Coupling between Multiple Inputs-TINA and TINACloud Resources

Coupling between Multiple Inputs

When more than one input signal is connected to either the inverting or the non-inverting input to the op-amp, coupling between the inputs can result. This is frequently a disturbing problem, since a variation in one channel can produce an unwanted input into an adjoining channel. Consider the dual-input, non-inverting op-amp of Figure 9.47(a), where each channel is driven with a voltage source in series with a source resistance (the internal resistance of the voltage source).

Figure 47 – Coupling between multiple inputs

The ideal source voltages, v₁ and v₂, have series resistances, r₁ and r₂ respectively. We begin by writing the equations for the effective voltages, v’₁ and v’₂, into the summing amplifier. We assume that zero current enters the op-amp. With v₂ set equal to zero, as shown in Figure 47(b), the voltage into the op-amp, v’₂, is

(98)

When v₁= 0, the v’₁ voltage is [see Figure 47(c)],

(99)

Notice that the v’₂ voltage comes from v₁ and the v’₁ voltage comes from v₂. This coupling effect produces undesirable cross-talk between the two inputs. The effect can be eliminated by designing a system with r₁ and r₂ 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.

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