I acquired voltage by NI USB-6009. And I set Reference single-ended for input terminal configuration and connected the +2.5 External References to AI0. I acquired one sample every time and got the following data:
2.490707, 2.493252, 2.495797, 2.498342, 2.500887, 2.503432
Since 2.503432 - 2.500887 = 2.500887 - 2.498342 = ... = 2.493252 - 2.490707 =0.002545,
I think the voltage resolution should be 0.002545 V.
But according the user guide of NI USB-6009, the Input resolution is 13 bits and the input voltage range is +/-10V for Single-ended terminal configuration, then the voltage resolution is 20/(2^13)=0.002441 V.
Obviously, 0.002545 is bigger than 0.002441. Why I cann't get correct voltage resolution for NI USB-6009?
Thanks!