The common-mode input signal rejection ratio (CMRR) of a differential amplifier is a metric used to indicate its ability to reject two signals or noises (common-mode noise) that have the same amplitude and phase at VIN(-) and VIN(+). It is expressed by the following equation. Figure 3-5 shows a test circuit for the common-mode input signal rejection ratio.
The common-mode input voltage range (CMVIN) is defined as the input voltage range in which the prescribed CMRR is satisfied under the prescribed conditions. The datasheets for Toshiba’s op-amps specify the CMRR value under DC conditions.
where VIN1 and VIN2 are the maximum and minimum CMVIN values, respectively, and VOUT1 and VOUT2 are the output (VO) voltages at VIN1 and VIN2, respectively.
As can be seen from Figure 3-5, the input offset voltage (VIO) discussed in the previous section is the CMRR value under a special condition (VIN = VDD/2).
The op-amp operates as described in Section 1.2. The differential input pair of a typical op-amp is composed of P-channel MOSFETs as shown in Figure 3-6. As the VIN(+) and VIN(-) voltages increase, the drain-source voltage of Qp3 in the current source decreases, causing the current flowing to the differential input pair and the current mirror to decrease slightly. Figure 3-7 shows an example of ID-VDS curves for a discrete P-channel MOSFET. Suppose, for example, that VDS = -1.5 V and ID = 80 mA initially. As the drain current of the MOSFET decreases, its drain-source voltage changes nonlinearly in the saturation region.
Although the internal devices of an op-amp IC are placed and fabricated in such a manner as to make the MOSFETs uniform, they are not perfectly symmetrical at the micro level. In addition, semiconductor chips are mounted on a metal frame in a package and soldered onto a printed circuit board. Therefore, the mechanical stress applied to each element of an op-amp IC differs slightly. These factors cause a slight difference in threshold voltage among the P-channel MOSFETs of the differential input pair. Therefore, a decrease in drain current causes different nonlinear changes in their drain-source voltage. As a result, the input offset voltage has a slope with respect to the input voltage (VIN) in the common-mode input voltage range as shown in Figure 3-8. Therefore, the CMRR values are calculated at the maximum and minimum common-mode input voltages.