2.7.3.8 - 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, …).  A 10 kΩ resistor will pull the analog input readings to within 1 binary count of any desired voltage, but obviously degrades the input impedance to 10 kΩ. For the specific case of pulling a floating channel to 0 volts, a 100 kΩ resistor to GND can typically be used to provide analog input readings within 100 mV of ground.