Mixed Signal Design for the Next Generation of Devices

Key Takeaways

• How to layout your circuit board for mixed signal design.
• A discussion on limiting electromagnetic interference (EMI).
• Learn about ground and reference plane setup for reliable signals. 

Mixed signals are now found on virtually every new electronics device. Having a strong foundation is critical for the development of the next generation of devices.

In the past, electronics used to consist of different individual circuit boards, each with their own specific function. However, as devices are further minimized and increase in  functionality, these multi-board designs of the past are being phased out with single boards containing both analog and digital information – also known as mixed signal boards.

Modern electronics are recording and digitizing different elements of the real world now more than ever; everything from temperature to motion and more. These different elements are captured as analog signals which must be processed and converted through analog-to-digital converters to a digital signal in order to interface with computers and servers.

Mixed signal boards present their own unique challenges of ensuring the integrity of these analog signals without introducing significant noise on a board that also contains digital signals – this is the key to mixed signal design. Let’s delve into this type of design and take a look at how to deal with sensitive analog circuitry, routing, power delivery, and EMI shielding.

Mixed Signal Design: The Basics

There’s a lot that goes into a circuit board with mixed signals. Using an advanced CAD program will help keep you organized as you lay it out.

Many of the mixed signal design tips we’ll be discussing fall into one of two categories: the first is reducing noise such as electromagnetic interference and the second is to make the designs as impervious to existing noise as possible.

Before routing your copper, it’s important to develop a strong floorplan for your mixed signal design. A good floorplan will help with component placement and routing down the line. Doing this ensures that each block of the circuit that serves different functions has its own contained area on the board. To do this, group together similar purpose analog components with each other and do the same for digital components.

Most importantly, separate analog and digital parts from each other. The further the digital parts are from the analog the better. Separation can include using the top and bottom of a double sided board or the left and right of a single board – as long as there is significant space between them.

In general, analog signals are where you need to take most of these precautions. These signals exist in a continuous range, whereas digital signals are binary – this, in theory, allows for a larger margin of error compared to analog counterparts. Following design for manufacturability (DFM) rules is required to place parts for the best signal and power integrity. This will help ensure the manufacturing of your board is efficient with the highest yields.

Mixed Signal Component Placement and Layout

Setting up a good layer stackup can assist in eliminating EMI from your board.

Once your floorplan is laid out, component placement is the next step to good mixed-signal design. As previously mentioned, maintaining the partitions between analog and digital circuitry will help reduce crosstalk and EMI while increasing the integrity of analog signals.

Keep Heavy Duty Circuitry Centered

For heavy duty digital circuitry–such as large processors and memory components–that generate a lot of heat, center them on your board for better thermal dissipation. As they’ll have a lot of routing connecting them, centering the components will allow them to be more easily accessible. Other digital components like ADCs should be kept close by and similarly centered.

Set Up Bypass Capacitors

Once you’ve placed all your major and large components, set up your bypass capacitors. Place these capacitors as close to your digital circuitry as possible. In the case of ground bounce or power spikes, they can help keep reliable power to your devices.

Allow for Direct Routing

Finally, as a rule of thumb, place your parts to allow for the most direct routing between them. Avoid creating situations where you’ll need to route analog circuitry between digital circuitry and vice versa. This will further help in reducing noise and allow for shorter traces, which we’ll be delving into in a later section.

Layer Stackup

The success of your board relies on how your layer stackup is configured, specifically your power and ground planes. Before you route, set up your layers to be easily accessible. Specifically, set up a reference plane on an adjacent layer to the component requiring it for a shorter signal return path. This will help minimize noise and improve signal integrity. When configuring your layer stackup, keep your floorplan in mind to ensure you’ll have appropriate space for routing down the line.

The stackup layer configuration is also essential in limiting EMI. Although reducing the layer count may save on manufacturing, it may create an end result with overall signal integrity being damaged. High speed signals, sensitive signals, and noisy power circuitry should be isolated from each other. A good solution is creating additional layers for the stackup and providing enough layers for ground planes to shield these signals from EMI.

Routing Guidelines

Keeping your traces wide and short is a major key in laying out a mixed signal design.

Once you’ve laid out your components on the board with a good ground system in place, most trace routing will naturally fall into place. In general, the two essential rules for routing are:

1. Keep signal paths short and direct.
2. Keep traces from the digital circuitry away from the analog circuitry.

In regard to signal path lengths, this should apply to all your circuit blocks. Traces from your power supply should be kept especially short and be as wide as possible to reduce inductance. Ensure that there is a short signal return path from the reference plane to reduce wandering. Keep your routing with minimal transition layers, as this also increases length.

For high speed circuits, follow the signal path of the schematic as close as possible. When routing traces and vias, there’s also a possibility of creating antennas, so focus on reducing your loop size. Routing your analog traces with significant vias can also create inductance, so minimize your layer transitions for this reason as well.

Protecting your analog signals will be crucial in device reliability. Metal planes in the stackup can provide a good amount of shielding. It is therefore good practice to route sensitive signals between two plane layers in a stripline configuration. If there isn’t enough room for wider spacing, use guard traces to prevent crosstalk between two parallel analog traces. This can also act as a shield between analog and digital traces. Finally, a shielded border or fence between functional partitions of circuitry can be formed with vias. Via fences are both effective and easy to put in, but note that they use up a lot of board space.

Using advanced PCB software can really help in designs like these. Placing components tightly to specific spacing widths with various trace widths requires detailed database management. Using your CAD system’s design rules to manage these constraints will help you stay organized.

Power Delivery for Mixed Signal Boards

Having reliable power is integral to strong circuit performance, especially for mixed signal designs. Place the circuitry of each power supply such that it is isolated from sensitive analog and digital circuitry while also being near the components themselves.

High-speed PCBs often suffer from various problems, such as transient ringing, due to their power delivery networks (PDNs). To deal with this issue, include decoupling capacitors in the design near the power supply and place ground and voltage layers adjacent to each other in the stackup to provide high interplane capacitance.

This image shows a ground plane with a large gap for vias in the center. This large gap in the ground plane significantly extends the return signal path, creating all sorts of instabilities in your circuit.

The layout of the planes themselves is also crucial to your board's operation. Signals should not be routed across areas where the ground plane is broken (see the image above). Gaps in the plane or sections with many vias can block the return path for signals. This may cause the return signal to wander before getting back to the source, which is a major cause of EMI and reduced signal integrity. Ensure the signal return path is as short as possible for the best board performance.

Shielding Your Board From EMI

A couple examples of large EMI shields that you may want to mount on your board. 

Signals can suffer from a variety of issues including ground bounce, crosstalk, power supply noise, and most notably, electromagnetic interference (EMI). EMI can completely change the function of an otherwise normally operating circuit board. If EMI isn’t properly dealt with, any of the following issues may arise:

• Communication disruptions
• Interference with wireless devices
• Corruption of sensor data
• Component malfunctions
• Software errors or failures

The best way of dealing with already-existing EMI is to use metal shielding. Make the critical area of the board less susceptible by covering it with metal from above and on all four sides, with a ground plane a layer below creating a faraday cage. This has the capability of shielding from a lot of unwanted EMI.

However, shielding can come at a cost: note that EMI shields aren’t flat and need to allow for accessing the components underneath them. Additionally, heat can get trapped under these shields unless they’re made perforated. They also result in an overall more complex board, making debugging and testing more difficult.

Ideally, the use of these shields would block out all incoming EMI. In reality though, there needs to be openings for thermal cooling, solder points to the boards, and enough room for potential small adjustments. Common materials for PCB shields include tin-plated rolled steel, time-plated copper, stainless steel, and more.

Another final method of further shielding is using differential pairs, which act like a twisted pair of wires in a phone cable. In this way, they can reject common mode noise better than single-ended transmission lines.

Reducing Noise Sources

Noise can come from a variety of sources. Most notably, from oscillators (crystals) or clock lines and the electromagnetic fields produced around large inductors and power supplies.

Large voltage swings of any kind can create issues, unless the device that is pulling the current contains the necessary capacitors, ferrite beads, diodes, and termination resistors with routing kept to a minimal length. This length is the path from the source of the power through the load and then back to the ground plane. In general, smaller loop lengths will produce less noise, creating less EMI.

RF energy can be absorbed by nearby metallic objects such as printed antennas. For this reason, placing ground vias around antennas may help reduce unwanted interference. To further reduce EMI, print the antenna on an outer layer with underlying layers directly below it that are void of copper.

A final note: interference from one part of the circuit or another device can couple with existing copper loops in unpredictable methods. In other words, any conducting loop can become an unwanted antenna. To ensure this doesn’t happen, use simulation tools to help you characterize your board and see what’s going on.

Ground and Reference Plane Tips

An example of a split plane on a printed circuit board.

With all the sensitive electronics on your board, having a well-designed ground plane is essential. In order to make them as reliable as possible, do not cross blocked areas of the plane with traces, as routing through them can create noise by degrading the return path.

In theory, split planes can create better isolation between analog and digital areas of circuitry. However, in practice, using split planes, cutouts, or holes in reference planes can result in EMI and create further obstacles for signal return paths. If possible, avoid using split planes. In the off-case a split-plane is essential to the mixed signal design, ensure that the two planes connect to each other at a single point, as multiple connection points can create antenna loops prime for radiating EMI. Instead of a split plane, when a complete ground plane is used, and the analog and digital sections are routed separately, you can produce a clear return path, reducing overall EMI.

Using ECAD Tools to Organize Your Design

Using intelligent ECAD software can help you a long way in your mixed signal designs. For example, in trace routing, PCB tools allow users to set up trace routing widths and other critical design rules to ensure their design meets specification. Especially with mixed signal designs, this can be useful with all the various trace widths required for standard routing, e.g., differential pairs, impedance controlled traces, different sensitive analog signals, power, and ground planes.

Other useful features include 3D viewing, which is especially helpful if you’ve added an EMI shield to your design. Cadence has a host of simulation tools (among those mixed signal simulation tools), that can be used while the design is laid out. With these advanced tools, designers can handle more complex designs and complete them faster and with fewer errors.