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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.

2 comments

We want to perform one high precision differential measurement on AIN0 and AIN1. The voltage range will be 10 volts bipolar.

What should we do with the remaining floating channels?

Because there are the following opportunities:

- Connect them to GND

- Connect them to VS

- Leave them floating

You can just leave unused analog inputs floating.  They provide meaningless readings, but there is nothing wrong with leaving them floating.  If you would rather they read something predictable, connecting them to GND is fine.

As for your differential measurement on AIN0 and AIN1, make sure both signals are referred to ground somehow and your common-mode voltage is valid.  See the Differential App Note.