The THD is around -110dB. I thought that the spectrum analyzer is loading it adervsaly so I decided to terminate the input of the spectrum analyzer with a 1Meg ohm with 200pF cap in parallel. After I did this the results improved dramatically. not the second harmonic is down around -100dB. See Attachment. I believe I can get better results by terminating the input of the ADC with 1Meg ohm with 200pF cap in parallel.
