3rd-Order Sallen-Key Low-Pass Filter

Cascade: a 1st-order RC stage (R3, C3) feeds the 2nd-order unity-gain Sallen-Key shown above (R1, R2, C1, C2).

Design from specification

3rd-order all-pole filter realised as a 1st-order RC stage + a 2nd-order Sallen-Key. Pick an approximation and cutoff; all six R/C values are computed and rounded to the chosen E-series.

Approximation: Cutoff f_c [Hz]: C [F] (optional): R series: C series:

Components (analyse / override)

R1 [Ω]: R2 [Ω]: C1 [F]: C2 [F]: R3 [Ω]: C3 [F]:

Results

Components: R1 = — R2 = — C1 = — C2 = — R3 = — C3 = —

2nd-order section: f₀ = — Hz Q = — ζ = — passband gain = —

Transfer function:
H(s) = —

Equivalent block diagram

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Poles

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Phase margin

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Oscillation (ringing) frequency

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Overshoot of the step response

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Final value of the step response

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