Book Analog Interfacing to Embedded Microprocessors
V1 ¥ R2 ( Vo - V2)R3
V- = V- ; =
R1 + R2 Rf + R3
Expanding and solving for Vo, we get:
R2 Ê Rf ˆ Rf
Vo = V1 1+ - V2
R1 + R2 Ë R3 ¯ R3
We get the same result in all cases, but using the V+ = V- relationship is usually
easier.
Nonresistive Elements
So far, we have looked only at resistors in the opamp circuit. It is possible to use other
elements, such as capacitors and inductors, to produce frequency-dependent circuits.
The equations work the same way, with the following cautions:
• You must substitute impedances, using complex numbers, to calculate the
output.
• The frequency characteristics and roll-off of the opamp must be taken into
consideration.
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