Common mode input impedance

Question

I was looking at the datasheet of the THAT 1510 mic preamp IC, they suggest the following application circuit:

In the datasheet it says that the T network formed by R1, R2, and R7 gives a common mode input impedance of R1+2R7 = 45K to the right of the coupling capacitors (thats without counting R3, R4, R5 and R6), however I dont know how they got the 45K figure.

According to my analysis a common mode input voltage is the one applied to both inputs, something like this:

^{simulate this circuit – Schematic created using CircuitLab}

In that case it is clearly seen that R1 and R2 are in parallel and both are in series with R7, so the common-mode impedance should be 22500 ohms, what am I doing wrong?

Also, just for the sake of argument, the differential input impedance should be around 2K.

Answer 1

what am I doing wrong?

Nothing! The datasheet is wrong. (Yeah, that happens, we're humans and all make mistakes).

The common mode impedance is R1//R2 + R7 = R1 / 2 + R7 = 22.5 kohm.

I think the "2" for placing two resistors in parallel (R1 and R2) somehow got attached to R7 in the datasheet resulting in a wrong value.