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Changing (Decreasing) Values in LJLogUD.exe as more # Channels are Added

applications channels floating values unexpected readings

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#1 Heckler511

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Posted 20 February 2014 - 04:55 PM

Hello Labjack!
 
I've been using my Labjack U6s for a couple of months now and I'm starting to get the hang of it (I have two, will be getting more soon).  I agree with the reviews that I've read, it's an excellent product.

 

I've been observing a behavior in the LJLogUD.exe application that I don't know how to explain.  Please have a look at the attached pictures.

 

I have an Omega thermistor attached to the AIN0 and the GND of the Labjack.  I also have a jumper from the 200uA current to the AIN0.  The first picture shows the voltage reading I would expect to get, around 1.75V.    

 

Things remain normal as I increase the amount of channels (please see pic 2). 

 

However, after increasing the # Channels value above 4, the voltage reading for AIN0 immediately drops down to around 0.35 volts (pic 3).  I've noticed when using other computers, I only have to increase the numbers of channels above 2 or 3 to observe the voltage drop.  Nothing else has changed.

 

Why am I getting these differing voltage readings?  The application LJControlPanel.exe seems to always show the voltage I would expect.  

 

Thanks for the help!

 

 

Attached Thumbnails

  • LJ_pic1.jpg
  • LJ_pic2.jpg
  • LJ_pic3.jpg
  • LJ_wire4.JPG


#2 LabJack Support

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Posted 21 February 2014 - 01:19 PM

Perhaps you have a settling problem due to too much resistance.  What is the resistance of your sensor in this condition?

 

To do some testing, set #Channels=9.  For the 1st 8 rows set +Ch to your sensor channel, and set the 9th row to a channel that you jumper to VS or GND.  A classic settling problem will show up as the 1st row being wrong and as you move towards the 8th row they get closer to the correct value, although sometimes there can be overshoot & oscillation.  Start with ResolutionIndex=1 and SettlingFactor= 1 or 9, but then experiment with other values:

 

http://labjack.com/s...s-guide/5.2.5.3



#3 Heckler511

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Posted 21 February 2014 - 04:06 PM

Well let's see.  Using R = V / I I believe the resistance is 8750 ohms at around 12 deg C.  The resistor lists as having 5000 ohms resistance at 25 deg C.

 

So that's R = 1.75V / 0.0002 = 8750 ohms.  I'm getting around 1.75 volts at AIN0 and I'm using the 200uA current source.  

 

Is that what you mean?  Is that too much resistance?  

 

I'll set up that test you've described, and get back to you with the results. 

 

Thanks!



#4 LabJack Support

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Posted 21 February 2014 - 05:19 PM

With autosettling (settingfactor=0), the U6 meets all specs at any combination of gain, range, and resolutionindex, if the source impedance is less than 1000 ohms.  For higher source impedance you can start to see noticeable errors for some combinations of gain, range, and resolutionindex.

 

I did a test similar to yours, but with a fixed 10k resistor.  So I put a jumper from 200UA to AIN0 and a 10k resistor from AIN0 to GND.  No other connections.  I did not see the exact same results as you, but similar.  What was repeatable was bad readings if I set #Channels=16, ResolutionIndex=1, and SettlingFactor=1.  I changed the first 15 rows to AIN0, and in the attachment you can see that the first few readings are no good.  With SettlingFactor=4 or higher it appears that all readings are settled.

 

 

Screenshot 2014-02-21 17.10.47.png Screenshot 2014-02-21 17.17.18.png




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