Filter First Order Lowpass I non-inverting

Butterworth

C1 = 1 / (2 · pi · Fc · R1)

Opamp is just buffer, No gain

First Order Lowpass II non-inverting

Butterworth

C1 = 1 / (2 · pi · Fc · R1)

Gain = (R3 / R2) +1

First Order Lowpass III inverting

Butterworth

C1 = 1 / (2 · pi · Fc · R1)

Gain = (R2 / R1)

(SK) Second order Lowpass I non-inverting

Butterworth

R1 = R2 = R
C2 = Sqrt[2] / (2 · pi · Fc · R)
C1 = C2 /2 = (1 / (Sqrt[2]) / (2 · pi · Fc · R)

(SK) Second order Lowpass II non-inverting

Butterworth

R1 = R2 = R
C2 = Sqrt[2] / (2 · pi · Fc · R)
C1 = C2 /2 = (1 / (Sqrt[2]) / (2 · pi · Fc · R)

Gain = (R4 / R3) +1