Help Needed: USB-6008_Error between Input & Output Value. Thanks!!

[Dolala]

I am using NI DAQ device USB-6008. I use 18 AWG wire connecting AO0 & AI0 two ports to do the self test procedure. In the self-test process, when I set up the analog input value as "3", the output gave me "1.6". When input value is "0", the output value is "-1.4". There is always "1.4" difference between input and output value. How do I fix this problem? Thanks! 

[Henrik_Volkers]

AI in differential mode?   

If you look at the manual picture 7 the inputs are biased.

Maybe it's wise to provide two cables 😉  because: “ground” is a convenient fantasy