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Tips for Routing Mixed Signal PCBs

Mixed signal Oscilloscope

CC BY 2.0 Dave Jones


Tips for Routing Mixed Signal PCBs

Designing a PCB correctly requires designers to consider a myriad of elements; overall layout (component spacing, placement, orientation, etc), grounding, heat management, interference, and more. In short, designers have plenty of things to think about. That list gets even longer when working with mixed signal circuits, so here are some tips to get you started on the right track with your design.

First off, always make sure that you define your requirements early and clearly before selecting critical elements to support those requirements. Parts like op-amps and  PA/LNA devices have a variety of options, so it’s important to make your selections solid before you proceed with routing.

Have you got a plan? Here are six tips to help you through the layout of your mixed signal circuit board.

Tip #1: Keep Analog and Digital Components Separate

The best way to build a successful mixed signal PCB is to keep your analog and digital components separated. You can do this by identifying each group, then dividing the circuit board with a shared ground plane. You could also use seperate grounds for the analog and digital sections of your board.

Tip #2: Ground Wisely

A single, solid ground plane is the simplest option to ground a mixed signal PCB. A PCB-wide copper ground avoids interference, so long as you don’t route high speed digital signals onto the analog section of the board. However, there could still be crosstalk between the analog and digital return currents along a shared ground board.  

In contrast, keeping grounds seperate for your digital and analog circuit sections has some benefits. If you’re particularly worried about noise from spikes in voltage and currents then keeping grounds separate might be the best solution.

Whether you’re using a single ground or separate ones, it’s important to keep the digital and analog currents adequately separated. Analog currents are smooth and continuous, in contrast to the abrupt behavior of digital currents and pulses. They will both interfere with one another without enough distance keeping them apart.

Tip #3: Place Analog to Digital Converters Carefully

When PCBs are divided into digital and analog sections the logical placement of the converters would be along the borderline between the two sections. It is possible to place analog to digital converters straddling both planes, however using a common ground enables the simpler solution of placing the converter along the dividing line.

Tip #4: Hold Your Boundaries

Once you’ve defined the digital and analog portions of your PCB make sure you follow the rules.. You don’t want to route analog signals through the digital portion, or vice versa. If your design requires you to trace a signal from one side to the other then make sure that the signal trace is over the ground plane of the board.

The main goal is to make sure that analog and digital currents don’t share any portion of their return paths.

Tip #5: Make a Cut When You Must

There may be times when the mechanical demands of your PCB design make it hard for you to keep currents from interfering with one another. A solution to this issue is to place cuts in the ground plane to keep currents from passing where they would cause performance degradation. It’s not ideal, but it can work when your other design constraints don’t leave you with many options.

Tip #6: Follow Your Current

Almost any issue that your PCB design may experience can be identified by simply following your current. Basic principles apply here, including the fact that DC and low frequencies typically move in a straight path of least resistance between source and load, while high frequency signals follow the path of least impedance directly under the signal trace. Frequencies that fall in between? They may take either route. Following your current is the surest way to make sure that your current design will be successful.