Troubles with analog input with ni6014

  I'm using ni6014, with the comedi drivers on debian to get some points of analog data and plot it.
   When I make the acquisition of a wave, if I use few points. like 1000, it's ok, everything correct. But, if I use some more points, like 10000 for example, the period plotted seems bigger (much bigger) than it is!! 
   Seems that there is a cumulative error, and, each more points I get, the error is bigger.
   And it's not a plotting error, the error is in the data.
   Someone knows something about it?
   

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