FPGA Sampling problem

hi everybody,

i have a problem with timing on my FPGA,  i would like to acquire a signal at 400khz, and the  analogique/digital converter( NI5731) is working at 40MS/s, so one period of my sinewave should be described by 100 points, but what i see ( picture attached) is that one periode is described  with 23 points. So it seems like the analogic signal  is not sampled at 40MS/s as it should be. Any help ?

Thank you !

Regards

Jean