TMS320F280049: PGA issue with Distortion

Tool/software:

Hi expert,

Customer is using F280049 PGA for AC sampling, connection as below (green block is the F280049 internal PGA, pin is PGA3-IN and PGA3-GND):

The configuration is as below:

The issue is, PGA output is distorted like this (normally it should be a sine wave):

Could there be any problems in HW or Sysconfig? And do you have any suggestions on differential input signal into PGAx-IN and PGAx-GND? 

Thanks!

Joyce