THD
Sankudey has a point,
Distortion is a non-linear effect usually caused by amplifiers, mixers, etc... THD power should increase faster then the input signal, so if the input signal is increased by 1dB, then THD power should increase by more then that.
Now an A/D's sampled signal has "distortion", but I'm used to it referred to as sampling noise. It's power is generally flat and is a function of the LSB, or the biggest difference between two adjacent steps. It always thought it's classified as a noise, and therefore not part of THD. The THD component would be caused by the sample-hold anmplifiers.