Can you please post your complete project, so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
You do not wait for a conversion ready, use ADC_IsEndConversion()
Your ADC is 12 bits wide, you need to shift right by 4 to get an unsigned short int
The ADC averages 8 measures, thus reducing the frequency.