Analog Interfacing to Embedded Microprocessor Systems, 2nd Edition

Now, if the opamp is an LM318 with a gain of 100,000, and using the nonideal

equation, we get this:

Vo ¼

Av  V1  R1

R1 þ R2 þðAv  R1Þ

þ

Av  V1  R2

R1 þ R2 þðAv  R1Þ

¼

100; 000  1:2V 10k

10K þ 50K þð100; 000  10kÞ

þ

100; 000  1:2V 50k

10K þ 50k þð100; 000  10KÞ

¼ 7:199568V

This is close to the ideal 7.2 V result, but it is off by 432 mv. This error would

not be a problem in an 8-bit, 0–5 V system, but it is 5 steps in a 16-bit ADC system.

The problem gets worse with higher closed-loop gain. Suppose that R2 is 110 K

(gain ¼ 12) and the input is 0.6v. The ideal output is still 7.2 V. The actual output

with a gain of 100,000 is 7.199136, which is 864