"Connect DAC Reference to Ground" property causes other channel to stop working

[lindope]

Can I set the Allow Connect DAC Reference to Ground property on one channel and not the other?

I have one analog output channel of a PCI-6251 set so the DAC reference can be set to Ground.

I understand that this will only work if the DAC reference is set to internal. Since this is the default I have not set it.

I write to the output then set the output to ground.

I try to write to the other channel on the same board and it does will not work. Sometimes, it changes momentarily but goes back to 0 volts. Most of the time it just doesn't change at all.

If I remove the aforementioned property and action off the first channel, the other channel works fine.

Can I not use this property independently? Or is there some way to correct this?

[EricS. · Accepted solution]

The Connect DAC Reference to Ground property can not be set for individual channels.  In the LV Help for that property (right click on the property in the property node and go to Help For AO.DAC.Ref.ConnToGnd to go to the specific help page) it states:

Grounding the internal DAC reference has the effect of grounding all analog output channels and stopping waveform generation across all analog output channels regardless of whether the channels belong to the current task.

So you cannot limit which output channels are going to be grounded, it will ground all of the channels that have the reference set to internal.  If you try to change the "Allow Connect DAC Reference to Ground" property to False for some of the channels and True for other channels, you will get error -200106: Property must have the same value for all channels on this device.  

One workaround for this is to instead of setting the DAC reference to ground, set the source to external, and use APFI 0.  What you could do is externally wire AO GND to APFI 0 (pin 20).  Then set the DAC reference to external.  The problem with this is that unlike the Connect DAC Reference to Ground property, which can be changed while the task is running, the reference source property cannot be changed while the task is running.

[lindope]

Thank you for clearing this up. And thanks for the suggestion for the work-around. That helps a lot.