Can I use SINAD for THD+N measurements? Are they inverse of each other?

1. Is SINAD the same as THD N?
2. Why is SINAD not important?
3. What is acceptable THD N?

Is SINAD the same as THD N?

For a given input frequency and amplitude, SINAD is equal to THD + N, provided the bandwidth for the noise measurement is the same for both (the Nyquist bandwidth).

Why is SINAD not important?

Since SINAD is a ratio to the sum of noise and distortion, it can be 'dominated' by one of the two if it is much higher. Because a 20dB difference is a tenfold change, the ratio doesn't change much between an amplifier with -80dB THD and -100dB noise, and one with -80dB THD and no noise at all.

What is acceptable THD N?

THD+N ratio is usually expressed either in percent or dB. Percent is commonly used for THD+N levels of 0.01% or higher. For very low distortion, dB units are easier to work with. For example, the typical residual distortion of an APx555 audio analyzer's analog I/O (Figure 2) is -120 dB or 0.0001%.