NI myDAQ Output Changes When New Task Starts

Updated Dec 19, 2017

Reported In

Driver

  • NI-DAQmx

Other

myDAQ, LabVIEW

Issue Details

When I try to run two AO tasks on my NI myDAQ that are at different references (2V and 10V) I get bad data on one channel or I receive an error.

Also if I stop an AO task and then start a new AO task at a different reference I see the original channel change its output. Why is this happening?
 

Solution

The two NI myDAQ analog output channels can be configured for either a 2 Volt or 10 Volt range. These ranges are implemented by supplying a respective reference voltage to the Digital-to-Analog-Converter (DAC) in hardware. The measurement reference voltage is the voltage level at which the measurement is referenced to as explained here

Both analog output channels share the same reference signal. Therefore, if a user configures a task with the analog output on the first channel using a reference of either ±10 V or ±2 V and then a second task that uses the other reference signal, the first channel will scale its output based on the new voltage range of the second channel, thus unexpectedly changing the output value and resulting in corrupted AO data. 

If an analog output task is stopped, the channel(s) will continue to output the last value. If a new task is created and the reference voltage is changed, however, any stopped channels will scale their output based on the new reference voltage. 
 

Additional Information

For NI-DAQmx 15.5 and later versions, a software change causes an error to be thrown if a user attempts this configuration. 

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