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4461 Analog output performance

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I am generating a 1kHz sine wave at 1.5V DC offset and 800mVp waveform.  I using this signal to test a 12 bit ADC.  As shown in the attachment the best I can get is -70dB on the second harmonic which equates to being able to only test a 11 bit system.  I need better than -75db on the 2nd harmonic. I am running the output into a high impeadance active probe through a HP  Being a 24 bit DAC I thought it would be able to acomplish this task.  What am I missing?  I also attached the LabVIEW code I am using.

 

John

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I am able to use your VI, with slight modifications (such as adding a wait before stopping the task), to output a sine tone long enough to make a good measurement of spectrum. 

 

Here is what I see:

4461_output.png

 

Some considerations when using DAQmx for analog output:

Make sure the task doesn't stop while generating the test signal.

Set the buffer size appropriately for your test.

Make sure that the analysis does not include endpoints of the generation where there are known discontinuities.

Finite Output with Endpoints.png

Doug
NI Sound and Vibration
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Do you have LabVIEW code on this that you can send with the inputs, like sample rate and buffer size?  In my code I start the acqusition and then add a while loop with 100ms delay so my signal is continusally running until I click on stop to end the while loop then I stop the acquistion and clear the task.

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I would encourage you to use the input channels of your 4461 to verify the outputs if only so you can also inspect both the time waveforms and the frequency spectra. In my earlier post, I used the DSA Soft Front Panel to acquire the data and compute the spectra. You can use the DSG and DSA Soft Front Panels to verify your hardware and configuration.

Doug
NI Sound and Vibration
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look at the time domain signal that is output. If you don't ensure that you have an integer number of cycles in the output buffer, regeneration will produce discontinuities in the output.

Regeneration Discontinuities.png

 

For an example of continuous generation where the application continuously updates the output buffer with the desired signals, check out the example: Voltage (non-regeneration) - Continuous Output.vi

Doug
NI Sound and Vibration
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I am using the same input data using the example VI and getting the same results.  My sample rate is 128k and my sample size is 256k.  The frequency is 1k.  What settings are you using?  I can try and duplicate what you have.  I am using integer numbers no fractions.

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I looked at my time domain and took a large sample of data.   looked through it and couldn't find any discontinuties in the waveform.  It looked very clean.

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Try your VI modified to continuously update the buffer. Note, the modified example explicitly disables regeneration.

 

Doug
NI Sound and Vibration
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I didn't notice any differnces.  I am still getting the same results.  I did notice that when I observe the spectrum using the 4461 it looks much like how you have it displayed but when I run it through my spectrum analyzer it is not as clean.  It seems that the loading of the spectrum analyzer is having a negative effect.  I figgured that as long as the input impedance is greator than 600 ohms I would be in good shape.  The input impedance of the spectrum analyzer is 1Meg ohm and 12pF cap. What kind of loading should I use?

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Given the high input impedance, I don't think the spectrum analyzer is drawing too much current. What is the THD of your spectrum analyzer?

Doug
NI Sound and Vibration
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