ADXL326 frequency response drop seriously

Hi ADI guys，

  We test ADXL326 recently as our new project application for low-frequency vibration ，and find there is some  frequency response problem，

  We test X axis ,when the frequency  exceed 500HZ, the acceleration magnitude drop seriously, as below:

           The actual bandwidth is much lower than 1.6KHz from the spec,How can we improve the ADXL326 BW? We set the  the Cout of the RC filter about 3nf .

     Thanks a lot!