Floating/Unconnected Analog Inputs (App Note)

No Volts ≠ 0.0 Volts

"No volts" is not necessarily 0 volts, but rather "no volts" means the voltage is undefined.

With quality high-impedance analog inputs the reading from a floating channel (no external connection) will almost never read 0.0.  The reading will usually trend towards the rails (e.g. -10 or +10 volts), but because it is so sensitive will be easily influenced by previously sampled channels, EMI, and any other influence.

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.  The analog inputs on the U6 and T7, for example, have an input impedance of 1 GΩ and bias currents on the order of 20 nA, but if you add a 1 MΩ resistor from the input to GND (to hold it at 0.0 when no signal is driving) the input impedance is now 1 MΩ and the bias current with a 10 volt signal is 10000 nA.  This lower input impedance and higher bias current can be a problem for signals with weak drive or high source impedance.

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, ...).  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.  With the U6 or T7 a 1 MΩ or even 10 MΩ resistor will work.

Note that the high voltage analog inputs on the U3-HV and T4 (AIN0-AIN3) and all the analog inputs on the U12 have a medium input-impedance resistive input network that does cause floating inputs to sit at a known voltage of about 1.4 volts.