When the primary design goal is a linear amplifier with high CMRR, the design must remain within the input common-mode voltage range (VICR) of an op amp. These ranges are referenced off of each supply as an input headroom requirement. Ensured operation at 25°C is maintained to the negative supply voltage and to within 1.3 V of the positive supply voltage. The common-mode input range specifications in the Electrical Characteristics table use CMRR to set the limit. The limits are selected to ensure CMRR does not degrade more than 3 dB below the minimum CMRR value if the input voltage is within the specified range.
Assuming the op amp is in linear operation, the voltage difference between the input pins is small (ideally 0 V); and the input common-mode voltage is analyzed at either input pin with the other input pin assumed to be at the same potential. The voltage at VIN+ is simple to evaluate. In the noninverting configuration of Figure 62, the input signal, VIN, must not violate the VICR. In the inverting configuration of Figure 63, the reference voltage, VREF, must be within the VICR.
The input voltage limits have fixed headroom to the power rails and track the power-supply voltages. For one 5-V supply, the typical linear input voltage ranges from –0.2 V to 3.8 V and –0.2 V to 1.5 V for a 2.7-V supply. The delta headroom from each power-supply rail is the same in either case: –0.2 V and 1.2 V, respectively.