Is 'absolute accuracy' typical or worst case?

[Sean.C]

For the PXI-6225, the absolute accuracy (for 10V range) is given as 3100uV.  I'm assuming this is worst case given the conditions specified. 

Is there any data on nominal/expected accuracy, instead of worst-case accuracy?  What does the distribution look like?  By that I mean, do we really expect all units to cluster at 3100uV error or is it evenly distributed from +/-3100uV?  Is it a normal distribution centered at 0, but extending to 3100uV after 2-3 standard deviations?

If anyone can lend me some insight on this I would appreciate it.

Thanks,

Sean

[stilly32]

The accuracy specifications for M Series are based on 3 standard deviations. The 3100uv is aboslute accuracy at full scale, meaning reading 10V in a 10 V range. Look at the rest of the specs to calculate for a specific voltage measurement. You can also check the assumptions it makes as far as temp change from last self calibration.

Cheers,

Andrew S

[Sean.C]

Thanks for replying stilly32,

For what I was working on I assumed a gaussian distribution with a mean of 0V with +/-3100uV @ 3 standard deviations. 

For future reference though... Do you (or anyone else who might read this) know if the accuracy is biasless or not?  (i.e. Is there a correlation between the error of different cards such that if I took a measurement independently using 100 different daq cards and averaged the results the accuracy would be of the form "value + X +/- Y" where X is less than 3100uV and Y is less than 310uV instead of "value +/- 310uV".)

Also, is the assumption of a gaussian distribution valid?  Is there a published measurement uncertainty budget for the cards somewhere?

Thanks,

Sean

[ChrisR.]

There will be a bias, if for no other reason than that all the boards use the same type of voltage reference, and its tempco has a characteristic shape.  There is probably also similarity between ADC nonlinearities.  Probably all you can count on being completely uncorrelated between boards is the noise.

Why do you want this information?  What are you trying to do?

Chris