Differential input voltage range is normally specified in data sheets as an absolute maximum. Figure 5-4 illustrates this.
If the differential input voltage is greater than the base-emitter reverse break down voltage of input transistor Q1 plus the baseemitter forward breakdown voltage of Q2, then Q1’s BE junction will act like a zener diode. This is a destructive mode of operation and results in deterioration of Q1’s current gain. The same is true if VIN_DIFF is reversed, except Q2 breaks down.
Some devices have protection built into them, and the current into the input needs to be limited. Normally, differential input mode voltage limit is not a design issue.