RC and LC Filter Designer with E-Series Values

Enter a corner frequency and pick a series. The calculator searches real E-series values, shows the corner you actually get from each candidate pair, and puts the tolerance band next to it — which is usually several times wider than the rounding error.

Runs entirely in your browser. Nothing is uploaded, stored, or sent anywhere.

The nominal is the least interesting number

Every filter calculator gives you R and C from a corner frequency. The arithmetic is one line, and it is not where designs go wrong. What actually matters is that you cannot buy the values it returns, and that the parts you can buy have tolerances several times larger than the rounding error you just worried about.

So this tool searches the E-series for combinations that land near your target, shows the realised corner for each, and then puts the tolerance band next to it. Once you see that a 0.4% rounding error sits inside a ±10% tolerance band, the question stops being "which pair is closest" and becomes "does the whole band still work".

Note how tolerance propagates differently. In an RC, the corner is inversely proportional to the product, so R and C tolerances add and a pair of ±10% parts gives roughly ±20% on the corner. In an LC it depends on the square root of the product, so the same parts give roughly ±10%. Second-order filters are less tolerance-sensitive in frequency — though their Q is a different matter.

Three things people get wrong about the corner

• At f_c the signal is already down 3 dB and phase-shifted by 45°. A corner placed "at the signal bandwidth" is attenuating and delaying the signal you wanted. If the signal must pass unaffected, put the corner a decade above it.
• One pole is 20 dB per decade, and that is not much. Rejecting an interferer by 40 dB one decade above the corner needs two poles. Making the capacitor bigger moves the corner down; it does not make the skirt steeper.
• Source impedance is part of the filter. An RC driven from a 600 Ω source has 600 Ω in series with its R, and the corner moves accordingly. This is why an anti-alias filter that measured correctly on the bench can be wrong in circuit — the tool includes the source impedance for exactly this reason.

Choosing where in the range to sit

The R and C can trade over many decades for the same corner, and the choice is not free:

• Large R, small C is cheap and small, but the resistor's thermal noise rises with √R and the node becomes high-impedance, so bias currents, leakage and stray capacitance all start to matter. Above about 100 kΩ the stray capacitance of the PCB itself is a meaningful fraction of a small C.
• Small R, large C is quiet and low-impedance, but loads whatever drives it and gets expensive — and a large ceramic capacitor loses a lot of its value under DC bias.

For anything feeding an amplifier or ADC, the op-amp error budget shows what that source impedance costs in bias-current error, and the divider calculator models the resistor's own noise.

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