ADLINK DAQe-2006 - Manual (Page 40)

28 Signal Connections
Differential Connection for Floating Signal Sources
Figure 3-3 shows how to connect a floating signal source to
DAQ-/DAQe-/PXI-2016/2010/2006/2005 card in differential
input mode. For floating signal sources, you need to add a
resistor at each channel to provide a bias return path. The
resistor value should be about 100 times the equivalent source
impedance. If the source impedance is less than 100 ohms,
you can simply connect the negative side of the signal to
AGND as well as the negative input of the Instrumentation
Amplifier, without any resistors at all. In differential input mode,
less noise couples into the signal connections than in single-
ended mode.
Figure 3-3: Floating Source and Differential Input