To design a second order low pass filter?

Although there are many filter types and ways to implement them, here's an active low-pass filter that's greatly simplified if R1=R2 and the op amp stage is a unity gain follower (RB=short and RA=open). Designing a 2-pole Butterworth filter requires just a few steps. 1. Choose a cutoff frequency fo (Hz). As an example, select fo=10 kHz to reduce a noise signal at 50 kHz and pass your desired signals below 5 kHz. 2. Pick a convenient cap value C2 between 100pF and 0.1 uF. Suppose you've got plenty of 1000pF caps in stock, select this value for C2. 3. Make C1 = 2 x C2 C1 = 2 · C2 = 2000pF 4. Calculate R1 = R2 = 0.707 / (2 · π · fo · C2) R1 = R2 = 0.707 / (2 · π · 10kHz · 1000pF) = 11.2 K ohms ......Rajiv......(scet)