noisy DAQ output

[Soran]

Hi,

Can anyone tell me why a DC signal generated by my simple LabVIEW code is much noisier than the same signal generated by an analog function generator? I have attached a picture of both cases and also a screenshot of my code.

Any help is appreciated.

[JÞB]

Why are you "Starting" and "Stopping" the DAQmx Task inside the While loop?

Don't do that!.  seriously

[Soran]

Thanks Jeff, I will fix it. Any idea about the noise?

[NIquist]

Put the Start Task before the loop and the Stop Task after and see if the noise diminishes.  What you have now is like what would happen to your analog generator if you clicked the power button on and off REALLY fast.  It would probably be noisy too.  If that doesn't help, post a diagram of your wiring as well.

[natasftw]

If all that fails, what DAQ device are you using? Which function generator?  This would be helpful to know if we are looking at potential accuracy of hardware.

[Dennis_Knutson]

And please learn how to create a snippet if you insist on posting an image of your block diagram. It's silly to use a phone and attach a 1Mb file.

[Soran]

Thank you natasftw and NIquist,

I did what you said about putting "Start" and "Stop" outside of the loop and the signal is the same noisy.

I have attached my modified VI and aslo its screenshot. The DAQ card I am using is PCIe6351 and the analog function generator is here:

http://www.berkeleynucleonics.com/products/model_645.html

Unfortunately the computer that I am working with at the lab is not connected to network and my flash driver is damaged, that is why I had to take "real" photo this morning.

[NIquist]

Did you get this figured out?  If not, post a diagram of how you have it wired including whatever breakout box is connected to the 6351.  That's a nice DAQ and I use them all the time.  I can usually see right down to the bit level noise when they're connected properly.

[Soran]

Thank you for your message.

No, I did not figure it out. I am using the following connector block:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/203462

A BNC cable has connected the output ao1 of the connector block and the osciloscope. That is all I have.

[Logan_K · Accepted solution]

My guess would be a ground loop exists between the DAQ board and the oscilloscope that's contributing to the noise. I'd look at the grounding jumper for the DAQ board's terminal block - this manual will give you some guidance on how to configure the jumper properly based on your particular grounding configuration: http://www.ni.com/pdf/manuals/372101a.pdf

Take a look at what the DAQ board and oscilloscope are plugged into... sometimes the ground supplied by the outlet can be noisy, which can bleed into your signal.