Trystan Casey
Electrical noise is a common issue in industrial facilities, and it can have detrimental effects on the functionality, performance, accuracy, and consistency of circuits. Noise can arise from either external or internal sources, and it refers to any undesirable signal in a circuit, such as powerline hum, shot noise, atmospheric noise, EMI from switching power supplies, and radio transmitters. To reduce noise in electrical circuits, various methods can be employed, including the use of decoupling capacitors, filters, and grounding techniques. For example, establishing a ground plane can help minimize electrical noise by providing a zero-potential reference. Additionally, anti-aliasing filters can remove noise above a certain frequency, and digital filters can be added to software to filter out lower-frequency noise. Other strategies include using a low-pass filter, which is a combination of a resistor and a capacitor, and employing a CLC-style pi filter or a low-noise regulator to the power supply output.
Characteristics and Values of Reducing Noise in Electrical Circuits
| Characteristics | Values |
|---|---|
| Establish a ground plane | All circuits within a system have the same reference potential for comparing different signals and voltages |
| Utilize anti-aliasing filters | Remove noise at frequencies above the filter cutoff frequency |
| Use digital filters | Filter out noise at frequencies lower than the Nyquist frequency |
| Use decoupling capacitors | Large decoupling capacitors ranging from 0.1uf to 10uf |
| Use surface-mount capacitors | Have low ESL and ESR |
| Use LC or RC filters | Reduce noise in conjunction with an LDO |
| Use a low-pass filter | Allow quick voltage changes to stop before the controller while passing slower deliberate changes to the input terminal |
| Use a CLC-style pi filter | May be effective in reducing noise |
| Use a different power supply | Sharing a common ground can help reduce noise |
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What You'll Learn
Use decoupling capacitors, filters and shielding
One of the ways to reduce noise in electrical circuits is to use decoupling capacitors, filters, and shielding.
Decoupling Capacitors
Decoupling capacitors, also known as bypass capacitors, are used to mitigate high-frequency noise signals and reduce power ripples. They provide high transient currents to an Integrated Circuit (IC) and help maintain stable voltage levels. These capacitors are placed near the power pins of an IC to ensure that voltage variations remain within the device's tolerance range. For low-impedance shunting, a single electrolytic capacitor is sufficient, but for signal stabilization, two different types of capacitors are required. Decoupling capacitors are also used to isolate or decouple two circuits, separating AC signals from DC signals or vice versa.
Filters
Filters are used to remove unwanted parts of a signal, such as blocking undesirable frequencies near a radio receiver to reduce radio frequency interference. There are different types of filters, including low-pass, high-pass, band-pass, and band-stop filters, each designed to affect specific frequency ranges. For example, low-pass filters block high frequencies while allowing low frequencies to pass through the circuit. Anti-aliasing filters, a type of low-pass filter, are used in analog-to-digital conversion to condition the analog signal. Capacitors can be used in band-stop circuits to protect sensitive electronics from harmonics in the power supply.
Shielding
Shielding is another effective method to minimize noise in electrical circuits. Electric fields are relatively easier to shield than magnetic fields, and the use of shielding at one or more points can disrupt electrical fields. To shield against magnetic fields, the key is to reduce the loop area by using a twisted pair or returning the current through the shielding. Grounded shielding is typically used above the cut frequency to prevent radiation on a conductor. Additionally, adequate shielding can act as a Faraday cage, providing protection from electromagnetic interference (EMI) and reducing operational costs and problems.
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Use anti-aliasing filters
Anti-aliasing filters are an effective way to reduce noise in electrical circuits. They are typically designed as higher-order active filters using a low-noise op-amp. The goal when designing an anti-aliasing filter is to achieve unity gain across the passband and to set the --3 dB cutoff frequency to be equal to the Nyquist frequency, which is half the intended sampling rate.
The purpose of an anti-aliasing filter is to remove noise and unwanted signals from the ADC input, attenuating them so they do not adversely affect the circuit. This is particularly important when trying to sample a perfect sinusoid, where the sampling rate needs to be at least double the signal frequency.
In the case of optical image sampling, as used by image sensors in digital cameras, the anti-aliasing filter is also known as an optical low-pass filter (OLPF), blur filter, or AA filter. The mathematics of sampling in two spatial dimensions is similar to time-domain sampling, but the filter implementation technologies differ. In digital cameras, for example, two layers of birefringent material, such as lithium niobate, are used to spread each optical point into a cluster of four points.
Anti-aliasing filters can also be used in conjunction with oversampling, where a higher intermediate digital sample rate is applied. This allows a digital filter to sharply cut off aliasing near the original low Nyquist frequency, improving phase response. The higher sampling rate can also moderately improve the signal-to-noise ratio as some noise is averaged out.
For signals that are bandwidth-limited but not centred at zero, a band-pass filter can be used as an anti-aliasing filter. For example, when sampling an FM radio broadcast, an appropriate anti-aliasing filter would be centred on 87.9 MHz with a 200 kHz bandwidth, and the sampling rate would need to be no less than 400 kHz.
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Establish a ground plane
Establishing a ground plane is an effective way to reduce noise in electrical circuits. A ground plane is a layer on the PCB stack-up that is maintained at 0V potential. It acts as a reference plane and a sink for return currents circulating through the PCB.
To establish a ground plane, it is important to ensure that all ground points across the PCB are connected to a common ground point, often called a chassis ground. This common ground point is connected to the metallic enclosure of a device, providing electric shock protection and physical shielding. It is crucial that all ground points have the same potential to prevent ground bounce noises, which occur due to potential differences between various ground points.
When designing a ground plane, it is recommended to cover as much of the board area as possible with ground planes, preferably reserving one layer solely for the ground plane. This helps to minimize the common impedance coupling between different subsystems. Additionally, it is important to design well-defined return paths to the ground, especially for high-frequency signals, avoiding interruptions or excessive use of vias. If space or budget constraints make it challenging to create a full ground plane, a single-point grounding for low frequencies or a star grounding for high frequencies can be employed, connecting all the ground traces at a common ground point.
The ground plane reduces the inductance of the signal return path, minimizing noise from transient ground currents. It also helps to prevent the formation of ground loops, which can induce EMF and cause disruptions. By following these guidelines, the ground plane will contribute to reducing noise and enhancing the performance of the electrical circuit.
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Use a low-pass filter
Noise in electrical circuits can be a difficult problem to deal with. It can arise from either an external or internal source and can have detrimental effects on the functionality, performance, accuracy, and consistency of the circuit. One effective method of reducing noise in electrical circuits is to use a low-pass filter.
A low-pass filter is a type of circuit that allows low-frequency signals to pass through while attenuating or blocking high-frequency signals. The key term in a low-pass filter circuit is the cutoff frequency, which is the frequency above which no variation of voltage with respect to time may enter the circuit. For example, if a low-pass filter has a cutoff frequency of 30 Hz, any interference associated with line voltage (60Hz) would be filtered out, but a signal of 25 Hz would be allowed to pass.
Low-pass filters can be constructed using either inductors or capacitors, with each having its own advantages and disadvantages. Inductive low-pass filters are often preferred in AC-DC power supplies to filter out the AC "ripple" waveform created during the conversion of AC to DC, resulting in pure DC output. This is because inductive low-pass filters do not require additional series resistance, which is undesirable in high-current circuits. On the other hand, capacitive low-pass filters may be more suitable when low weight and compact size are higher priorities.
Low-pass filters can be placed on the signal wires between a signal and an electronic device, such as a DAQ module, to prevent high-frequency noise from entering the device's inputs. They can also be used in conjunction with an LDO (low-dropout) linear regulator to further reduce noise in power supplies.
In addition to their use in electrical circuits, low-pass filters are also used in various other applications, such as audio noise reduction, signal conditioning before analog-to-digital conversion, smoothing sets of data, and even in finance as a moving average operation.
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Use a different power supply
Using a different power supply can be an effective way to reduce noise in an electrical circuit. Noise, or interference, can have detrimental effects on the functionality of electronic circuits. It is important to first understand the different types of noise that can occur in a circuit. Power-supply noise is typically a combination of unwanted periodic ripple and spikes, along with random noise from external or internal sources. This can include electromagnetic interference (EMI) and conducted noise.
One way to reduce noise is to use a different type of power supply, such as switching from a conventional linear power supply to a switching power supply. While switching power supplies have excellent features such as small size, lightweight, and high efficiency, they also have a unique weakness: they generate high-frequency noise due to the rapid switching of the current. To address this, various noise suppression measures can be applied, including EMC (Electromagnetic Compatibility) strategies such as reflection, absorption, bypassing, and shielding.
Another approach is to utilize filters to remove noise from the power supply, similar to how a filter can remove noise from a signal. Output capacitors can be used as part of the filtration process as they react against the output impedance of the power supply circuit. Increasing the output capacitance will result in noise mitigation. It is important to consider that capacitors have both an equivalent series resistance (ESR) and an equivalent series inductance (ESL), and selecting capacitors with lower ESR and ESL values will help lower noise levels.
Additionally, when designing circuit board layouts, careful consideration must be given to the placement of components with coils, such as transformers and chokes. The leakage of flux from these coils can induce noise in nearby components. By minimizing the length of leads on electronic components and utilizing surface-mounted SMD components, such as multilayer ceramic chip capacitors, the potential for noise generation can be reduced.
Furthermore, establishing a ground plane can help reduce noise by ensuring that all circuits within a system have the same reference potential. This provides a low-impedance ground-fault return path to the power source and minimizes electrical noise from high-frequency RF sources. Proper grounding techniques, such as symmetrically constructed grounding conductors for motor cables, can also contribute to noise reduction.
In summary, using a different power supply in combination with careful component placement, filtering techniques, and effective grounding methods can significantly reduce noise in electrical circuits, leading to improved performance, accuracy, and consistency.
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Frequently asked questions
Circuit noise is any undesirable signal that you don't want in your circuit. It can be a powerline hum of 60 Hz in the US and 50 Hz in the rest of the world, or "shot noise" produced in active devices from the electron flow through the device.
Circuit noise can arise from either an external or internal source. Noise comes from inductors, which are electrical components that store energy as a magnetic field. When the source voltage changes, a shift in the magnetic field occurs, causing any nearby wires to receive an induced voltage.
There are several ways to reduce circuit noise, including:
- Using decoupling capacitors, filters, and shielding
- Establishing a ground plane to ensure all circuits within a system have the same reference potential
- Using anti-aliasing filters or other digital filters to filter out noise at certain frequencies
- Using a low-pass filter, which is a circuit combination of a resistor and a capacitor
