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Passive vs. active filters: which to build, and the pitfalls by application

"Active is just higher performance" isn't right. An active filter can't be used on a power rail or at RF, and a passive filter alone isn't enough ahead of an ADC, so the signal's frequency, power, and impedance decide the topology.

Last updated: 2026-10-28 FiltersActive filterSallen-KeyOp-amp

What's actually different

A passive filter is built from resistors, capacitors, and inductors alone. The impedance on either side becomes part of its response.

An active filter is built from an op-amp plus R and C. The op-amp isolates (buffers) the source from the load, so it's unaffected by what's on either side, and it can provide gain. The op-amp's bandwidth, slew rate, output swing, and noise become the ceiling.

Passive vs. active
Passive (R, L, C)Active (op-amp + R, C)
Gain1 or less (loss only)Can exceed 1
Input/output impedanceDepends on the surrounding circuit; shifts responseHigh in, low out. Isolates source from load
Upper frequencyUp to GHz (LC)Set by op-amp GBW; a few MHz is the practical ceiling
Power / current handlingHigh current and voltage OKLimited to op-amp output (tens of mA, within supply rails)
2nd order and aboveNeeds an inductor (large, nonlinear)R and C alone
SupplyNot neededNeeded. Draws quiescent current
NoiseResistor thermal noise onlyOp-amp voltage/current noise adds
Component sensitivityLowSensitive to tolerance at high Q
Passive and active filters shaping signals
Passive and active filters shaping signals. AI-generated conceptual illustration; see the article’s diagrams and text for circuit details and conditions.

The passive pitfall: the surrounding circuit changes the response

An RC low-pass's fc = 1 ÷ (2πRC) assumes zero source impedance and an infinite load. A source resistance Rs adds to R, and a load RL appears in parallel with C, shifting both the DC gain and fc. A passive filter's response isn't fixed on its own, only together with whatever it's connected to.

In LC filters, the termination resistance sets Q. Design an LC filter for a 50Ω system and connect it to a high-impedance circuit, and a peak appears in the passband, wrecking the cutoff response. That's why an EMI filter's datasheet says "attenuation in a 50Ω system": a real power line's impedance (a few Ω to a few hundred Ω, varying with frequency) gives attenuation far from the catalog figure.

Stack the same RC twice and the second stage loads the first, so you don't get −40dB/decade. Either make the second stage's R at least 10x the first, or put a buffer between them, at which point it's an active filter.

The lack of gain is also easy to overlook. Cascade three passive stages and halve the signal, and a following amplifier needs 2x gain to make it up, at 2x the noise.

An RC low-pass is set together with its surroundingsTop: an RC low-pass with source output resistance Rs, resistor R, capacitor C, and load resistor RL. Rs adds to R and RL sits in parallel with C, so both DC gain and fc change. Bottom: the same RC stacked twice. The second stage loads the first, so the response is not −40dB/decade. An RC low-pass is set together with what it's connected to fc = 1 ÷ (2πRC) assumes a 0Ω source and an infinite load Source Rs R C RL Output resistance Rs adds to R RL sits in parallel with C DC gain and fc both change The same RC stacked twice R C R C Load on stage 1 Not −40dB/decade Fix 1: make the 2nd R 10x or more Fix 2: put a buffer between them
Figure 1: an RC low-pass is set together with the source's output resistance and the load. Stacking the same RC twice makes the second stage a load on the first.

The active pitfall: the op-amp's limits show up directly

For GBW, a second-order Sallen-Key with fc = 100kHz, Q = 0.7 wants an op-amp GBW at least 100x fc (10MHz+). Rule of thumb: GBW ≥ 100 × fc × Q. A general-purpose op-amp with GBW = 1MHz is good for roughly fc = 10kHz and below.

A Sallen-Key (VCVS) topology ties its feedback capacitor to the op-amp's output. The op-amp's output impedance rises at high frequency (tens to hundreds of Ω past the GBW), so from around 100x fc the input leaks through that capacitor to the output, and attenuation stalls around −40 to −60dB, then comes back up. In an ADC anti-aliasing filter, sampling-frequency harmonics get through. The fix is to add a small RC stage after the op-amp, or to use a multiple-feedback (MFB) topology, which is inverting and has no such leakage path.

A large-amplitude signal needs slew rate SR ≥ 2π × f × Vpeak: 3.1V/µs for a 5V peak at 100kHz. The output can only swing to just inside the supply rails, so a single-supply design needs a mid-rail bias.

A sharp response like Q = 5 shifts Q and fc by a few percent for a 1% error in R or C. Splitting it into lower-Q cascaded stages is more stable. Use C0G or film capacitors, because X7R shifts with DC bias and temperature.

The op-amp's voltage noise (a few to a few tens of nV/√Hz) also adds in. µA-class low-current op-amps have only tens of kHz of GBW.

Ideal 2nd-order Sallen-Key response, and the high-frequency leakageX-axis 1kHz–100MHz, y-axis 0 to −100dB. The ideal fc = 10kHz, Q = 0.707 response keeps falling at −40dB/decade, but leakage from the op-amp's output impedance stalls the attenuation from around 1MHz and it climbs back up above that. A curve with a trailing RC stage keeps falling. Calculated: −40dB/decade. Real: stalls around 100x fc fc = 10kHz, Q = 0.707. Leakage magnitude depends on the op-amp and layout 1k 10k 100k 1M 10M 100M 0dB -20dB -40dB -60dB -80dB -100dB Frequency Ideal Real (leakage via output Z) Trailing RC stage fc, −3dB Why it leaks C1 ties to the op-amp output; output Z rises at high freq, so it leaks. Where it bites ADC front-ends. Sampling-freq harmonics get through via this leak. Fix Add an RC stage at the output, or switch to MFB.
Figure 2: a second-order Sallen-Key's calculated response versus leakage from the op-amp's output impedance. Attenuation stalls around 100x fc and climbs back up. An RC stage after the op-amp fixes it.

By application: which to choose, and what to watch for

Topology and caveats by application
ApplicationTopologyCaveats
ADC anti-aliasingActive (2nd to 4th order)Set the order from the attenuation needed at half the sampling frequency. Add a trailing RC with Sallen-Key. A ΔΣ ADC often needs only a single RC
Power lines (DC-DC output, IC supply pins)Passive only (LC, ferrite bead + C)An op-amp can't handle the current. An LC filter can resonate with load transients and worsen ripple, so add a damping resistor or a capacitor with real ESR
RF (tens of MHz and up)Passive LCOp-amp bandwidth isn't enough. Design for 50Ω termination; in the build, inductor self-resonance and capacitor ESL set the response
Sensor signals (a few Hz to tens of kHz, mV level)ActiveFeed a high-impedance source to the non-inverting input, with gain and filtering in one stage. Watch offset and 1/f noise
Audio (20Hz–20kHz)Mostly activeUse film capacitors to avoid ceramic distortion
EMI suppression (cables, connectors)Passive only (common-mode choke, ferrite bead, Y capacitor)Attenuation swings with the surrounding impedance, so measure it
Low-current, battery-powered circuitsPassive if possibleIf active, pick the lowest-current op-amp that hits the needed fc
Smoothing digital signals (debounce, edge softening)Passive RCFollow it with a Schmitt-trigger input
Notch (removing 50/60Hz)Active (twin-T plus op-amp)A passive twin-T has Q = 0.25, too broad. The notch drifts, so a digital filter is often more stable
Application and frequency mostly decide the topologyX-axis is the signal or the frequency to be blocked Application and frequency mostly decide the topology X-axis is the signal or the frequency to be blocked Small signals: sensors, audio Active (op-amp + RC) Passive LC ADC anti-aliasing Active 2nd-4th order + trailing RC Power lines (current flows) Passive only: LC, ferrite bead + C Cable/connector EMI Passive only: common-mode choke, Y cap Digital signal smoothing Passive RC + Schmitt-trigger input 1Hz 1kHz 1MHz 1GHz Active Passive Passive + handled in logic
Figure 3: a map of topology by application and frequency. Power, EMI, and RF have no choice but passive; small signals and ADC front-ends call for active.

How to decide

  1. If the ratio of signal frequency to blocked frequency is under 10x, a first-order filter isn't enough.
  2. If current exceeds a few mA, voltage exceeds the supply rails, or frequency exceeds 1MHz, go passive (LC).
  3. If the source is high-impedance or the load varies, go active (or passive plus a buffer).
  4. In the prototype, measure passband flatness, stopband attenuation (especially above 100x fc), and distortion at large amplitude. Wherever it doesn't match the calculation, it's almost always one of these three.

Frequently asked questions

Is an active filter pointless below second order?
No. Even at first order it isolates source from load: a voltage follower plus an RC is the simplest case.
How do I choose between Sallen-Key and multiple feedback (MFB)?
Sallen-Key is non-inverting and easy at unity gain, but has the high-frequency leakage. MFB is inverting and has none. MFB where stopband attenuation matters, as ahead of an ADC; Sallen-Key where it doubles as a unity-gain buffer.
Should I avoid passive filters that use an inductor?
At low frequency (kHz and below), yes: they're large, expensive, and bring DC resistance and saturation. For power or RF, there's no alternative.
If I'm using a digital filter, do I still need an analog one?
An analog anti-aliasing filter is mandatory ahead of any ADC. Anything above half the sampling frequency folds down once digitized and can't be removed afterward. The analog filter can be gentle, with the sharp response left to the digital side.

Standards and references

  • TI, Active Filter Design Techniques (SLOA088) — Design equations and component sensitivity for Sallen-Key and MFB
  • Analog Devices, MT-222 / MT-223 — Op-amp requirements for active filters (GBW, output impedance)
  • A. B. Williams, F. J. Taylor, Electronic Filter Design Handbook — Passive LC and active filter design

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