CompactDAQ onboard timer accuracy

[jdub584]

Hello,

The 9215 modules for the CompactDAQ 9172 chassis say it can sample at a rate of 100 kS/s.  I would like to know if the onboard timer on the chassis can accurately time stamp at this rate as well.  I don't have the CompactDAQ chassis yet to test the onboard timer so if someone knows where I can find this information, it would be a big help to me.

Thanks in advance!

Jason

[Jarrod_B.]

Hello Jason,

Timestamping typically refers to the actual time value that is associated with each data point in an acquisition.  If this is what you are referring to, the CompactDAQ chassis itself has nothing to do with timestamping the data.  This time information comes from the computer.  In other words, when you program a DAQ application in LabVIEW, the DAQmx Read function can return a waveform data type that includes an initial time (T0) and a delta time (deltaT).  These time values were not transferred from your DAQ device.  The initial time is a timestamp taken from your system clock, and the delta time is based on your sample rate. 

If I've misunderstood your question, please let me know so that I can make sure your concerns are addressed.

Best regards,

[jdub584]

Hello Jarrod,

I am aware that the DAQmx read function uses the system clock to perform the time stamping.  I also know that the system clock is not very accurate, ~30-50 ms resolution.  Since delta t is based off of the collection rate, time 0 is where the time stamp values would vary by 30-50 ms from the actual time if you use the system clock.

A coworker ordered the compactDAQ for this project and told me the chassis has an onboard timer that can be used to obtain more accurate time stamps.  I'm not exactly sure how to use the onboard timer to generate time stamps but I've created a DAQmx task that uses an external timing source with the source being the \compactDAQ\ai\sampleclock.  If this is not correct, is there a way I can use the compactDAQ's onboard timer to time stamp the data points with an accuracy better than +/- 1 ms?  I ask this because the customer wants to know the fastest data collection rate with the condition that the time stamps for the data points must be accurate.  They want to at least be able to collect data every 1 ms with each data point having a unique and accurate time stamp.  So if I can use the chassis timer to time stamp the data, I wanted to know the resolution of that timer so I can inform the customer.  Hopefully that clarifies what I'm asking.

Jason

P.S - I'm using LV 8.2 and winXP pro

[Jarrod_B.]

Hello Jason,

You are correct that the CompactDAQ chassis has an onboard analog input sample clock.  We can see of page 66 of the NI cDAQ-9172 User Guide and Specifications that the timing accuracy of this clock is 50 ppm of the sample rate that you specify.  Since timestamps are calculated directly from the sample rate in your program, the accuracy of the sample clock will determine the accuracy of the timestamps relative to each other.  If you are acquiring data at 1000 Hz, the accuracy of the sample clock will be ± 1000 Hz * 50 ppm or ± 0.05 Hz.  You can also view this as a sample period accuracy of ± 1 ms * 50 ppm or ± 0.00005 ms.  I hope this helps!

Best regards,