Greetings,
Setting the analog input trigger for Digital A will setup a digital start trigger, and so you won't want to to that if you're wanting to trigger off of an analog value coming from an analog output channel.
I would take a look at a LabVIEW shipping example entitled 'Cont Acq&Graph (buffered) Hard A-Trig VI' that shows how to configure an analog trigger.
The analog trigger source can either be PFI 0 or the first analog channel in the scan list. So, if you are wanting to trigger off of analog channel 3 and you are reading from analog channels 0,1,2 and 3, channel 3 would have to be first in the scan list (3,0,1,2 for example).
The AI Start VI is the VI with the inputs for configuring triggering. The shipping example I referenced above shows how t
o configure all of the inputs for the AI Start VI to set the trigger.
I hope this helps. Let me know if you have any other questions.
Regards,
Todd D.
NI Applications Engineer