We typically only use high-impedance states on PFI and Digital IO lines. The user manual for your card only mentions that capability in the context of PFI and DIO lines as well.
You may set the "idle-state" of the analog lines to 0 volts using a channel property node.
Could you tell me a little more about your application, and why you would like to set the analog output lines to a high-impedance state?
those two analog output connect at different points on the same channel.
I want to transmit sine wave at the beginning of the path,from AO_0 ,and after that with AO_1 at the middle of the path.
The problem is that AO_1 drop the voltage down when it is not in use.
AO_1 always controls the output voltage of the channel.
From the description of your project, I'm envisioning two probes attached to different points along a single trace on a PCB.
It is certainly not recommended to have two analog output lines attached to the same channel. As you have experienced, the behavior is unpredictable. In a worst-case-scenario, one could potentially damage the AO line that is not actively driving the channel.
So we do need to find a configuration that makes the unusued AO channel electrically separated from the channel-under-test. One option would be to introduce an electromechanical switch. This will add some complexity to your setup and your code, but it will permit you to send the signal from AO-0, disconnect AO-0, connect AO-1 and send a signal from AO-1 — all in a very short amount of time.
I was trying to avoid this solution.
Can you put series resistors at each AO connection? You get a voltage divider action, not disabling, but depending on the load on the line and the voltage that you set on the "idle" output, you might be able to get something to work withou needing activ intervention.
If you post more details, I might be able to help.