# What “rule of thumb” to use for mapping -3dB frequency to ideal response for 2nd order Sallen-Key?

For a first order passive filter (RC circuit), we often reach for a the familiar 10X rule. If the -3dB corner frequency is f, then "ideal" frequency response starts at 10f.

What similar factor can be used for a second order Sallen-Key filter which is critically damped (C2 = 2C1)?

If, say, the -3dB roll-off of such a low-pass filter is at 25Khz, until where do we consider the frequency response to be as flat as we would by the 10X rule for RC filter for the same corner frequency?