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.
I think I figure it out, However, whenever I sample it and convert it back to analog, the analog voltage seems to be about half of the input VDAC vaue.
Seems that it caused by SAR ADC Vref selection?