AI signal timing (PCI-6025E)

[Jim_Marihew]

I have inherited an ATE that uses a PCI-6025E (rated for 200KS/s) and CVI.  The analog input is initiated by:

error1 = AIStartAcquisition (1, "1,2,4", 5000, 10000, 10.0, 0.0, &scan_rate, &id);

I could not find any documentation stating if this is a multiplexed of simultaneous sampling device, but since it is listed as a "low cost" device I am assuming multiplexed.  NI shows multiplexed input looking like this

Does this mean that I will get 3 samples every 0.1ms, just not at the same moment in time?  The data seems to indicate that the programmer was of the opinion that the 3 samples would bs 0.1ms apart.

[Kevin_Price · Accepted solution]

Yes, the 6025E is a multiplexing device, not simultaneous sampling.  

In NI's terminology, 1 sample means 1 ADC conversion value for each channel contained in the task.  With a 10 kHz sample rate and 3 channels in the task, the sample clock will cycle at 10 kHz while the convert clock runs at a higher rate to fit 3 multiplexed conversions into the available 0.1 msec.   The default behavior is to spread them out to use most of the available 0.1 msec (which minimizes the influence of one channel on the next by giving the converter more time to bleed off the charge from the previous channel before capturing the next one). 

   There are ways to manually choose a different convert clock rate or to query for what DAQmx chose for you.  But I only know how to do it from LabVIEW, I don't know the text syntax.  Either way, each 0.1 msec, you'll get fresh ADC conversion values for each of the 3 channels.

-Kevin P

[Jim_Marihew]

Thank you Kevin, for a very clear picture of the process.