(Signal Voltage + maximum common mode voltage) should be < documented voltage specification

The A/D module inputs are differential in nature but not isolated from system ground. This means that the (+) and (-) legs must be referenced to a system ground.

Example: If (+) leg is at +20 V with respect to the system ground, and the (-) leg is at -20 V with respect to the same system ground, the channel will measure 40 V differential.

Any common mode voltage introduced could bring either the (+) or (-) leg out of the (common mode + signal voltage) range. This would saturate the differential operational amplifiers because the amplifiers themselves are not isolated.