2.6.3.8 - Floating/Unconnected Inputs [U3 Datasheet]

The reading from a floating (no external connection) analog input channel can be tough to predict and is likely to vary with sample timing and adjacent sampled channels.  Keep in mind that a floating channel is not at 0 volts, but rather is at an undefined voltage.  In order to see 0 volts, a 0 volt signal (such as GND) should be connected to the input.

Some data acquisition devices use a resistor, from the input to ground, to bias an unconnected input to read 0.  This is often just for “cosmetic” reasons so that the input reads close to 0 with floating inputs, and a reason not to do that is that this resistor can degrade the input impedance of the analog input.

In a situation where it is desired that a floating channel read a particular voltage, say to detect a broken wire, a resistor (pull-down or pull-up) can be placed from the AINx screw terminal to the desired voltage (GND, VS, DACx, …).  A 100 kΩ resistor should pull the analog input readings to within 50 mV of any desired voltage, but obviously degrades the input impedance to 100 kΩ. For the specific case of pulling a floating channel to 0 volts, a 1 MΩ resistor to GND can typically be used to provide analog input readings of less than 50 mV. This information is for a low-voltage analog input channel on a U3.

Note that the four high-voltage channels on the U3-HV do sit at a predictable 1.4 volts. You can use a pull-down or pull-up resistor with the high-voltage inputs, but because their input impedance is lower the resistor must be lower (~1k might be typical) and thus the signal is going to have to drive substantial current.