"Over-The-Rail" inputs are similar to the "Fail-Safe" inputs, but the difference is the valid input voltage range extends up to the specified maximum input voltage (e.g, 5.5 V) and is independent of supply voltage and proper output switching will still be maintained. The inputs will also remain high impedance even when the comparator supply voltage is removed.

This functionality does come at a cost. When the input voltage is greater than the supply voltage, the input bias currents may reverse direction and increase considerably (100x to 1000x). Offset voltages may also shift value and/or polarity. Propagation delay may also vary. The individual datasheet graphs and applications section of the particular device should be consulted.

The "Over-the-Rail" input is ideal for high-side measurement applications, such as monitoring a battery voltage without a divider while powered off the regulated processor supply voltage. An example would be measuring a 4.2V battery supply directly while the comparator is powered from the digital logic supply for proper output logic levels. Or other applications where the source input voltage is continuous and cannot be loaded down when the comparator power is removed.

Because these inputs lack the upper input clamp, please see the Precautions for "Failsafe" and "Over-The-Rail" inputs section.