Crosstalk in Mixed-Signal PCB Traces and Ground Planes
Digital and analog elements, if not properly arranged, mix about as well as oil and water. While many consumer electronics operate in the purely digital regime, there are many devices that could not function without mixing digital signal and analog signaling within a single PCB. Just like in purely digital or purely analog PCBs, crosstalk and EMI can occur in mixed signal PCBs.
These potential signal integrity problems can create a major headache for electronics designers and ruin the integrity of both types of signals. Fortunately, there are some relatively simple design strategies, applicable in certain cases, to suppress crosstalk between analog and digital elements in your PCB.
Most discussions on crosstalk in PCBs focus on using differential routing to suppress crosstalk. Crosstalk is suppressed in these traces due to counter propagating magnetic fields between the traces, which effectively cancel each other. This reduces any current that one trace can induce in the other. This issue with crosstalk applies in traces carrying digital or analog signals.
In traces, the solution is to route signal and return traces as differential pairs, where one trace carries the signal and the other trace carries the return. The level of crosstalk between a pair of traces and the required impedance matching depends on the pitch between them.
However, crosstalk can also occur between digital and analog elements. When both types of signals are used on a single circuit board, there is a possibility of high-speed digital logic interfering with low-level analog circuits. It is often said that the only solution is to separate digital and analog ground planes, but this is not always the case.
Analog and digital components on a PCB
Power, Grounding, and A/D Conversion
Crosstalk between analog and digital elements in a PCB can occur due to the power and ground plane arrangement as well as routing. Crosstalk between digital and analog elements, as well as EMI, can occur in your PCB if ground planes and traces aren’t properly arranged.
A common strategy in mixed signal PCB layout is to segregate analog and digital elements into functional blocks on the PCB. This requires separating the digital and analog ground planes and power planes into two different copper planes in the same layer.
When the analog signals and components can be confined entirely over the analog ground plane, each analog signal trace induces its return current directly in the ground plane. Assuming there are no anomalous sources of impedance, like a mismatched trace or resonance due to via coupling, the induced return current flows back to the ground reference point. The same applies to digital signals and their ground plane.
When digital and analog ground planes are separated, a problem arises when you need to route between the digital and analog section. For example, you might need to route an analog signal to an ADC that sits in the digital section. You will inevitably have to route over an area that is not covered by either ground plane. Split ground planes will also act as slot antennas and radiate.
The result is EMI and the potential for crosstalk. Once you route an analog trace over the digital ground plane, an analog return current is induced in the digital ground plane, and vice versa for digital signals. This creates crosstalk between analog and digital signals. Signals also radiate in the region between the two ground planes.
You don’t always need shielding to suppress EMI
Split Ground and Power Planes
If you need to use an ADC IC in the above example, it would need to straddle the digital and analog ground planes, and the digital/analog pins would need to be placed on each side of the IC package. Most ICs are not arranged in this manner.
Rather than using a pair of split planes, it is better to use a single ground plane that is partitioned into digital and analog sections. Any ADC/DAC can straddle the dividing line between each section, and this would not require an IC with a special pin arrangement. This gives you some flexibility with component options.
Some more complex PCBs and component arrangements force you to split ground planes to accommodate your layout and routing requirements. If you must route between the analog and digital sections, you can create a bridge between the two ground planes and route traces over the bridge on your signal layer. This will simultaneously prevent crosstalk between digital and analog elements and suppress EMI.
If the ground plane is split or bridged, then the same should be done to the power plane. Always be mindful that the return current will propagate in the adjacent plane, regardless of whether it is a ground or power plane. Thus, traces should never be routed over the split in a power plane, as this will also increase crosstalk and EMI.
PCB design is as much an art as it is a science, and designing mixed signal devices requires top-quality PCB design software. The advanced CAD and simulation tools in Altium Designer 18.1 make it easy to arrange components in your next PCB and diagnose signal integrity issues. Download a free trial and find out if Altium Designer is right for you. Talk to an Altium expert today to learn more.