An
adder circuit can be made by connecting more inputs to the inverting
op amp (Figure 3–4). The opposite end of the resistor connected to
the inverting input is held at virtual ground by the feedback;
therefore, adding new inputs does not affect the response of the
existing inputs.
Figure
3–4. The Adder Circuit
Superposition
is used to calculate the output voltages resulting from each input,
and the output voltages are added algebraically to obtain the total
output voltage. Equation 3–6 is the output equation when V1 and V2
are grounded. Equations 3–7 and 3–8 are the other superposition
equations, and the final result is given in Equation 3–9.
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