Book 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
is dependent on the open-loop gain of the opamp.
The input impedance range of the LM318 is fairly low as opamps go; the
MAX400 has an input resistance at least 30 MW. Clearly, choosing a better
opamp will alleviate errors caused by input resistance. Another approach is
to change the impedances; changing to smaller resistors for Rf and R1 will
reduce the error caused by input resistance. Of course, this increases the
output current of the opamp and may affect other parts of the circuit.
We ignored the input resistance of the noninverting input in this example.
If the application calls for connecting the opamp input to a high-impedance
source, then the input resistance of the noninverting input must be taken into
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