Noise, Grounding and Decoupling

Introduction

A circuit that works perfectly on paper can misbehave in real life: random resets, jumpy readings, interference with radios. The cause is usually noise, unwanted signals that sneak into the circuit. Learning how noise travels is the key to stopping it.

Sources of noise

• Switching: digital chips, motors, relays and switching power supplies create fast current changes and voltage spikes.
• Power supply ripple and drops when loads suddenly demand current.
• Radio and mains interference picked up by wires and tracks, which act as small antennas.
• Crosstalk between neighbouring tracks or wires.
• Thermal and component noise, inherent in resistors and amplifiers, which matters in sensitive measurements.
• Static discharge (ESD) from people and equipment.

How noise travels

• Conduction: through shared wires, power rails and ground connections.
• Capacitive coupling: between close conductors at changing voltages.
• Inductive coupling: a changing current in one loop induces voltage in a nearby loop.
• Radiation: radio waves from fast-changing signals and long wires.

Often a problem is a mix, so work systematically.

Decoupling

When a chip switches, it demands a sudden burst of current. A long power track has inductance and resistance, so the voltage at the chip dips and bounces. A decoupling (bypass) capacitor placed close to each chip's power pins supplies those bursts locally.

• A common choice is 100 nF (0.1 µF) ceramic across the power and ground pins of each digital IC, with the shortest possible tracks.
• Add a larger bulk capacitor, such as 10 µF, where power enters a board or near noisy loads.
• Place the capacitor between the power pin and ground right at the pin, not at the far end of a track.

Grounding

Ground is the reference for all voltages, and also the return path for current. Poor grounding creates noise.

• Ground loops: if ground connects in more than one path, loop currents and induced voltages cause hum and errors.
• Keep noisy and sensitive grounds separate where they meet only at one point, a star connection, for example motor ground and sensor ground.
• On PCBs, a solid ground plane gives a low-impedance, short return path for most designs.
• Keep return paths short and directly under signal tracks. Current returns by the path of least impedance, and large loop areas pick up and radiate noise.
• Do not rely on a thin ground wire for large currents.

Filtering

A filter blocks frequencies you do not want.

• RC low-pass filter: a resistor in series and a capacitor to ground. It lets slow signals through and reduces fast noise. The cutoff frequency is f = 1 ÷ (2π R C). With R = 1 kΩ and C = 100 nF, the cutoff is about 1.6 kHz.
• Ferrite beads and inductors on power lines and cables block high-frequency noise.
• Shielded cables and twisted pairs reduce pickup and crosstalk.
• Software filtering: averaging several readings reduces random noise in a microcontroller.

Protection from spikes and static

• Flyback diodes on inductive loads
• TVS diodes and series resistors on inputs exposed to the outside world
• Handle static-sensitive parts carefully. Touch a grounded surface first, or use an anti-static wrist strap, and keep parts in anti-static packaging.

Electromagnetic compatibility (EMC)

EMC means that a device does not emit too much interference and tolerates the interference around it. Products sold commercially must pass tests under regulations that depend on the country and product. Good habits from the start make passing easier: decoupling, ground planes, short loops, filtered cables, shielding where needed and careful layout of fast signals.

A noise-hunting routine

1. Describe the symptom, and when it occurs. Does it match motor switching or mains cycles?
2. Look with an oscilloscope at the supply and the troubled signal, with a short ground lead.
3. Change one thing at a time, such as adding decoupling or moving a wire.
4. Isolate: run the circuit from a battery, disconnect the noisy load and see what changes.
5. Record what worked.

Try it yourself

Calculate the cutoff frequency of a low-pass filter with R = 10 kΩ and C = 10 nF (about 1.6 kHz). Then list where you would put decoupling capacitors on a small microcontroller board.

Summary

• Noise arrives by conduction, capacitive and inductive coupling, and radiation from switching, spikes and interference.
• Use decoupling capacitors near chips, good grounding with short return paths, and filters.
• EMC means both emitting little and tolerating interference, and good layout is the best start.