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Data generation and acquisition of high frequency problem

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The hardware system requires a sin signal (32.764 KHz)plus sawtooth signal(0.3 Hz)output to drive the laser and acquires the signal from the detector output. I use the NI USB-6356 DAQ with the speed limit of 1.25 MS/s.

I set the generation mode as "Continuous Samples", the Samples to Write/read of 1.2 M, and Rate (Hz) of 500K in the DAQ assitant. I also have tried 500K and 500K.

The vi block diagram is shown as the following figure.

Block Diagram.jpg

From the front plate, the generation signal is normal, which can be see in the left side of the following figure. But the acquisition signal has a lot of periodic fluctuations which is shown if the right side of the figure. Besides, the acquist siganl shift all the times. 


I think it may be some mistakes with my sampling frequency setting, I have tried a lot of time to change different rate, but it seems useless. 

Those fluctuations not only exist in the increasing period but alos can be seen in the decreasing period, which is shown in the following figure:

Details of the acquist signal.jpg


I have tried to solve this problems for almost a week and try a lot of different rate, but all fail. 😞

Could any one give me some suggestion please? Thank you very much!!

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Message 1 of 9

Forget that DAQ Assistant, and learn how to program using proper low level DAQmx functions. There are lots of examples via the Help menu option.


edit: this might help you:

see Figure 2...

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Message 2 of 9

Thank you.

You means that I should program the DAQ without DAQ assistant?

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Message 3 of 9


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Message 4 of 9

Thank you!

I guess the reason of the DAQ assistant cannot reach my require is that it without synchronization? 

I am study that now and will update the program to see what happen.

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Message 5 of 9

One problem with the DAQ Assistant is that, it hides from you what it does with the task. One possible problem can be like it always re-initializes the HW resource at certain conditions, which can cause delays/jitter. But simply said: do not use DAQ express VI for serious applications, specially when you need real control over DAQmx tasks and timing...

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Message 6 of 9

 Thank you very much for your nice explain. I have written the DAQ without DAQ assistant, which can be seen as follow:


And seeing the output signal, it seems better than before. However, the period flutuation still exist as following:

Acquicy Signal decrease.bmp

I wonder do I miss some parameter or the rate is not correct? Thank you. 

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Message 7 of 9
  1. You should attach your VI and not a picture of it, so people can check it and help you
  2. I see strange things in your picture, like why you start and stop the DAQmx tasks at every iteration? Did you look at the examples at the above links? You do things very different.

I would recommend you to take some days reading through the DAQmx tutorials. Your approach should not be this kind of "intuitive programming", but based on understanding what are the different DAQmx functions actually do 🙂

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Message 8 of 9
Accepted by topic author univercity460616

Thank you SOSOSO much for your immediately reply every time!!

It is my first time to write the Labview program with the DAQ 😞

The attachement is the Vi field. 

I will read carefully of the webpage you offer, thank you very much again!!

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Message 9 of 9