The number of reactive elements (capacitors or inductors) that contribute independently to the filter's order determines the order of the filter. Two capacitors (or inductors) generally lead to a second-order filter.
In an RC low-pass filter section, resistors are typically in series with the signal path and capacitors are in shunt to ground.
Sallen-Key is a popular active filter topology for implementing second-order filters.
The circuit shown is a Sallen-Key filter topology. It uses an operational amplifier (op-amp) and two RC sections.
The presence of two capacitors (and two resistors in the filter network) indicates that it is a second-order filter. Each RC section contributes one pole to the filter's transfer function.
The configuration of the resistors and capacitors (series R, shunt C, then series R, shunt C to the non-inverting input of the op-amp) is characteristic of a low-pass filter. At low frequencies, capacitors act as open circuits, allowing the signal to pass. At high frequencies, capacitors act as short circuits, attenuating the signal.
The op-amp is typically configured as a voltage follower (gain = 1) or a non-inverting amplifier with a specific gain to shape the filter response (e.g., Butterworth, Chebyshev). The feedback resistors \( R_1, R_1 \) shown are likely part of a non-inverting amplifier configuration, setting a gain of \( 1 + \frac{R_1}{R_1} = 2 \) if connected from output to inverting input and from inverting input to ground respectively. However, if the output \( V_o \) is directly connected to the inverting input, it's a voltage follower.
Regardless of the exact op-amp gain configuration, the RC network determines the filter type and order. This is a second-order low-pass filter.
