Having Noise in acceleration time history response of a single degree of freedom by Newmark's method

Hi dear friends.

I have written a code based on chopra's book (Newmark Linear Acceleration method). and I checked it with the example in the book and everything is exactly the same. Using the ground motion as the input I got noise in the acceleration response history. the displacement and velocity is exactly the same as the output of seismological software but the acceleration seems to have a lot of noise. I cross check it by the seismological output the trend is the same how ever my signal is not smooth. can anybody tell me the reason.

I appreciate your time in advanced.