I am not really familiar with this NI product, but have some experience with analog/digital converters.
In some cases the input lines need a dc path to AGND (analog ground) to make the converter work properly, since it needs a (very small) bias current at its input(s). If the inputs of such an ADC are just left open, you will see random values at best, and - depending on the input/grounding scheme - maybe extreme values, i.e. values close to the upper or lower end of the range.
You should check the manual whether it is necessary to connect resistors to AGND when using a floating differential non-ground referenced input. If so, you should connect resistors of, say, 1 M Ohm (Mega Ohms) from either input line to AGND.
If you use any kind of ground referenced input configuration (i.e. a configuration where the signal source is connected to AGND) the signal source usually will be able to supply the bias current provided that the signal source does not have an extremly high output impedance. In these cases you can refer the signal to as a resistor, and current will flow from/to AGND to the input lines via the signal source and its power supply.
