Using 2 DAQ Assistant to Measure 2 Sound Pressure with different Current Excitation Source

[popcorn]

Dear Support,

I need to use 2 DAQ Assistant that run sequentially to measure 2 sound pressure with different current excitation source. The block diagram as attached. When start run,  Labview run the first DAQ Assistant Aquire, the input is the expected input. Then, when the next DAQ Assistant Aquire run, the input value is not the right one. After stopping and start running again, the first DAQ Assistant Aquire will not aquire the expected input too. I tried to change the current excitation source of both DAQ Assistant, there's not problem occur. But, I need to have 2 DAQ Assistant with different excitation source for measurement of Equivalent Noise Level (use internal current excitation source) and Sound Pressure(use external current excitation source) . Please help me on how to accomplish this.

thanks

[Nick_F1]

Hi Popcorn,

Thank you for posting on the National Instruments forums.
It is possible to do what you are trying to do in a single DAQ Assistant instead of two.  Place a DAQ Assistant on your block diagram and select Custom Voltage with Excitation.  Setup the DAQ Assistant with the excitation that you need.  Now, on the configuration tab, click on the + icon and you can create another channel.  You can select Custom Voltage with Excitation again and select another channel.  On that channel, you can select a new excitation value.  Now you have one DAQ Assistant that can use 2 different excitation levels.

[popcorn]

Dear Nick,

Thank you for your reply.

The Custom Voltage with Excitation is not supported by the card I am using which is PXI-4461. The 2 inputs of different excitation source are taken from the same input channel.  So, it is not possible for me to add them on the same DAQ Assistant. Is it possible to do it sequentially? I know it is possible if the same excitation source is used because i tried it before. How about with different excitation source.

Thanks.

[Nick_F1]

Hi Popcorn,

I apologize, I misread your original post.  Yes, you can run them sequentially.  You will just need to make sure you wire a true value into the Stop terminal, that way after the DAQ Assistant has acquired once, it will close the resource and allow you to run the second DAQ Assistant.




Message Edited by Nick F on 11-16-2007 04:48 PM

[popcorn]

I tried connecting the error out of first DAQ Aquireto error in of second DAQ Aquire. But, the result is still the same. The first DAQ Aquire can aquire the right result. Then, the second DAQ Aquire which has different excitation source as the first DAQ Aquire will aquire not the right one. There's also a 'click' sound coming from the pxi-4461 card when changing from one DAQ Aquire to another. What this click sound indicate?

Thanks

[Nick_F1]

Hi Popcorn,

The relay you are hearing is for the IEPE Excitation.  Here is a knowledgebase article explaining about IEPE Excitation

How Do I Enable IEPE Excitation on My DSA Device in DAQmx?

For the DAQ Assistants, are you wiring the true constants to the Stop (T) terminal?  This will ensure that the task is stopped when the DAQ Assistant is done, so you can start another DAQ Assistant with a different excitation.  The other question is what are your settings for each DAQ Assistant.  Can you post a screenshot or list the settings of the DAQ Assistants. It's possible that there is a setting in there that is causing an issue, but I have a feeling that wiring the true constants to the Stop (T) terminals will fix your issue.