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U3-HV DAC0 and DAC1 output config

dac0 dac1 u3 output

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#1 jdeters79

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Posted 01 July 2014 - 01:08 PM

I have a LabView vi in which I am trying to setup both DAC0 and DAC1 as ouptuts.  I had previous success with DAC0 only as an output, however, I have not been able to figure out how to configure both as outputs.  I have attached a screenshot of the code for review.  

 

Thanks

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  • LabJack_DAC0_DAC1_config.PNG
  • LabJack_DAC0_DAC1_config.PNG


#2 LabJack Support

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Posted 01 July 2014 - 01:17 PM

You are wanting to put together 2 requests, but your iotype array only has 1 element.  You want 2 elements where both are LJ_ioPUT_DAC.  See "U3 Multiple IO Example.vi".

 

Note that DAC1 can only be disabled on old U3s with hardware version 1.20 and 1.21 (pre March 2008).  On hardware version 1.30 both DACs are always enabled.



#3 jdeters79

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Posted 01 July 2014 - 01:32 PM

Thank you.  I saw that shortly after I posted and was able to resolve it that way.  I appreciate your response.



#4 jdeters79

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Posted 07 July 2014 - 02:51 PM

With the previously discussed config for DAC0 and DAC1 fixed (i.e. LJ_ioPUT_DAC has two elements now), I'm running into an issue with the voltage being output.  I was telling it to output 3.6 V, but when I measure the output with a meter on the LabJack, it was ~2.0 V.  

 

Any help would be appreciated.



#5 LabJack Support

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Posted 07 July 2014 - 03:11 PM

Loading will cause the voltage to droop, so use a DMM to measure DACx to GND with nothing else connected.  Make sure the DMM leads are securely connected to the inside of the screw-terminals.

 

Make sure the DMM measures about 5V for VS/GND, to confirm that the DMM works right.

 

One other thing that can cause confusion is if you lower the timer clock frequency.  See the 4th paragraph on the DAC page of the U3 User's Guide:

 

http://labjack.com/s...users-guide/2.7



#6 jdeters79

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Posted 08 July 2014 - 08:30 AM

I measured DAC1 to GND (with no wires connected) and the voltage increased back up to 4.9V.  The device I was trying to supply power to from DAC1 will work when supplying power from VS.  Is there a lower current limitation on DAC1 than VS that would cause the device to draw the voltage down on DAC1?  I was hoping to supply power to the device via DAC1 so I can control when it is powered ON and OFF.  

 

Thanks for your help.



#7 LabJack Support

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Posted 08 July 2014 - 08:59 AM

Yes, the DACs cannot provide as much current as VS and will droop more than VS at the same current.  The DACs have 50 ohms of source impedance, so if you draw 10 mA you will lose 0.5V just due to that.

 

http://labjack.com/s...uide/appendix-a



#8 jdeters79

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Posted 08 July 2014 - 10:35 AM

Thanks.  I found a workable solution.  I'm supplying power to the device via VS, and using an NPN transistor as a switch with DAC1 connected to the base of the transistor.





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