Removing Noise From Analog Signals in Your PCB
On my 12th birthday, I received my very own stereo system, complete with 4 speakers, a CD changer, and a tape deck. My parents found my taste in 80s hair metal pretty annoying, but I couldn’t get over the annoying hum coming from the speakers after a CD ended.
Noise in analog signals can present serious problems in a number of applications where precise measurements are required. One example is in audio circuits, where noise can manifest itself as an annoying, audible hum. Another example is in sensors that output DC or AC signals, as noise from supporting electronics makes readings less accurate. The right design choices in these systems can prevent noise from degrading signal quality.
White Noise, Pink Noise, and Amplifiers
These two noise sources are important in analog signals and they have two different characteristics in the frequency domain. White noise is uniformly distributed throughout the frequency domain, i.e., this noise has a flat spectrum. Pink noise, also called 1/f noise or flicker noise, has a frequency dependence that is inversely proportional to frequency.
Both noise sources are important in analog devices as these circuits typically involve one or more amplifiers. Pink noise dominates at low frequencies, but it eventually falls off to the point where white noise is the dominant noise source.
The best way to remove white noise from an analog signal is with bandwidth reduction. This is equivalent to reducing the measurement time for a signal and gathering more measurements, followed by averaging results. In terms of analog signals sent to an amplifier circuit, you should opt for a lower bandwidth amplifier. You can reduce the RMS noise fluctuations due to white noise by a factor N if you reduce the bandwidth of the amplifier by a factor N-squared.
If there is some other factor limiting the signal bandwidth, then there is already no point in using an amplifier with higher bandwidth. If you cannot reduce the bandwidth of the amplifier, you can decrease the bandwidth of an analog signal further using filtering. A simple RC or LC filter will only provide filtering up to first order, but it is simple enough that it can be quickly implemented on an analog signal trace.
Eliminating pink noise is challenging to implement at the PCB level. This may require external equipment, depending on your board’s application and the signals involved. One common approach is chopping, which can be implemented at the IC level. In this technique, the reference signal is modulated at a higher frequency and amplified with a summing amplifier. The higher frequency components are removed with a filter, leaving behind a low frequency or DC component with better SNR. Implementing this purely on a PCB may require the use of a frequency synthesis technique (e.g., with a phase-locked loop).
Crosstalk Between Analog Signals with Different Frequencies
Many designers who build boards with analog signals are only working at a single frequency, but many modern devices will incorporate elements that operate at multiple analog frequencies. A perfect example is a smartphone or tablet. These devices include RF capabilities for wireless communication, as well as components for playing and capturing audio.
Compared to digital circuits, analog circuits are quite sensitive to noise. Many devices contain digital and analog circuitry that must be separated. This prevents noise in the digital portion from interfering with analog circuits. As digital logic ICs are run at saturation, and in the absence of severe noise from a switching power supply, they have significant noise immunity. This is not the case with analog circuitry.
In these types of devices, you’ll want to place digital and analog portions of the board with different frequencies into different areas with their own ground planes. In devices with RF wireless capabilities, there is a common problem where RF signals will induce audible noise in an audio circuit through demodulation. Separating these components into their own sections with their own ground planes helps prevent this coupling in two ways.
First, placing greater distance between these sections reduces the electromagnetic field strength between these sections. Second, when analog sections with different frequencies share a ground plane, return signals with one frequency can propagate very close to circuitry that runs at a different frequency, causing crosstalk.
Finally, in order to decrease the strength of induced signals, analog signal lines should be routed over the shortest possible distance. While it is appropriate to place splits in ground planes in order to properly ground components, you should never route a signal trace over a split; always route over a solid ground plane. This ensures that the loop area associated with this current path is as small as possible, thus minimizing parasitic inductance.
Another reason that analog signal lines should be as short as possible is that these traces can act as antennas. Longer traces will have lower resonance frequencies and closely spaced harmonics, allowing external fields to easily induce a current in these traces.
It is common that a board with ubiquitous analog functionality will also require some digital functionality in order to interface with other modern devices. Arguably, the most important point in this system involves the arrangement of your digital and analog ground planes. Analog and digital portions of the devices should be split into different sections, and the ground planes should be split to match this arrangement. This helps prevent crosstalk between digital and analog sections of the device.
Determining the best layout to ensure analog signal integrity is much easier when you have access to a great simulation and analysis package directly in your PCB design software. Altium Designer® contains the best schematic design, layout, signal analysis, and deliverable generation tools in a single interface.
Download a free trial of Altium Designer today to learn more about the signal integrity and simulation tools. You’ll also have access to the industry’s best design, simulation, and verification features in a single program. Talk to an Altium expert today to learn more.
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