How can I calculate precisely the roll-off frequencies of this configuration? The passband seem to be between 3.1Hz to 78.1Hz. I tried using the equations for the inverting active bandpass filter but they gave different results. I can't seem to be able to get these values with any combination of values, what am I missing out?
The passband seem to be between 3.1Hz to 78.1Hz
C4 and R6 define the LF cut-off and that equals 3.38 Hz
C3 and R4 define the HF cut-off and that equals 72.34 Hz
If you are measuring a difference in your sim then this is likely due to imperfections in the LM324 i.e. non-infinite input resistance. There will also be a slight effect due to C6 and R10.