This is the datasheet of AD8232 https://www.analog.com/media/en/technical-documentation/data-sheets/AD8232.pdf
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.

 

What you're showing is a variant of the two pole active filter. Typically the critical frequency (fc) is where the filter will give you 3dB roll off.

Google two pole active filter... also the datasheet offers a 3 pole active filter for further rejection of lower frequency.

 

What you're showing is a variant of the two pole active filter. Typically the critical frequency (fc) is where the filter will give you 3dB roll off.

Google two pole active filter... also the datasheet offers a 3 pole active filter for further rejection of lower frequency.
View attachment 180814

yes it is an two pole active filter but
doesn't the high pass filter pass the input signal on the capacitor first? but why does this circuit pass the input signal on the resistor (R2) first and instead like a low pass filter
while on the datasheet it's said that this circuit is a high pass filter

 

I see what you're getting at... the diagram is a little difficult to follow and the block diagram is only a representation of the circuitry... you will have to go through the datasheet and see if you can figure out:







They go further into detail: