why 32bit have better THD then 16bit?

[mini-k]

i have 2 usb sound cards, one 32bit in, and second have only 16 bit in. i make 2 identical measures , it have identical levels, but have completely different THD after 800Hz.
Source in both measures the same- motherboard sound card and external amp

 

[John Mulcahy]

When looking at the distortion graph you also need to look at where the noise floor is. The green trace has many grey areas where the harmonic level is below the noise floor, so what you are seeing there is noise, not distortion.

 

[mini-k]

can you please advise-what should be the conditions for measuring distortion and not noise?

 

[John Mulcahy]

All measurements have noise, if the distortion is lower than the noise it can't be seen. Noise can be reduced by measuring in a quieter environment and using longer sweeps.