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I am trying to understand the input impedance of this circuit, which is a part of the schematic of this project.

schematic

simulate this circuit – Schematic created using CircuitLab

The circuit explanation in the linked page says: "Note however that the input impedance is 100k resistance between the - input and ground, and 100k between the + input and the - input."

I don't understand why it is so. Does the sentence refer to the maximum impedance you have when the rotary switch is in position 4? If it were so, why aren't R6 and R7 taken into consideration for the calculation? And why do R6 and R7 need to be precisely 49.9kOhm? What is the meaning of these two resistors?

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The input impedance between + and - input is 100k regardless of rotary switch position. It is the sum of all ratio resistors = 100k. The input impedance of noninverting opamp is huge, so you can wipe them off for easier calculation.

R6 and R7 are overcurrent protection resistors in case of overvoltage input in original schematics (Please, do copy/paste the original schematics, instead), where the current flows through input protection diodes. The precision of R6 and R7 is not critical.

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  • \$\begingroup\$ I updated the schema adding the missing diodes. Thanks. \$\endgroup\$
    – Enrico
    Sep 10 '17 at 10:05
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Input impedance of a opamp is ideally infinite. So R6 and R7 are in series with infinite resistor. It means you can ignore it in your calculations.

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