Noise on the DAQ

[no_brain]

I am using an NI-9223 to read data from a photodetector and an NI-9262 to control the laser with a sine waveform. The resulting graph shows noise spikes occurring five times per half cycle, as seen in the attached image. These noise spikes persist even when the waveform frequency is changed, and the voltage signal still exhibits the noise.

Additionally, when the same LabVIEW code was used a few months ago, such noise did not occur.

Although I cannot share the code due to circumstances, what could be causing this issue?

[littlesphaeroid]

Have you measured the noise period? What has changed about your setup? And new sources of vibration or electrical noise? It looks too fast to be line noise.

[no_brain]

Thank you for your comment. 

The noises appear 5 times in a half cycle at any frequency of waveform.

Thus, I think it is not power noise or mechanical vibration too..

[Yamaeda]

Without seeing code we can only guess. It could e.g. be a race/speed problem so you get an old sample. Add a Low pass filter (after checking for #1).

[Bob_Schor]

Some observations, thoughts, and questions:

• I only see 4 "noise spikes" in the picture.  There might be one at 0.0027, but the red trace (which is the "top" trace) doesn't really show much.
• What is your sampling rate?
• What are your sampling parameters?  Frequency?  Sampling 2 channels simultaneously?  [I haven't read the specs on your A/D hardware ...]
• Have you tried taking data with the Laser turned off?  Do you get two straight lines with no teeny-tiny bumps?
• Has anything changed in the wiring?  Are lines properly shielded?
• What changed to your setup, or to the code, from when you ran a few months ago?  Can you try the code from a few months ago?

Bob Schor

[NIquist]

Since the spikes are time-related to the frequency of the wave, the first thing I would check is that the sine wave circuit driving the laser is not generating the noise.  Can you read the laser drive with another DAQ channel or better, an independent o-scope?