EMC and Noise at Board Level
Where noise comes from and how it gets in: grounding, decoupling, return paths, filters, ESD and surge protection, with worked numbers.
On this page
- What EMC means
- How noise gets from source to victim
- Return currents and the ground plane
- Decoupling capacitors
- Grounding and ground bounce
- Traces, crosstalk and edge rates
- Filters: ferrites, LC and common-mode
- Cables and connectors
- ESD, surge and transient protection
- Layout rules that matter most
- Finding and fixing noise problems
- Common faults
- FAQ
What EMC means
Electromagnetic compatibility (EMC) is the ability of a circuit to work in its electrical environment without disturbing it and without being disturbed by it. Two halves: emissions (what your board radiates or sends down its cables) and immunity (what it survives: radio fields, fast transients, static discharge). Most noise problems in a project, from a microcontroller that resets when a motor starts to an ADC that jitters, are small EMC problems. This page gives the practical rules and the numbers behind them. For power supplies see Power Electronics: Buck, Boost and Thermal Design, and for noise in industrial wiring see Cabling, Grounding and Noise.
| Term | Meaning |
|---|---|
| Emission | Noise leaving the product: radiated (through the air) or conducted (along cables) |
| Immunity (susceptibility) | How much noise the product tolerates before it misbehaves |
| Source, path, victim | Every problem has all three; remove any one to cure it |
| Differential mode | Noise current flowing out on one conductor and back on the other |
| Common mode | Noise current flowing the same way on both conductors and returning through ground or the air: the main cause of radiation from cables |
| Standards | CISPR 32 / FCC part 15 (emissions), IEC 61000-4 series (immunity tests), IEC 61326 (measurement equipment) |
How noise gets from source to victim
| Coupling | How it works | Typical cure |
|---|---|---|
| Conducted (shared impedance) | Two circuits share a supply or ground wire: one’s current causes a voltage that the other sees | Separate returns, a star ground, decoupling, filters |
| Capacitive (electric field) | A fast-changing voltage on one conductor couples through the stray capacitance to a nearby one | Spacing, a ground guard trace, shielding, lower impedance on the victim |
| Inductive (magnetic field) | A changing current makes a magnetic field that induces a voltage in a nearby loop | Smaller loop areas, twisted pair, spacing, magnetic shielding |
| Radiated (far field) | A loop or cable acts as an antenna | Smaller loops, slower edges, filters on cables, shielded enclosure |
The sources on a typical board are fast edges (digital clocks and switch nodes), large di/dt (a processor core, a motor driver) and cables that act as antennas. The size of the problem grows with frequency, loop area and current: halving the loop area of the worst loop is often the single best fix.
Return currents and the ground plane
- Every signal needs a return, and at low frequency the current takes the path of least resistance, but at high frequency (above about 1 MHz) it takes the path of least inductance: right under the trace in the plane below it.
- Use a solid ground plane under as much of the board as you can. On a 2-layer board, pour ground on the bottom and route most signals on top.
- Never cut the plane under a fast signal. A slot, a gap between pours or a wide via field forces the return to detour and creates a loop antenna.
- If a signal changes layers, put a ground via next to the signal via so the return current can change layers too.
- Skin effect: at high frequency the current flows only near the surface of the copper, to a depth δ = √(ρ ÷ (π f μ)). For copper that is 65.2 µm at 1 MHz, but 6.5 µm at 100 MHz: the effective resistance of a trace rises with √f, but the plane directly below stays the best return path.
Decoupling capacitors
Each chip that switches draws sharp pulses of current. The supply and its wiring have inductance and cannot respond that fast, so the chip takes the pulse from a decoupling capacitor right next to its supply pin. But a real capacitor has series inductance (ESL) from its package, leads and the board traces, so above its self-resonant frequency fSR = 1 ÷ (2π √(L C)) it acts as an inductor and stops helping.
Worked example: why the loop length matters
- A 100 nF capacitor with 1 nH of ESL has fSR = 1 ÷ (2π √(100 nF × 1 nH)) = 15.9 MHz; a 10 nF capacitor with 0.8 nH is 56.3 MHz; a 1 µF capacitor with 2.5 nH is 3.2 MHz.
- A trace has about 1 nH per millimeter. A 10 mm connection between the capacitor and the chip therefore adds 10 nH, whose reactance at 100 MHz is 2π × 100 MHz × 10 nH = 6.3 Ω: ten times worse than the capacitor itself. That is why decoupling capacitors go next to the pin, with vias straight to the ground plane, not at the end of a long trace.
- Good practice: one 100 nF (0402 or 0603) per supply pin, placed between the pin and the via, plus a 1–10 µF capacitor per few chips, and a bulk capacitor at the regulator. Mixed values help a little, but the placement matters more than the value.
- Several capacitors in parallel have a lower inductance, but their resonances can create a parallel resonance (a peak) between two different values: use the same value, or damp it with a small resistance, if a peak is a problem.
Grounding and ground bounce
The ground connection inside a chip package and on the board has inductance. When several outputs switch at once, the current change through that inductance produces a voltage: ground bounce, V = L × di/dt, which shifts the reference seen by every input on the chip.
Worked example: eight outputs switching together
A 74HC244 drives eight outputs, each switching 20 mA in 2 ns, through a ground pin and lead of 5 nH. The current step is 8 × 20 mA = 160 mA, so V = 5 nH × 0.16 A ÷ 2 ns = 0.4 V. That is a large part of a 1.5 V low-level noise margin. Slower edges (a series resistor on each output), fewer outputs per chip, more ground pins and a lower-inductance path reduce it.
- Single point, star or plane? At low frequency (audio, sensors, below 1 MHz) a star ground keeps noisy returns away from quiet ones. At high frequency use a plane: the return finds its own low-inductance path. Mixed boards: one solid plane, with the analog and digital sections kept apart so the digital return currents do not flow under the analog part.
- Join analog and digital ground at one point (usually under the ADC or at the power entry), never with a gap that a signal must cross.
- High-current returns (motors, LEDs, switching converters) should go directly to the supply, not through the sensor ground.
- Connector shields and chassis bond to the plane through a low-inductance connection (several vias), often with an RC or ferrite to the signal ground.
Traces, crosstalk and edge rates
Noise radiates from fast edges, not from the clock frequency itself: a 1 MHz clock with a 1 ns edge makes noise out to hundreds of megahertz. The usable bandwidth of an edge is about 0.35 ÷ trise, so a 1 ns edge has content up to about 350 MHz. Slow the edges down as far as the function allows (series resistors of 22–100 Ω, lower drive-strength settings, slower slew-rate options).
Worked example: spacing to reduce crosstalk
- Crosstalk between two parallel traces falls rapidly with spacing. The 3W rule says to space traces at least three trace widths apart (center to center) to keep the coupling low (about 70 percent less than at one width).
- With 0.2 mm traces, the 3W rule is a center-to-center spacing of 0.6 mm, which means a gap of 0.4 mm: well within the board’s manufacturing minimum, so it costs almost nothing.
- Over a ground plane, the coupling also depends on the height above the plane: a thin dielectric between the trace and plane (0.1–0.2 mm on a four-layer board) confines the fields and reduces crosstalk much more than spacing alone.
- Put a guard trace (grounded at both ends and every few centimeters) between a clock and a sensitive line if they must run together.
- Route fast and sensitive signals over an unbroken plane, and keep clocks short.
- Avoid long parallel runs of fast and slow signals; cross at 90°.
- Terminate long fast traces (a series resistor at the source close to the driver) to stop reflections: see PCB Design and Layout.
- Keep high-impedance nodes small (op-amp inputs, ADC inputs, crystal pins) and away from the switch node.
Filters: ferrites, LC and common-mode
| Filter | How it works | Typical use |
|---|---|---|
| Ferrite bead | An inductor that is lossy at high frequency, so it absorbs noise instead of reflecting it: e.g. 600 Ω at 100 MHz | In a supply line to an analog or radio section, on a cable, on a clock output |
| RC low-pass | Resistor and capacitor; first-order roll-off | Slow signals, ADC inputs, reset pins (see the RC Filter Calculator) |
| LC low-pass (π or T) | Inductor and capacitors; second-order roll-off, 40 dB per decade | Supply inputs, the output of a switching converter |
| Common-mode choke | Two windings on one core: transparent to the wanted differential signal, high impedance to common-mode noise | USB, Ethernet and mains inputs, long cables |
| Feed-through capacitor, EMI filter module | A capacitor through a panel wall: very low inductance | Enclosure entries |
Worked example: an LC filter on a supply
- A 10 µH inductor with a 1 µF capacitor has a cutoff fc = 1 ÷ (2π √(10 µH × 1 µF)) = 50.33 kHz.
- A 100 kHz converter ripple is a factor of 100 kHz ÷ 50.33 kHz = 2 above the cutoff, so a second-order filter attenuates it by about 40 × log10(2) = 12 dB (a factor of about 4).
- Take care: an LC filter can ring at its resonance and make the supply worse. Add a small damping resistance (an electrolytic capacitor with ESR, or a resistor in series with the capacitor), and check the inductor’s saturation current.
- A ferrite bead of 600 Ω at 100 MHz in series with a 100 nF capacitor makes a simple, well-damped filter for a noisy rail: its impedance is mostly resistive at the frequency of the noise.
Cables and connectors
- Cables are the antennas. A cable only a few microamperes of common-mode current can fail a radiated emissions test, and the same current picks up noise from outside.
- Filter every cable at the connector: a common-mode choke or ferrite and capacitors to the chassis ground or the ground plane, close to the connector.
- Bond shields 360° to the enclosure or the board ground at the connector, not through a pigtail wire.
- Twisted pairs and balanced signaling (RS-485, CAN, Ethernet) cancel the magnetic coupling and reject common-mode noise.
- Keep a separate return for each signal group where you can, and put the return pin next to the signal in connectors (ground pins between signals on a ribbon cable).
- Unscreened cables for power and screened for signals, laid apart from each other.
ESD, surge and transient protection
A static discharge from a person is a pulse of tens of amperes in a nanosecond. The IEC 61000-4-2 test uses a 150 pF capacitor discharged through 330 Ω from up to 8 kV (contact). Every connector that a person can touch needs protection.
Worked example: the current from an 8 kV discharge
- From the test network, the peak current into a short circuit is 8000 V ÷ 330 Ω = 24.2 A (the real test waveform has about 30 A in its first nanosecond because of stray effects).
- A bare 3.3 V pin cannot take this. A TVS diode to ground with a clamping voltage of 15 V at 16 A turns it into a pulse the following chip can survive, and a series resistor (100–330 Ω) limits the current that reaches the pin and its protection diodes: with a 330 Ω series resistor and a 15 V clamp, the current into the clamp is (8000 − 15) ÷ 330 ≈ 24.2 A for a very short time.
- Place the TVS at the connector, with a short, low-inductance path to ground (the pad and a via), before the series resistor, so that the pulse does not run along the board.
| Threat | Waveform | Protection |
|---|---|---|
| ESD (IEC 61000-4-2) | 8 kV contact, nanosecond rise | TVS or ESD diodes at every external connector; series resistors; a ground plane |
| Electrical fast transients (IEC 61000-4-4) | Bursts of fast pulses on power and signal lines | Filters, common-mode chokes, a good ground |
| Surge (IEC 61000-4-5) | Lightning or switching: microseconds, hundreds of volts to kilovolts | Varistors (MOV), gas discharge tubes, larger TVS on power and long cables |
| Supply spikes and reversal | Automotive load dump, hot-plug ringing | TVS on the input, reverse-polarity protection, a series resistor or polyfuse (see Switches, Connectors and Protection) |
| Radiated fields (IEC 61000-4-3) | 3–10 V/m at 80 MHz–6 GHz | Small loops, filtering, a shielded enclosure |
Layout rules that matter most
| # | Rule | Why |
|---|---|---|
| 1 | Keep the high-current, fast loops tiny (switching converters, clock drivers, motor drivers) | Radiation and ringing scale with loop area |
| 2 | Put decoupling capacitors next to the pins, with vias directly to the plane | Each millimeter of trace is a nanohenry |
| 3 | Use an unbroken ground plane; never route fast signals over a split | Return currents follow the signal |
| 4 | Separate noisy and quiet circuits; put the connectors at one edge of the board | Noise stays away from sensitive nodes and cables |
| 5 | Filter and protect at the connector, not across the board | The cable is where noise enters and leaves |
| 6 | Slow the edges that need not be fast | Bandwidth is the enemy |
| 7 | Keep sensitive analog nodes small and guarded | High impedance picks up noise easily |
| 8 | Keep antenna-like stubs short (unused pins, test points, long traces to nothing) | They radiate and receive |
Finding and fixing noise problems
| Tool | What it shows | How to use it |
|---|---|---|
| Oscilloscope with a short ground spring | Ripple, spikes and ringing on rails and signals | Probe at the chip pin, not at the connector; use the ground spring, not the long clip lead, to avoid adding noise |
| Near-field probes (loop and E-field probes) | Where on the board the noise is strongest | Move a probe over the board with a spectrum analyzer or scope and find the hot spot |
| Current clamp on a cable | Common-mode current on a cable | Clamp around the cable (all conductors): it should be near zero |
| Spectrum analyzer or an SDR | The frequencies of the emissions | Find the clock harmonics and the converter frequency |
| Experiments | Whether a change helps | Add a ferrite, a capacitor or a shield to a cable; change a clock speed; unplug a cable |
| Pre-compliance testing | Likely to pass or fail the standard | Test in a shielded box or at a lab before the formal test |
Common faults
| Symptom | Likely cause | Try |
|---|---|---|
| Microcontroller resets when a motor or relay switches | Supply dip or spike, shared ground | Bulk and 100 nF capacitors, separate supply, a snubber or flyback, a TVS |
| ADC readings jitter | Switching noise on the supply or reference, a high source impedance | Filter the input, decouple the reference, separate the analog ground, average |
| Fails an emissions test at a clock harmonic | A loop antenna, a long cable, no filter | Find the hot spot with a probe; add a ferrite, shorten the loop, spread the clock |
| Random lock-ups when a person touches it | ESD entering through a connector or a case | TVS diodes, a series resistor, a bonded enclosure, a watchdog |
| Communication errors only with a long cable | Common-mode noise, a ground difference, no termination | Differential signaling, a common-mode choke, shield bonding, isolation |
| Noise changes when a cable is moved | The cable is a significant antenna or loop | Filter the cable at the connector; route it away from the source |
| Audio hum | A ground loop through shields or supplies | Break the loop; balanced connections; bond at one point |
| A radio or sensor on the board does not work with the converter running | Converter noise on the supply or in the air | LC filter on the supply, a different frequency, shielding, distance |
Related
- Design, Testing and Troubleshooting: the overview of this guide.
- Capacitors: capacitor types, ESR and decoupling basics.
- Power Electronics: Buck, Boost and Thermal Design: switching converters, the biggest noise source.
- Switches, Connectors and Protection: fuses, TVS and protection parts.
- Driving Motors and Actuators: flyback and PWM noise from drives.
- PCB Design and Layout and Cabling, Grounding and Noise.
FAQ
Where should I put a decoupling capacitor?
As close as possible to the chip’s supply pin, between the pin and the capacitor’s via to the ground plane, with the shortest traces. A capacitor a few centimeters away is far less effective at high frequencies.
Do I need a ferrite bead on every supply?
No: use one where a noisy rail feeds a sensitive circuit (an ADC, a radio, an op-amp) or where a cable leaves the board. A bead in series with a heavily loaded supply drops voltage and can cause resonance.
How do I know whether a design will pass emissions tests?
Follow the layout rules, then measure with a near-field probe and a spectrum analyzer, or test in a pre-compliance lab. The loop areas and cable filtering usually decide it.
Is a metal enclosure always better?
A bonded metal enclosure helps both emissions and immunity, but only if every cable is filtered or shielded at the entry; an unfiltered cable carries the noise through the wall.
What is the difference between analog ground and digital ground?
Usually none on a solid plane: they are the same copper. The aim is to keep noisy digital return currents from flowing under the analog circuits, by placing the circuits apart and joining them at one point.
References
- Ott, H. W., Electromagnetic Compatibility Engineering, Wiley.
- Montrose, M. I., EMC and the Printed Circuit Board, IEEE Press.
- IEC 61000-4-2, -4-3, -4-4 and -4-5 (immunity test methods); CISPR 32 and FCC 47 CFR part 15 (emissions).
- Texas Instruments, Understanding Ground Bounce and Decoupling capacitor selection application notes; Analog Devices, A Practical Guide to High-Speed PCB Layout.
Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. This page gives general guidance; compliance with legal EMC requirements needs testing of the actual product.