When the primary design goal is a linear amplifier, with high CMRR, it is important to not violate the input common-mode voltage range (V_ICR) of an op amp.

The common-mode input range specifications in the table data use CMRR to set the limit. The limits are selected to ensure CMRR will not degrade more than 3 dB below the CMRR limit if the input voltage is kept within the specified range. The limits cover all process variations, and most parts will be better than specified. The typical specifications are 0.2 V below the negative rail and 1.1 V below the positive rail.

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 V_IN+ is simple to evaluate. In noninverting configuration, Figure 8-1, the input signal, V_IN, must not violate the V_ICR. In inverting configuration, as shown in Figure 8-2, the reference voltage, V_REF, must be within the V_ICR.

The input voltage limits have fixed headroom to the power rails and track the power supply voltages. For one 5-V supply, the linear input voltage ranges from –0.2 V to 3.9 V and –0.2 V to 1.6 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 1.1 V.