We live in a digital age, and smart technology is rapidly replacing even simple tasks, such as turning on and off a light switch. Our children’s children will puzzle over antiquated objects, such as a checkbook register or a VHS tape, as they rely on more modern digital services. Another casualty of this advancement is the good old analog clock, which is steadily being replaced by its digital counterpart. I wonder how long it will be until the term “sweep second hand” passes into obscurity.
Yet, even as modern technology pushes analog devices such as the regal grandfather clock aside in favor of the more modern and sleek wall-mounted digital readout, at the heart of many of these devices is still analog circuitry. Even two resistors combining to make a voltage divider can be considered analog circuitry, and usually, there are a lot more than those two resistors for designers to work with. Just as digital circuitry has specific guidelines for layout, analog circuitry does, too, for those circuits to operate the most efficiently. Here are some of those analog PCB layout guidelines that designers should know.
Analog PCB Layout Guidelines for Layer Stackup and Component Placement
Printed circuit board materials and layer stackup configurations play an important role in analog design. PCB materials will need to be selected that can support the desired analog performance characteristics for speed, isolation, and voltage breakdown. Meanwhile, the layer configuration will need precise placement of power planes to support sensitive analog components. Additionally, analog circuitry is usually combined with digital circuitry in mixed-signal designs.
Therefore, the selection of the board materials and the configuration of the layer stackup must support the needs of both the analog and digital circuitry. This requires extreme care in setting up the design correctly before the layout is started to avoid having to retrofit it all later.
To reduce noise in analog circuitry, start first with physically separating analog and digital components in a mixed-signal design. This will allow you to plan out a cleaner power delivery system for your analog circuits that won’t mix in with the digital power.
As the power and signal sensitivity of analog circuitry increases, the need to maintain separation from digital circuitry becomes more important. You will also want to keep noisy and sensitive components away from the edges of the board whenever possible, as the board will have better grounding in the middle. You should also look to using surface mount components as much as possible to reduce the amount of drilling required and improve the signal integrity with the smaller parts.
Consider also the manufacturing needs of your design. Analog components often have greater metal contact than digital components. You will want to plan your component placement and power and ground routing so there is an even absorption of heat during the soldering process. Ball grid arrays (BGAs) especially need to have similar thermal loads on the solder balls to avoid problems during reflow solder.
Routing Considerations for Analog Designs
The first step in routing analog circuitry is to fully set up your design rules and constraints. This is extremely important in mixed-signal designs where you will have different routing rules for the analog and digital circuitry. You may also want to establish specific routing rules for those signals that require more isolation from other circuitry.
Here are some important considerations when routing analog circuitry:
- Keep higher frequency traces short to help prevent transmission line effects. These effects can result in signal reflections that can cause electromagnetic interference in nearby components.
- Minimize the use of vias on critical signal trace routing. The more that you can route these traces on a single layer and as short as possible, the better.
- Analog signals that belong to different functions should also be kept separate from each other to prevent one from causing noise for the other.
- It is very important to avoid routing sensitive signals over the boundaries between analog and digital circuitry regions.
- The use of wider traces on analog circuits will help to keep their impedance lower.
- As always, it is important to route sensitive traces over a continuous ground plane for an unbroken signal return path.
Good power and ground plane design are especially critical for analog circuitry as it is very vulnerable to the noise from fast digital current spikes. Therefore, it is important to have separate ground return and power planes in mixed-signal designs to prevent the digital noise from corrupting the analog performance.
It is also important that these two planes do not overlap with each other, as that could cause capacitive coupling of noise from the digital ground into the analog. By carefully placing your components, you can establish regions for your separate circuitry that will allow you to not overlap your planes.
How Your Contract Manufacturer Can Help with Your Analog Designs
There is a lot to consider when designing sensitive analog circuitry, but fortunately, there are some helpful resources available to you that can help. Many PCBA contract manufacturers have years of experience supporting, designing, and building analog circuit boards. For those with dedicated engineering personnel, you will find that they are very knowledgeable in the different intricacies of analog design. These would include EMI and EMC issues, as well as the regulatory compliances that are required for these designs.
At VSE, we have the analog design and manufacturing expertise that you are looking for. Our engineering staff has years of experience working with analog designs that give us the unique empirical knowledge you need. We can work together with you to refine and optimize your PCB layout and assembly or design the board ourselves to meet your analog performance challenges.
