NI 6115/6120 only NI 6115/6120 only
The following figures show how to connect a floating signal source to a channel on the NI 6115 and NI 6120, respectively.
The figures show a bias resistor connected between AI 0 − and the floating signal source ground. This resistor provides a return path for the bias current. A value of 10 kΩ to 100 kΩ is usually sufficient. If you do not use the resistor and the source is truly floating, the source is not likely to remain within the common-mode signal range of the instrumentation amplifier, so the instrumentation amplifier saturates, causing erroneous readings. You must reference the source to the respective channel ground.
Common-mode rejection might be improved by using another bias resistor from the AI 0 + input to AI 0 GND. This connection gives a slight measurement error due to the voltage divider formed with the output impedance of the floating source, but it also gives a more balanced input for better common-mode rejection.
If a signal source is truly floating, you can use a bias resistor with a smaller value to reduce noise. You can further reduce noise by putting a capacitor in parallel with the bias resistor.