Diagnosing noisy analog output

[codejon]

Hi,

I have a 6221 PCI DAQ.  I'm trying to diagnose noise in the analog output signal.  When I give a static output I measure 2mV RMS with my oscilloscope (and a similar figure when recording the output via an AI input).  This seems much too noisy, and I'm trying to determine whether this is a problem with our board or I am somehow picking up a lot of noise from the environment.  I'm using the SCB-68A breakout board, and I have it contained inside of a steel box and wired to BNC outputs.

Sorry if this isn't the right place for this kind of question.  If I've understood correctly then I would have to pay NI to answer this question, so I thought I would start here first...thanks!

[PezDeSpencer]

Hello codejon,

Due to the specifications there is a possibility for noise. You can calculate the AO absolute accuracy as per the equation included in the specification sheet.

6221 Spec Sheet:

http://www.ni.com/pdf/manuals/375303c.pdf

Otherwise you may want to look into how you are wiring up your signal (grounded, differential) which could introduce noise. Look into this article on Analog Noise Considerations

Field Wiring and Noise Considerations for Analog Signals

http://www.ni.com/white-paper/3344/en/#toc1

Also if you have a visual of your signal noise that you can post that may be helpful. 

Spencer R | NI

[codejon]

Hi Spencer,

Thanks for you reply!  I reviewed the spec sheet again, and the only equation that I see for analog outputs is the "AO Absolute Accuracy Equation."  My understanding is that this equation is usefulf for calculating the accuracy of a reading but not the spread of readings.  I would like to know how consistent is the output over time when the output that I'm asking it to produce is fixed.  Maybe I'm misunderstanding and the figure I need is in the spec sheet somewhere, but I don't see it.

As for the analog input wiring, I didn't expect this to matter very much when reading the AO with an oscilloscope.  However I also tried recording the AO using the DAQ in both differential and RSE configurations, and this had no significant effect on the noise.

I will post back with an FFT of the noise soon, I don't have access to the device at the moment.