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I'm looking for a clarification, which I haven't really been able to find. It's probably obvious but I'd like to be absolutely sure of what I'm calculating.

I noticed that the formula that calculates THD is:

$THD = \frac{\sqrt{H^2_2 + H^2_3 + H^2_4 + \dots + H^2_n}}{H_1}$

This is seen as the power ratio of the harmonics to the fundamental.

The sinad function in MATLAB calculates the power ratio of the fundamental to the harmonics and noise floor.

$SINAD = \frac{P_{fundamental}}{P_{noise+distortion}}$

Wouldn't THD+N be the inverse of SINAD, since for calculating THD+N we essentially need:

$\frac{P_{noise+distortion}}{P_{fundamental}}$

Both the measurements seem to ignore DC. I use this MATLAB tutorial page as a reference: https://www.mathworks.com/help/signal/ug/distortion-measurements.html

I guess I'm wondering this because I often see SINAD and THD+N listed as separate measurements, but I don't see how they are different.

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So I guess I should have looked around a little better. I've found that SINAD and THD+N are reciprocals.

These references were useful:

https://www.ap.com/technical-library/more-about-thdn-and-thd/

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.476.1903&rep=rep1&type=pdf

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