# Working principle of the AD8232 high pass filter

This is the datasheet of the AD8232.

On page 22 in the section "Additional High-Pass Filtering Options" is this diagram:

It is said that the cut off frequency is located at

I tried to find out where this formula came from.

I tried to make the equivalent circuit of this filter in hopes I could find the transfer function of this circuit.

What I get is not in accordance with the high pass filter circuit and looks like a low pass filter:

I would be happy if someone could explain to me how to get the cut off frequency and equivalent circuit of this high-pass filter.

EDIT: Thanks Andy Aka, so if there is a low-pass filter in the negative feedback of an amplifier the circuit performs as a high-pass? And I still don't know how to get the cut-off frequency formula.

As far as I know the formula for the cut off frequency is $$fc=1/2\pi\sqrt(R1C1R2C2)$$