Example Passive 100khz Low-Pass LC Filter Design Guide

To build an example passive 100khz low-pass lc filter, you need to match your inductor and capacitor values to the specific resistance of your circuit. If you ignore the source and load impedance, your filter will either “ring” like a bell or fail to cut the noise you are trying to kill.

Building a 100kHz Low-Pass LC Filter

A standard 100kHz low-pass LC filter for a 50-ohm system uses an 80µH inductor in series and a 32nF capacitor in parallel to ground.

In practice, a low-pass filter is a gatekeeper. It allows slow, low-frequency signals to pass through while slamming the door on high-frequency noise. Unlike simple resistor-capacitor (RC) filters, an LC filter uses an inductor (L) and a capacitor (C). This combination creates a “sharper” cutoff. While an RC filter rolls off at 20dB per decade, an LC filter drops off at 40dB per decade. That means it is much better at cleaning up messy power supplies or radio signals.

The math is simple, but the implementation is where most people trip up. To find the cutoff frequency (fc), you use this formula:

fc = 1 / (2 * π * √ (L * C))

If you want a 100kHz cutoff and you are working with a 50-ohm load, you also need to consider the characteristic impedance (Z). You calculate this with:

Z = √ (L / C)

Let’s say you want to hit exactly 100kHz with a 50-ohm impedance. That means:

• Inductance (L): ~79.6 µH (Standard value: 82 µH)
• Capacitance (C): ~31.8 nF (Standard value: 33 nF)

Component Selection Guide

Choosing the right physical materials for your inductor and capacitor is just as important as the numbers on their labels.

Not all components are created equal. If you use a cheap ceramic capacitor with a “Y5V” dielectric, your 100kHz filter might shift to 60kHz just because the room got warm. For a stable 100kHz filter, you want parts that stay consistent.

Component	Recommended Type	Why?
Capacitor	C0G / NP0 Ceramic	Zero temperature drift; stays accurate.
Inductor	Ferrite Core (Shielded)	Prevents magnetic interference; high Q factor.
Circuit Board	Ground Plane PCB	Reduces stray capacitance that ruins the filter.

For example, let’s look at the inductor. If you pick a tiny “chip” inductor, it might have a high Direct Current Resistance (DCR). That resistance acts like a hidden resistor in your circuit, turning your sharp LC filter back into a sluggish RC-style filter. Always check the datasheet for the DCR. You want it as low as possible—ideally under 0.5 ohms for signal filtering.

How to Calculate Your Custom Filter

To calculate your own values, you must first define your load impedance and then solve for L and C using the resonance equations.

Let’s be honest: you probably don’t have an 82µH inductor and a 33nF capacitor sitting in your parts bin right now. You might have a 47µH inductor. Here is how you adjust.

1. Start with your Inductor: Let’s use 47 µH.
2. Rearrange the formula: To find the capacitor needed for 100kHz, the math looks like this: C = 1 / ( (2 * π * 100,000)^2 * 47e-6 ).
3. The Result: You would need a 54nF capacitor.
4. Check the Impedance: Z = √ (47e-6 / 54e-9) = 29.5 ohms.

As a result, this filter would work perfectly if your circuit has an impedance around 30 ohms. If you try to use this “30-ohm” filter in a “1000-ohm” circuit, the signal will “ring” or overshoot at the cutoff point. This is called the Q factor. A high Q factor means the filter is “peaky” and might actually amplify noise right at 100kHz before it starts cutting it.

Step-by-Step Assembly Tips

Building this isn’t just about soldering parts. Here is how to do it right:

• Keep leads short: Long wires act like mini-antennas and add “stray” inductance.
• Avoid breadboards: At 100kHz, a breadboard’s internal metal strips add enough capacitance to shift your cutoff by 5% or more.
• Watch the current: If this filter is for a power supply, make sure the inductor is rated for the current. A saturated inductor loses its “L” value and stops filtering entirely.

The Hidden Truth: Why Your Filter Might Fail

The biggest “secret” in filter design is that the source impedance and the load impedance are part of the filter itself.

Most textbooks show you an “L” and a “C” and call it a day. That’s a trap. In the real world, the device sending the signal has its own resistance, and the device receiving it does too.

Think of it like this: your LC filter is a heavy swinging door. If the person pushing the door (the source) is weak, they can’t get the door moving. If the person on the other side (the load) is pushing back, the door won’t swing as intended.

Here is why that matters: if your load is “High Impedance” (like an Op-Amp input), the LC filter has nothing to “drain” its energy into. The energy bounces back and forth between the inductor and capacitor, creating a massive voltage spike at 100kHz. To fix this, you often need to add a “damping” resistor in parallel with the capacitor. This “wastes” a little energy but keeps the filter stable and smooth.

Another rarely discussed issue is the Self-Resonant Frequency (SRF) of the inductor. Every inductor has a tiny amount of capacitance between its wire coils. At a certain high frequency, the inductor stops being an inductor and starts acting like a capacitor. For a 100kHz filter, you want an inductor with an SRF of at least 2MHz. If the SRF is too low, high-frequency noise will simply “jump” over the inductor and ruin your day.

Summary of the 100kHz Design

In simple terms, an LC filter is your best bet for a clean 100kHz cutoff. By using an 82µH inductor and a 33nF capacitor, you create a robust barrier against high-frequency interference. Just remember to match your impedances and pick high-quality C0G capacitors to ensure your “clean window” stays clean.

Frequently Asked Questions