How to exclude DC in sinad calculation

[koolniran]

Hi 

I am using Sinad Analyzer VI for caluculating SINAD for my ADC data. I wanted to exclude the DC from FFT while calculating SINAD (ie exclude first point of FFT).  How could I do using SINAD analyzer VI? I am not seeing any option to set that. But when checked inside the VI mask I found options to turn it ON and OFF. Since its a built in fuction of labview I could not able to change the code. Is there any way to do it? or If I can do the same thing in some other way please help me. 

thank you,

[NiAchilles]

If you change the export mode to "residual signal" does that give you the data you want?

[koolniran]

I am not sure about the explanation of export mode. If some one can explain that would be great. Does tone means the frequency of the input signal? In that case I would be getting FFT minus my fundamental input frequency of the signal (correct me if I am wrong!) then I would be getting exactly oppsite! 

export mode selects the source of the signal and spectrum to export to exported signals.

0	none—Fastest computation
1	input signal—Input signal only
2	detected signal—Single tone
3	residual signal—Signal minus tone

[Richard2950 · Accepted solution]

You already stated how to do what you want: take an FFT, set the first point to zero, invert the FFT and use that as the input to SINAD. You could accomplish the same thing in the time domain by finding the DC average of your signal and subtracting it.

The DC option inside the vi you mentioned does not remove the DC completely, it only scales it by sqrt(2) if you request it.

Rich

[koolniran]

Thank you for the quick reply.

Is there any inbult VI can do the trick for me easily! I understand the method you explained previously. I am gonna try that.

I am just wondering any other possibility? For example Distortion Measurements Express VI also gves me a SINAD value but I am not sure the how it is calculated. I am almost getting the same value for sinad Sinad Analyzer VI so I assumed hey both are using the same method. I wish there is an inbuilt option to exclude DC!

[koolniran]

I am actually getting confused with this discussion. Can any one clarify this?

http://forums.ni.com/t5/LabVIEW/SINAD-analysis-VI/m-p/999762/highlight/true#M446755

[Richard2950 · Accepted solution]

The SINAD analysis gives SINAD (dB) and THD (rms) which do not depend on the DC component. Some of the output spectra do include the DC component. If you select the input signal for export you get the spectrum of the input signal, including any DC which may be present. If you choose "fundamental signal' for export, then you get the spectrum of the single tone only, and no DC. If you choose "residual signal" you get the spectrum of the input signal minus the fundamental signal, so that will include any DC that is present in the input signal. You can use the "Tones and Noise" Waveform Generation vi to make some sample signals and test out the behavior of the SINAD vi.

Rich

[koolniran · Accepted solution]

Now I understood.

But just to clarify, It is the exported signal (spectrum and signal) has the DC content but the SINAD and THD value which is the other two output of the VI is calculated excluding DC. Am I right?

[Richard2950 · Accepted solution]

Yes, that is correct.