Why does daq information become distorted on the chart graph as S/s are increased?

[engineer 202]

Using Labview 6, I am developing a simple analog IO program that merely sends a dc signal to one output channel, while collecting data points from one input channel of a 1 Hz, 1 V sine wave. The wave is displayed on a strip chart and works fine as long as my sampling rate is low (below 5kS/s). The more samples that I request, the more distorted and non-sine-wave-like the chart becomes. What am I doing wrong?

Thnx.

[Ben]

Sounds like you are not observing the Nyquist Criteria.

It dicates that you must sample at least 2X as fast as the highest frequency component you are trying to sample.

I am guessing that what are seeing is what is called aliasing.

Ben

[Randy_H]

engineer,

I tried to reproduce your issue using a continuous AO and continuous AI example VIs. I could not. Can you post a screen shot or your VI for your acquisition? Also, does this happen with any or all of the shippinf examples that use charts?

Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask

[engineer 202]

Thanks for you reply.

I am not sure what you are asking, but I attached the file. I am very new to and untrained in LabVIEW. Any help that you can provide is much appreciated.

Thanks again.

[Ben]

Sorry, read your numbers backwards.

New guess.

Getting worse when sampling faster could be an indication that there is not enough settling time.

This would happen the output impeadence of the signal source is not low enough to charge the input capacitance before the data is sampled.

Ben