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