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Analog Output with Offset

U12 AnalogOut

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3 replies to this topic

#1 beta

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Posted 16 June 2014 - 10:24 PM

I am using the analog outputs of a U12 to control a power supply. Sporadically, the voltage at both outputs is about 0.7 Volts higher than the written voltage, so if I set the analog output to 0.5 Volts, I get 1.2 Volts in real.

The Power Supply uses a 0-5V line as a remote with an resistance of more than 100 kOhm. After the first troubles I connected a 5k resistance in parallel.

 

for Information: Windows 7, Python 3.3 with changed u12.py, actual drivers



#2 LabJack Support

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Posted 17 June 2014 - 08:39 AM

Connect a DMM to AO0 & GND so you can monitor the voltage with the DMM at the same time it is connected to the U12 and power supply.

 

When the voltage jumps from 0.5 to 1.2, if you remove the connections to the power supply, so you are just left with the U12 and DMM, do the outputs still read 1.2?

 

If you are not running any software, do you still see the voltage jump?

 

If you run LJlogger.exe only, and don't run any other software, do you still see the issue?



#3 beta

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Posted 19 June 2014 - 10:54 AM

Sorry for the late reply.

Both outputs now have an offset of plus 0.68 Volts , doesn't matter if I connect the Power-Supply or not and no matter what . Measured with a DMM at AO 0 and AO 1.

I see this voltage when setting the voltage with LJlogger, too. I even see it while booting up the PC, before windows has started properly.

The AOs of a second U12 I am using are working in a correct way.

The Offset is not completely linear:

Voltage set in Software/Voltage measured

 

0V/ 0.7V

1V / 1.6V

2V / 2.4V

3V / 3.3V

4V / 4.15V

5V / 5V

 

I had an additional trouble, don't know if they are a possible reason for this behavior: I am using two U12 parallel, with a multi-thread-application. One day, the first U12 didn't start properly anymore. The green LED blinks some time then is switched off to start to blink again. The second U12 had the same trouble some beginning some days later. Resetting the Watchdog by wire and restarting it with the software removed this problem, but I'm not able to recalibrate the AIs. 



#4 LabJack Support

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Posted 19 June 2014 - 04:02 PM

Looks like there is a hardware problem with the U12 analog outputs.  Contact [email protected] to request an RMA number.

 

If you want to check further yourself, it is possible that there has just been damage to the output resistors.  Take a DMM and measure the voltage on both sides of R63 & R67 (AO0) and R62 & R66 (AO1), all versus GND.

 

 

I am not quite clear on your other problem.  When you have the problem, what are the symptoms?  Errors from software commands?  LED is off?





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