A Sallen-Key low-pass filter design toolkit - EDN Asia
transfer function - 2nd order passive low pass filter cutoff frequency with additional paralel resistor before the second capacitor - Electrical Engineering Stack Exchange
Sallen-Key Low-pass Filter Design Tool
Theory
Active Low Pass Filter Circuit Design and Applications | Filters, Electrical circuit diagram, Circuit design
Second Order Filters | Second Order Low Pass Filter
Analog Tutorial Lesson 8: Designing Cascaded Active Op-Amp Filters - Emagtech Wiki