Book Analog Interfacing to Embedded Microprocessors
(typical). Just for simplicity, we will assume that the source driving the non-
inverting input has a low enough impedance that the current is negligible.
This will allow us to examine the inverting input only.
Figure 9.2 shows the effect of input resistance, Ri, from the inverting to
noninverting inputs. If we assume that the input resistance is equal to the
typical value, 3 MW, then the opamp output equation looks like this:
R1Ri + R2Ri
Vo =
R1Ri + R2R1+ R2Ri
+ R1Ri
Av
Figure 9.2
Opamp input resistance.
216 Analog Interfacing to Embedded Microprocessors
If Av is very large, then the /AV term goes to zero, the Ri term cancels out,
and the result is equal to the ideal equation. So the effect of input resistance
annaabi.ee 
