Silence is Golden: A Practical Guide to Noise Reduction in Electronic Circuits

For our everyday electronic devices to function, they must transmit their internal electrical signals cleanly and reliably. Electrical noise is an ever-present hindrance, but there are ways to mitigate and reduce it through the design of electronic systems.

Electrical noise is any irregular amount of voltage or current that finds its way from an external source into a conductor that’s transmitting electricity. It can affect power, signal, and data circuits, though signal and data ones are more vulnerable due to their lower voltage. Low-voltage circuits are more susceptible to small amplitudes of noise signals and thus face more hazards that can prevent clean transmissions.

What causes electrical noise? Some of its common sources are:

• Capacitive coupling: This is when a higher-voltage line discharges some voltage to a lower-voltage line through an insulating medium, such as air. This turns the lines and air into an effective capacitor. Surges or switching in the higher voltage line can cause this, and low-voltage lines at high frequencies are more vulnerable. An extreme, visual example is lightning discharging from clouds to the ground through the air
• Inductive coupling: Another wireless entry of unwanted voltage, this occurs when a magnetic field is generated by any conductor carrying a current (the effect electric motors rely on). Wires, especially coils, carrying large alternating currents can induce voltages in nearby lower-voltage wires, forming an effective transformer. Inductive coupling is a form of electromagnetic interference (EMI)
• Conduction: All noise is conducted once it reaches a conductor, but noise can originate from conducted sources directly. An example is shared neutrals and grounds meeting each other and causing current leakage
• RFI: Another form of EMI, radio frequency interference is when wires become effective antennas for sending and receiving signals. RFI can originate from everyday communication signals at radio frequencies or appliances such as laptops and monitors
• Ground bounce: With the rapid switching of high-speed digital circuits, signals may not have time to return to their reference state, which builds current and noise

In electronics, PCBs are particularly vulnerable to ground bounce and capacitive and inductive coupling (forms of crosstalk). Without designed noise mitigation, PCB electronics can experience corrupted data, distorted signals, general malfunctions, or total system failure.

Grounding is the best way to remove acquired electrical noise, but it should be done intentionally. By assuming a component’s chassis or frame has infinite ground capacity, electrical noise can remain in the system and affect PCB electronics. To discharge noise, carefully consider signal grounds (reference points) separately from earth grounds for shock hazards.

Here are some grounding practices for reducing noise:

• Design adequate grounding into circuits based on components and operations, and avoid ground loops
• Thoroughly define ground return paths, such as with separate power and ground planes covering the entire board size or by connecting all ground traces to a common ground point
• Ensure coaxial cables or other shielded signal cables have grounding at just one end
• Use larger-gauge grounding wires to prevent noise from forming in them
• Separate signal grounds from power grounds to prevent signal lines from picking up the noise from high-current devices like motors
• Use bypass capacitors to reduce ground bounce, and place them close to device supply inputs

Shielding is a good measure when noise can’t otherwise be eliminated. Use it to form barriers between noise sources and sensitive components.

Custom sheet metal shielding cans envelop components without contacting them and then direct their noise to ground. They form effective Faraday cages and can be mounted with soldering or through-holes. Shielding cans should be grounded to the main ground plane.

EMI shielding gaskets or compounds are isolation layers placed between PCB components, layers, or boards to protect them from EMI noise. These gasket materials use conductors like nickel, copper, or aluminium to absorb electrical noise. EPDM rubber options are available for resistance to the elements, while silicone ones resist UV light and chemicals.

Another shielding option is ferrite plates. Place them in enclosures near noise-source components to filter switching noise and suppress radiating EMI. This measure avoids adding filters to the circuit.

Filtering is generally a measure for preventing electrical noise from reaching PCB electronics in the first place. It’s a gatekeeper for letting only specific signals through. Filtering helps prevent image distortion on screens, disruption in industrial control systems, and poor signals in critical medical devices like MRIs.

Noise filters combine inductors to impede high-frequency signals and capacitors to impede low frequencies, while allowing low and high ones, respectively. Together, this can be fine-tuned to allow only the desired frequencies of the application. Filter types include those for EMI, RFI, and power line noise.

To choose the right filter for protecting a device, consider the device’s voltage rating, temperature range, power supply type, and impedance. Install the filter close to the noise source to reduce noise-carrying wire length and separate its input and output leads to prevent crosstalk between them.

Decoupling capacitors help protect PCB ICs. They minimise impedance between the power supply and ground so the IC can receive clean, low-ripple input currents. You can also use these capacitors to separate digital and analogue signals.

These capacitors maintain steady power if the input voltage drops and protect against excess current if the voltage surges. This is important, given the sensitivity of ICs to power fluctuations. Use electrolytic decoupling capacitors for low-frequency noise and ceramic ones for high frequencies.

To maximise its effectiveness, connect a decoupling capacitor directly to a low-impedance ground plane and as close to the IC input as possible. Place it in series with input/output traces to remove low-frequency noise and in parallel with a resistor to remove high-frequency noise. Ground the capacitor with a dedicated via rather than connecting to the IC’s own ground via, which can create interference.

Choosing the right electronic components can also help reduce electrical noise. For example, selecting linear rather than non-linear components ensures input signals are output at a set ratio, which improves signal predictability. Linear components include ideal resistors, inductors, and capacitors, and some linear circuits featuring them are simple differentiators, amplifiers, and integrators. Non-linear circuits are found in transistors like diodes.

An example of a linear circuit that can reduce noise is a capacitance multiplier. It combines capacitors, resistors, and a transistor into a linear circuit that removes voltage noise. It also acts as a larger capacitor overall than would typically be possible for its size.

Other components for reducing noise include low-noise amplifiers. They increase signal strength without introducing much noise, which is useful in radio communication, electronic testing equipment, and medical instrumentation. Also, use low-noise cables to withstand shocks like twisting or pulling, which can create capacitance and voltage noise from the moving cable layers charging the insulation.

Filtering and shielding mitigate generated noise, but it’s even better to design PCB electronics to minimise noise buildup. A circuit designed around noise reduction will run more efficiently and need fewer noise reduction components.

Besides the grounding measures discussed above, some other PCB layout measures for reducing electrical noise include:

• Making traces thin and wide to minimise capacitive coupling
• Spacing traces at a distance wider than their own width to avoid crosstalk
• Separating analogue zones from digital zones, which create digital noise at high frequencies
• Identifying and avoiding parasitics like high dI/dt and dV/dt nodes and loops
• For adjacent signal layers in a PCB stack, orienting traces horizontally on one layer and vertically on the other to reduce crosstalk
• Spacing traces away from oscillating components, including those on adjacent layers

While electrical noise sources and symptoms vary greatly, you can follow this general process for diagnosing them:

1. Look for noise sources: See what external sources (components giving off EMI or RFI) or internal sources (switching components, high-frequency clocks) could be generating noise
2. Analyse paths: Look for poor grounding, excessive trace lengths, and ground loops, all of which could induce noise
3. Measure noise levels: Validate the presence and strength of noise in signals. Use a spectrum analyser to assess noise in frequency ranges and an oscilloscope for high-frequency time-domain spikes
4. Determine risk level: Assess whether the PCB components can withstand these noise levels and the impact on operations
5. Introduce noise mitigation: Apply common measures like installing shielding and grounding for external EMI, decoupling capacitors to protect ICs from power supply noise, and filters to block specific frequencies and permit others
6. Test results: Measure noise levels again to determine the success of these mitigations

With all these measures in mind, you can successfully prevent electrical noise from interfering with your PCB electronics, ensuring they function correctly and safely. 

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