Harmonic distortion inquiry

THD is a ratio of voltages (harmonics to fundamental) so it can be expressed as dB. For 0.03% THD the distortion attenuation is:

dB = 20 Log [THD/100] or 20 Log [0.03/100] = -70.5

Now all you have to do is add the three devices (-70.5 * 3) and that comes out to -65.7 db. (You can easily google adding dB’s). To convert that to THD:

10^(-65.7/20)(100) = 0.05% THD

Ralph Karsten (Atmasphere), would be the person to either chime in or PM.

Bob

Whatever the answer is, your setup is at a level NOT to be concerned. 

Since THD is calculated as root-sum-square of rms voltages of individual harmonics as a percentage of fundamental I wouldn't just add THDs of two devices.  Most likely it would be again root-sum-square of two (or three) THDs.