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Why I cann't get correct resolution for NI USB-6009?

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!
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Hi hmqi,

You said that you are seeing a resolution that's about 0.000104 V off of what the stated specs are. However, it looks like you are slowly ramping up the voltage. What are you using to ramp your voltage up? Is it something like a DC power supply? If so are you sure that those aren't the steps used by that supply?

Some more details on how you are testing these specs could help clear up the apparent discreppancy.

Regards,
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