Acquiring a waveform over a Specific Pulse width

[Chris S.]

Hi I am having the following problem:

I have a 6025E series board and a counter board (I think its a 6602) and I wish to do the following.  My output signals are good it is the inputs that are tricky.  I have output 8 TTL counter signals at 20Hz and 10ms Pulse width but now I have to accept 8 waveform inputs that overlap those TTL counter signals.  For instance, I just want to acquire the waveform for the same period that the TTL signal is high on all eight counters.  I figured I could do this by routing the TTL signals to PFI inputs on the E series board but when I run it, I get no waveform and I know there is one there because I see it on the oscilloscope.  I have to capture this waveform (1000 samples) at the same rate of the TTL Frequency about 20-100Hz over the TTL pulse width can I do that?  But first if I could just capture the waveform over the TTL signal I can go from there.  Also, what configuration is best when reading in voltage values say from a Hall Current probe device RSE? NRSE? Differential?

Thanks For your Help

[TheWoost]

Hi Chris,

Routing your TTL signal from the 6602 to the 6025E to use as a trigger for your analog input acquisition is fine.  Are you wanting 1000 points to be obtained in the 10ms that the TTL signal is high?  This is possible on a single channel, but would require a sampling frequency of at least 100kHz.  You could have the analog input trigger off of the rising edge of your TTL signal and then sample 1000 points at 100kHz. 

If you are acquiring waveforms from 8 channels, however, the allowable sampling frequency will be reduced greatly.  In multichannel scanning mode the maximum conversion rate of the device is distributed among the number of channels scanned.  The 6025E has a maximum sampling rate of 200kS/s, and this would be distributed for multiple channels.

To address your question about the transducer configuration, the termination type depends on your application.  Please refer to an article on the NI Developer Zone here that provides more detailed information about the differences in signal sources.

Regards,
Andrew W
National Instruments

[Quiksilver]

Andrew

Yep I do want to aquire the waveform while the TTL is high but only for the first 2.5ms and sampling it 250 times for the 100k would be suitable.  However you are correct I do need to sample all 8 inputs and if what you said is true this may present a problem.  Though I am planning on staggering the TTL signals, in other words, I have 2 go at 0 delay then 2 at 5ms then 2 at 10ms then the final 2 TTL signals at 15ms.  If this is the case then I presumably could sample at 100K for two inputs at a time.  Is that correct? 

Thanks for your help

[TheWoost]

Hi Chris,

Yes, you could sample two analog input channels at the same time at 100kS/s if they were the only two channels in your analog input task.  However, you will need all 8 channels in the task, and the sampling frequency will be distributed.  Another issue with your application is that with a single analog input task, only one start trigger can be used.  In other words, you can only use 1 of your TTL signals to provide triggering in the acquisition and not 8 triggers each dedicated to their own analog channel.  You could use 1 TTL signal to trigger all 8 channels and programmatically provide the time delays that you want, but this would be non-deterministic.

Regards,
Andrew W