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This is the datasheet of the AD8232.

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

Figure 56

It is said that the cut off frequency is located at

enter image description here

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:

enter image description here

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 $$ fc=1/2\pi\sqrt(R1C1R2C2) $$

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HPSENSE - High-Pass Sense Input for Instrumentation Amplifier. Connect HPSENSE to the junction of R and C that sets the corner frequency of the dc blocking circuit.

HPSENSE is a negative feedback point for the instrumentation amplifier that processes the input and, if you connect the output of said IA to HPSENSE via a low pass filter (yes, that is what you analysed), the overall transfer function from the input of the IA to "the next stage" is one of a high pass filter.

I would be happy if someone could explain me about how to get the cut off frequency and equivalent circuit of this High-Pass Filter

It is a low pass filter but, due to it being in the negative feedback path of an amplifier, the whole circuit performs as a high pass filter.

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