# 11. Coupling between Multiple Inputs

*Coupling between Multiple Inputs*

The ideal source voltages, *v _{1}* and

*v*, have series resistances,

_{2}*r*and

_{1}*r*respectively. We begin by writing the equations for the effective voltages,

_{2}*v’*and

_{1}*v’*, into the summing amplifier. We assume that zero current enters the op-amp. With

_{2}*v*set equal to zero, as shown in Figure 47(b), the voltage into the op-amp,

_{2}*v’*, is

_{2}When *v _{1 }*= 0, the

*v’*voltage is [see Figure 47(c)],

_{1}Notice that the *v’ _{2}* voltage comes from

*v*and the

_{1}*v’*voltage comes from

_{1}*v*. This coupling effect produces undesirable

_{2}*cross-talk*between the two inputs. The effect can be eliminated by designing a system with

*r*and

_{1}*r*approaching zero. Hence, to eliminate coupling, each non-inverting multiple input should be driven with an op-amp that has zero (or very low) output impedance.

_{2}