1.2.8 Input Impedance
Two basic ways of specifying the input impedance of an op-amp are the differential and the common mode. The differential input impedance is the total resistance between the inverting and the non inverting inputs, as illustrated in Figure 9(a). Differential impedance is measured by determining the change in bias current for a given change in differential input voltage. The common-mode input impedance is the resistance between each input and ground and is measured by determining the change in bias current for a given change in common-mode input voltage. It is depicted in Figure 9(b).