Hello,
When channels are left unconnected static charges build up on the inputs. These voltages will change sporadically and sometimes saturate the amplifier in one direction or another. This is why these channels appear to "drift" when scanning them. However, usually this isn't an issue. (Why are you scanning unconnected channels?) Making a connection to a voltage source will eliminate the drifting and the DAQ board will then read just the voltage source. If this is otherwise undesireable or it is causing interference to other channels, then connect the unused channels to ground and they will always read 0.
I hope this helps!
Russell
Applications Engineer
National Instruments
http://www.ni.com/support
