Multiple AI each with different external sample clock

[L._Andrzejewski]

Hi there,

I want to acquire 2 to 4 signals incoming from photo diodes, I am interested only in the maximum amplitude of the signal for each, the Laser repetition rate is at 100Hz. So I was thinking of using a digital signal ,which triggers the laser, and a delay box to optimize the delay with the corresponding maximum of amplitude on the diode as read by an oscilloscope and send it as the sample clock for my Analog input task (for example on the PFI 0). This way each time the sample clock triggers an analog conversion it reads the maximum of the diode for each shot ~ 100 shots/s.

It works fine for one channel, but is it possible to use 2 to 4 different sample clocks each with it s own digital input (ex.: PFI 0, PFI 1 ..) for the other diodes, since the delays I want to use are not the same for each due to laser propagation time...

Or is it possible to control the acquisition/conversion  delay/trigger between each channel at ~50 nanoseconds precision…so I can delay the acquisition  internaly instead of using  2-4 external digital sources..

Any help is welcome

Regards

Lukasz

[tnek]

Hi Lukasz,

I was wondering if you could give the exact delay between the firing of the digital trigger to the laser and the analog acquisition. Since there is usually a slight delay between a trigger and when an acquisition begins, this should be taken into account since you are concerned about the 50 nanosecond range. This will largely depend on the hardware that you are using. Could you specify the DAQ card that is sending the digital signal and acquiring the analog data?

One way which you may be able to accomplish this task without the digital sources, would be to use counter tasks running at 100Hz and use that output as the sample clock for your analog acquisition. A good example that shows this is named Multi-Function-Ctr Pulse Train Generation for AI Sample Clock and can be found in the Example Finder. This example is listed browsing by task under Hardware Input and Output » DAQmx » Synchronization » Multi-Function.The delay can be configured using property nodes. Again, I'm not sure if this will completely meet your needs but it may be worth a try.

Regards,
Kent
Applications Engineer

[L._Andrzejewski]

Hi kent,

 The delay can be up to 10ms (1/100Hz) I don t really need to trigg on the same event I want  to measure, it can be the one before. The trigg comes from a masterclock device which allows us to have multiple TTL outputs delayed with an ns resolution, in synch with an reference source.

The delay will work with one counter but only for one channel, I need at least 2 channels each with it s seperate delay.
regards
Lukasz


Message Edited by -Smash- on 04-28-2008 04:24 PM

[tnek]

Hi Lukasz,

An important piece of information will be the exact card you are using for your data acquisition. The multifunction DAQ cards will not be able to have different sampling rates or different triggers for each channel. Each analog input task will reserve the use of the sample clock and multiplexer. If you try to use multiple tasks, you will get an error saying that the resources are reserved. More information about this error can be found here.

If multiple triggers on one board are necessary, you may want to look at the Intelligent DAQ devices that we offer. With these boards, you will be able to specify different clocks and triggers for each channel.

Regards,
Kent
Applications Engineer

[L._Andrzejewski]

Yes, I got that error trying

I see, well I wanted to use my USB-6251, since it s an temporary experiment  and they have an better sampling rate than the FPGA boards, If one delay for all diodes dosen t make it, I guess I use the R series I have for another project...

But still, it would be nice to have such a capability for much less $$ than the R series...
I guess buying multiple cheap usb daq boards, one for each diode would be the simplest way...

thx
Lukasz