to use LJStreamUD).