collecting data using Multi DAQ and computer

[Tony88a]

I'm using a USB 6251 and a Dell Latitude 2 Duo CPU 1.95 GB RAM and 2.00GHz

How does the computer affect how fast the samples can be processed after the DAQ collects the data?? I looked up specs on the DAQ card and it can sample

16 analog inputs (16-bit), 1.25 MS/s single-channel (1 MS/s aggregate)

I'm taking in an analog input that is reading voltage from a photo diode. I have the DAQ assist doing 40,000 samples at 20,000 per second.

I'm not firmilar enought to know how this all works and what my limitations are?

Whats the max I can sample using this hardware I have and doing a FFT of the information??

[John_P1 · Accepted solution]

Hi Tony88a,

Welcome to the NI forums!  While it would be difficult to put an exact number on the rate you can process the data on your computer, you should not have a problem reading 40,000 samples at 20 kHz.  Essentially, you need to ensure that your loop that is calling the DAQmx read is running fast enough to keep up with the data coming in.  If you read 40,000 samples at a time and are sampling at 20 kHz, this means the loop must run once every two seconds which should not be a problem.  Keep in mind that the loop rate will not be entirely consistent unless you are using a Real-Time OS. 

To further clarify, you could sample all the way up to the 1.25 MS/s (if using a single channel) specced on your card.  If using multiple channels you can sample all the way up to 1 Ms/s divided by the number of channels you are acquiring from.  Of course, you would need to increase the number of samples that you are acquiring per call to maintain an appropriate ratio so the loop can keep up.  Performing an FFT on a larger block of data will take longer as well, so this would be another consideration. 

You might want to consider using the producer/consumer architecture to move the processing outside of your while loop.  This will take advantage of your CPU's multi-cores and will allow the DAQmx read function to be executed more frequently inside its own dedicated loop.  If you have any questions about it be sure to let me know.  Thanks for posting and have a great day!

-John 

[Tony88a]

Thanks John P. I was also curious of all the analysis tools that I can use for my FFT of the data. I dont know of they apply but I see a couple articles on 'The fundamentals of FFT-Based Signal Analysis and Measurement' and one called 'Using FFT and Power Spectra in LabVIEW'.

Basically I'm pulling continuous analog data from a photo diode, then doing a Fourier Transform...we will eventually look to analyze the signal to determine different liquids and gases. Do any of the FFT tools in LabVIEW help with this analysis?

What other features would you recommend I look into?

I'm new to using the hardware and software together and analyzing the data using LabVIEW. I'm a student and dont have my LabVIEW for everyone book I just ordered!!!

Thanks,

Tony88a

[John_P1]

Sounds like a very cool application.  I'm not sure exactly what type of analysis you need to do, but we do have a FFT function that you will probably find helpful.  Provided you are not using the base version of LabVIEW, the function can be found in the functions palette under:

SIgnal Processing >> Transforms >> FFT

I hope this answers your question, but if you need any more information please let us know.  Thanks and have a great day!

-John