The Top 5 PCB Design Guidelines for Analog Circuits

Key Takeaways

• The importance of analog circuitry in your PCB design.

• Five PCB design guidelines for analog circuits.

• The design tools that you will need for laying out analog circuitry.

A clock composited on a printed circuit board.

When I was growing up, my parents had a cuckoo clock on the wall of our dining room. It was old, it was made of wood and other magical parts, and the little bird popped out and squawked annoyingly at us at regular intervals. It also had to be wound manually by pulling the free end of the chains down to pull the weights back up. With no batteries required, it was about as analog as you could get. And if something interfered with the smooth operation of those weights, like misbehaving little children, the clock would no longer tell the correct time.

Unlike its current digital counterparts, that old clock relied on being able to run without any interference. In the same way, analog circuitry on a printed circuit board requires an environment where it, too, can run without interference. When those conditions aren’t met, the circuitry may not perform as expected, causing a failure in the board. Here are some PCB design guidelines for analog circuits to help you lay out a board that will operate correctly.

The Difference Between Analog and Digital Circuitry, and Why It is Important

As you probably already know, digital and analog signals differ from each other in their values. Digital signals have just two values, on and off, while analog values are more diverse. Just as analog signals in nature, such as light or sound, will vary depending on how bright or loud the source is, so will electronic analog signals depending on their amplitude. 

Due to the on/off nature of a digital signal, there can be more margin for error in how they are transmitted, whereas the value of an analog signal must be precisely controlled for transmission and reception. This precision is why analog circuitry can be more difficult to design than digital.

Analog circuitry uses combinations of passive components and active parts to transmit and receive the required signal values. When receiving analog signals from sensors or other external devices, analog circuitry can receive the data directly, whereas digital circuitry has to have the analog data converted first. Therefore, a typical application of a mixed-signal board will involve receiving analog input signals, which are then converted into digital signals to go into the processors. This requires the PCB layout designer to keep the analog and digital circuitry separate to protect the analog circuitry from being influenced by extraneous digital noise.

Next, let’s take a look at five important design techniques that will help the PCB designer to accomplish this.

Tight component placement with short, direct routes is best for analog circuitry.

Five PCB Design Guidelines for Analog Circuitry Layout

While many of the rules of layout can be applied equally between analog and digital layouts, here are the most important ones to focus on when creating analog circuitry:

1. Component Placement Should Be Tight

Analog signals need to be short and direct, and the best way to accomplish this is to start with a good tight component placement for the analog parts. Not only will this simplify the direct routing of the traces, but it will help keep the analog and digital circuitry separate. Also, remember to place noisy analog parts towards the center of the board instead of along the perimeter. This will allow the ground planes and other circuitry on the board to help contain some of that noise.

2. Provide Separation Between Analog and Digital Circuitry

Since both the analog and digital components can influence each other with noise, the best thing to do is to separate them as much as possible when placing the board. With this separation of parts, you need to avoid routing high-speed digital traces through the areas of analog circuitry. The noise of the digital signals can affect the analog circuitry.

3. Signals Must Be Short

At higher frequencies, impedance mismatches can cause reflections in analog signals. These reflections can create electromagnetic interference (EMI) in other components on the board. The best way to guard against this is to route analog signals as shortly and directly as possible. Another good tip is to keep your traces on one layer, which will help to reduce the number of vias and lower the inductance of the circuitry.

4. Use a Dedicated Analog Ground

The digital ground plane in the board will be busy with all of the digital signal return paths as well as filtering the power spikes created by rapidly-switching high-speed digital parts. All of this activity can create a lot of interference for your analog circuitry, so the best thing to do is to create a separate ground plane. The digital and analog ground planes can be connected at a single point, but it is important to keep them isolated otherwise. 

5. Make Sure to Provide Clear Analog Signal Return Paths

Not only do you want to make sure that your analog signals are routed with traces that are short and direct, but their signal return paths on the analog ground plane must also be as unobstructed as possible. Too often, designers will block the direct return path flow of the signal with vias, plane splits, or cutouts and slots. This blockage forces the return signal to wander around the ground plane in an attempt to find the shortest path back to its source, creating unnecessary noise along the way.

Those are the five main points to keep in mind when laying out analog circuitry. Remember that even with these recommendations, you still need to observe design for manufacturing (DFM) rules to ensure that your board can be assembled without error.

The next step is to lay out the analog circuitry. For that, you need help from your PCB design tools.

Be careful of the location of holes in your ground plane, as they can block signal return paths

The Tools That Will Take You There

The first step to designing analog circuits is to arm yourself with PCB design tools that can help you get the job done. You will need to use circuit simulators while you are capturing the analog circuitry on your schematic as well as access to the latest analog components from online library services. On the layout side, you will need analysis tools to check your signal and power integrity as well as good solid place and route tools.

Thankfully, PCB design systems like these with all of the features and capabilities that we’ve talked about are already available for PCB designers like you. OrCAD PCB Designer, from Cadence, has the tools and functionality to expertly take your analog circuit design from concept to assembly files to final fabrication. With OrCAD, you will have everything that you need for success.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.