One DAQmx with two DAQ rates?

[gottfried]

Hi,

can I (how can I) get an the same board (a 6251) two different data rates? I need a 80kS/s channel and 4 10S/s (every 100ms) channels.

Thanks

Gottfried

[mross]

QUOTE(gottfried @ Oct 11 2007, 09:01 AM)

Hi,

can I (how can I) get an the same board (a 6251) two different data rates? I need a 80kS/s channel and 4 10S/s (every 100ms) channels.

Thanks

Gottfried

With E series boards you couldn't do it. But I don't think I ever tried and I haven't on my 6251 either. But you can do you slow DAQ with a second cheap USB board.

Irene He sells a little USB DAQ board for US$86. 32kS/s for 8 channels.

http://www.hytekautomation.com/Products/IUSBDAQ.html

If you try that I would like too know how you liked it. (I am in no way affiliated with Hytek Automation)

There are other inexpensive DAQ boards available as well.

If you must use NI then the low dollar choice is USB-6008 that does 150S/s:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/14604

Be sure to make sure the other specs for these are good enough for you.

Mike

[Neville D]

QUOTE(gottfried @ Oct 11 2007, 06:01 AM)

Hi,

can I (how can I) get an the same board (a 6251) two different data rates? I need a 80kS/s channel and 4 10S/s (every 100ms) channels.

Thanks

Gottfried

Short answer: you cannot do this in hardware. DAQ boards have only limited resources (scan clocks, timebases etc) that have to be shared.

Workaround: Sample at the highest rate, and then downsample one of the signals to your required lower sample rate. There are VI's available in LV that will allow you to change the sampling rate pretty easily.

Neville.

[gottfried]

HiQUOTE(Neville D @ Oct 11 2007, 06:53 PM)

Workaround: Sample at the highest rate, and then downsample one of the signals to your required lower sample rate. There are VI's available in LV that will allow you to change the sampling rate pretty easily.

Thanks Neville

Gottfried

Hi Mike,this are 12 Bit boards, but I have to measure with 16 bit.

Thanks anyway for the links

Gottfried

QUOTE(mross @ Oct 11 2007, 06:47 PM)

With E series boards you couldn't do it. But I don't think I ever tried and I haven't on my 6251 either. But you can do you slow DAQ with a second cheap USB board.Irene He sells a little USB DAQ board for US$86. 32kS/s for 8 channels.

If you try that I would like too know how you liked it. (I am in no way affiliated with Hytek Automation)There are other inexpensive DAQ boards available as well.If you must use NI then the low dollar choice is USB-6008 that does 150S/s:

Be sure to make sure the other specs for these are good enough for you.Mike

[crelf]

QUOTE(gottfried @ Oct 12 2007, 05:22 PM)

...this are 12 Bit boards, but I have to measure with 16 bit.

The Neville_D's answer is the one to go with - sample fast and resample in software. You need to think about what method you want to use to do the resampling, as there are several, and it really depends on the applicaiton. For example, if I wanted to acquire a slow signal at x Hz and then a fast one at 3x Hz, I acquire in hardware at 3 x Hz. Then, how do you get your x Hz signal? You can decimate the 3 x Hz signal to get the first value, the second value, the third value, a mean of all three values, the highest value, the lowest value, etc. You also may need to think about syncronisation - do you need to justify which data point that you're going to select? etc...

[Neville D]

QUOTE(crelf @ Oct 12 2007, 05:12 AM)

...You can decimate the 3 x Hz signal to get the first value, the second value, the third value, a mean of all three values, the highest value, the lowest value, etc. You also may need to think about syncronisation - do you need to justify which data point that you're going to select? etc...

True, but you can also use the Align & Resample VI's under Signal Processing>Waveform Conditioning. They give you a lot more flexibility than just throwing out unwanted samples (i.e decimation). You can resample at an arbitrarily lower sample rate and choose what kind of interpolation to use between the pts. as well as the alignment (starting pt) for the original and resampled waveform.

These are indeed powerful VI's with lots of flexibility and functionality. A good starting pt. is the Express VI, and then drill down from it to select the bits you really need.

Neville.

[crelf]

QUOTE(Neville D @ Oct 13 2007, 06:26 AM)

True, but you can also use the Align & Resample VI's under Signal Processing>Waveform Conditioning. They give you a lot more flexibility than just throwing out unwanted samples (i.e decimation). You can resample at an arbitrarily lower sample rate and choose what kind of interpolation to use between the pts. as well as the alignment (starting pt) for the original and resampled waveform.

Absolutley - but make sure you fully understand what these VIs are doing: decimation is a very different concept to interpolation (in fact, they can almost be thought of as the opposite), and there will be times when only one of them is appropriate for your situation. I also humbly suggest you have a quick read through this blog post.

[gottfried]

Thanks a lot especially for the links

Gottfried

[LAVA 1.0 Content]

QUOTE(gottfried @ Oct 11 2007, 08:01 AM)

Hi,

can I (how can I) get an the same board (a 6251) two different data rates? I need a 80kS/s channel and 4 10S/s (every 100ms) channels.

Thanks

Gottfried

There are several ways to do this, I have attached an example of one way. Import the .nce file into max then run the example. The important thing to remember is that there is only one A/D in a multifunction DAQ card, when you read in the documentation that there are X#Channels, they are really just talking about multiplexing capabilities. This means that there is really no such thing as simultaneous sampling with a single multi function DAQ card, so if you really need simultaneous sampling you need: more than one Multifunction daq card, a multiple channel digitizer card or a sample and hold card. If you protect your resource (A/D) with a semaphore and configure your task in MAX you can have multiple sample rates with one A/D, they are however not simultaneous, but that is not possible with a multifunction daq anyway.