2.6.3.8 - Floating/Unconnected Inputs

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 can be placed from the AINx screw terminal to the desired voltage (GND, VS, DACx, …), but obviously that degrades the input impedance.  The resistor value used depends on how close to the desired voltage you need to be, minimum allowable input impedance, sample rate, settling time, resolution, and adjacent channels.