Saw-Tooth Graph When Acquiring Low Frequency Sine Wave

[Jim_Browne]

Hello,

I am a fairly new LabVIEW user developing a DAQ system for strain gauge measurements.

I have been working through the tutorial book and have encountered an odd issue when acquiring a simple sine wave from my device's analog output.  I am using an 80ch NI USB 6225.  

The issue is that when I am acquring any signal with a frequency lower than 9Hz or so, the chart shows a saw-tooth pattern instead of a smooth sine wave.  Above 10Hz, I get a nice, smooth sine wave after changing nothing but the generated wave frequency.  I have tried different voltage levels and various generation and acquisition frequencies (up to 100KHz aquisition and 10KHz generation) with the same issue. I have tried various timing options including adding waits into my acquisition loop and changing the buffer size with no change in the behavior.

I have attached a screenshot of my VI's and outputs.  

Any help would be appreciated!

[ColdenR]

Hello Jim,

What device are you using to generate the analog output? 

Can you try replicating this issue in Measurement and Automation Explorer (MAX)? Go into MAX and right-click on your device to setup a DAQmx task. Then run through the setup in the same way you would for those Express VIs. If you're using an NI device for the analog output, you can setup the task in the same way and run it simultaneously.

It's unclear at this point if the analog output device is outputting a bad waveform or if the input device is reading it incorrectly. If you have a way to monitor the actual analog signal using another device, that would be helpful.