read channels from task indivudually

[untzi]

Hi,
I need to read two AI voltage simultaneously from two different instruments. I keep having the error 50103 at DAQmx Start Task.vi. From what I've read I cannot read two different channels from the same DAQ board. I've tried to crate a task with both channels but after that I don't know how to read each channel individually from the task.
I need to analyze each channel individually at the same time.

I'm using windows XP, Labview 7.1 and DAQPad-6015 for USB.

Thanks,
Untzi

[Dennis_Knutson]

You can read two channels from the same daq board but you can't have two separate tasks. You have to do like you said and create a single task with two channels. The way you extract the different channels is dependent on how you configure DAQmx Read by right clicking and going to the Select Type menu. Typically I choose Analog>Multiple Channels>Multiple Samples>1D Waveform. In this case, a single Index Array function expanded for two outputs will get both waveforms. Choosing a type of 2D DBL is similar except you would be using Index Array on a 2D array.

[untzi]

I've got both signals from the task and they are synchronize and in phase. But after some manipulation they are not in phase any more. I need to multiply both signal and cannot get them in phase. What would you recomend me?

Thanks,
Untzi

[Dennis_Knutson]

What manipulation are you doing? It sounds like somehow you've modified the timestamp of one or more of the waveforms. One way to fix this is the use Align Waveform Timestamps on the Waveform palette.

[untzi]

In my experiment I get a reference signal from a beam chopper and a voltage signal from a photodiode detector. The signals are acquired via a DAQPad-6015 USB board.
When I first get the signals waveforms from the channel they have the same timestamp, same frequency and they are in phase. Then I pass the photodiode (PD) signal through a Bandpass filter ("Butterworth.vi") and multiply the filtered waveform with the reference one. Because after the filter the PD signals waveform has change, it's not in phase with the reference and cannot multiply then and keep writing the program.

I've used "WDT Align Waveform Timestamps.vi" and there is no change in the waveforms.

I've sent the reference signal through a Lowpass filter with a higher cut off frequency than the actual frequency to check if I could create the same phase delay than in the PD signal. But I keep having a phase difference between waveforms.

I've tried to get the value of the phase difference using "Extract single Tone Information.vi" and even if the most of the time is between 110 and 125 degrees (sometimes 80 degrees or so), it keeps changing each time I run the program.

I've been trying to solve this phase difference for so long and try many many different VIs but I cannot get rid of this problem. I also tried "Align Waveforms (single shot).vi" but didn't work. This last one I'm not sure if I define it properly.

Do you know about any other VI I could try?
Thanks,
Untzi

[AnujD]

Have you tried doing these steps individually to see which one causes the singals to be out of phase?

"Then I pass the photodiode (PD) signal through a Bandpass filter ("Butterworth.vi") and multiply the filtered waveform with the reference one. "

Regards,
Anuj D.