Page 388 - Op Amps Design, Applications, and Troubleshooting

364 ARITHMETIC FUNCTION CIRCUITS

single exception of the feedback resistor. Selection of the common resistor value is
made by considering the following guidelines:

1. High-value resistances magnify the nonideal op amp characteristics and
make the circuit more susceptible to external noise.
2. Low-resistor values present more of a load on the driving circuits.

Resistor selection will determine the input impedance presented to the vari-
ous signal sources as expressed by Equation (9.1).

where R is the common resistor value and N is the number of inputs to the adder
circuit Since the design goal specifies a minimum input of 6 kilohms, let us sub-
stitute this value into Equation (9.1) and solve for the value of R.

This represents the smallest value that we can use for the input resistors and still
meet our input impedance requirement. Let us choose to use 5.6-kilohm resistors
for RI and for R 3 to R^.

Calculate the Feedback Resistor* The value of the feedback resistor must be
selected such that the voltage gain is equal to the number of inputs. In our case we
will need a gain of 4. Since we already know the value of RJ, we can transpose our
basic noninverting amplifier gain equation to determine the value of feedback
resistor.

Since voltage gain will always be equal to the number of inputs, this can also be
written as