Quite often a printed circuit board (PCB) design will contain both an analog section and a digital section. The analog section typically conditions a signal for digitization and the digital section converts the analog signal to a digital one and then acts on the now digital domain signal. Segregating these two blocks of a PCB design is very important to insuring the integrity of the analogue circuity. Analogue circuits are typically highly susceptible to noise signals and digital circuits are typically very electrically noisy. This article will try to shed a light on some general rules to avoid mixed signal layout issues and discuss the best approach to isolating your analog circuit portion from its digital counter-part.
As a quick review, it is important to discuss the return path of high speed AC signals. When examining the return path of a DC signal, said path with simply be the path of least resistance back to the originating component. AC signal return paths follow the path of least impedance though. This means that AC signal return path currents remain localized to the area bellow their originating signal traces. The exception to this rule is that when you break the ground plane below a high speed AC signal you will force the return current for said signal to create a radiating loop. This sort of loop is both a source of and sink for radiated noise and to be avoided when possible. This brief review should remind the reader of the one of the two basic rules of EMI (electromagnetic interference) reduction: Keep return paths as close to their originating signal paths as possible in order to avoid creating return current loops. The other basic rule of EMI reduction is to make sure that you use only one reference plane. If two are used, then the PCB will effectively become a dipole antenna. With this quick review in hand, let us proceed to the specifics of mixed signal layout.
Quite often the first inclination of a designer is to simply separate the analog portion of the board from the digital portion by using an analog and a digital grounding scheme. The issue with such a scheme is that when connections are made from the digital to the analog side of the board, the board (as discussed in the previous section) effectively becomes a dipole antenna. Any such design will be both inherently susceptible to electrical noise and similarly very electrically noisy itself.
Another common approach to this problem is to simply connect the analogue and digital ground together at a single point (quite often the negative rail of the power supply being used with the design). This is a very poor solution though, as any traces connecting the digital to the analogue side of the board will now form a loop antenna through the ground connection point which will both radiate from your design and receive into your design electrical noise. Additionally the traces connecting the independent ground portions of your board together will effectively create a dipole antenna. Both effects will produce a very noisy and noise susceptible design.
Yet another common (albeit slightly more effective) approach to designing a mixed signal board is a configuration where the analogue and the digital portions of the board are directly connected to each other by way of a "bridge". While the digital and analog grounds are directly connected to each other in such a scheme, all connecting traces from the analog to the digital side of the board are routed above the portion of the board where the analog and digital grounds are connected. In this way the high speed AC signals going between the two circuits will have a direct return path, but the grounds planes will still be somewhat segregated. This bridge type configuration will, in theory, allow the digital side of the board to have the same ground plane as the analog side of the board, but with some further isolation than simply having the two portions of the board share a continuous ground plane. While this type of configuration will typically result in a board that performs well, why use a bridge in the first place? The return currents of high-speed AC signal will inherently stay very close to their originating traces, so the need for a bridge can be avoided by mindful routing of digital signals.
The best and easiest approach to completing a mixed signal layout is to simply divide the circuit board into an analog partition and a digital partition. These two partitions can then share the same ground plane, which will be constituted of a PCB wide copper pour. Interference between the two sides can then be easily avoided by not routing the high speed digital signals onto the analog portion of the PCB.
It follows that in any of these configurations the dividing line where the partitions are separated will be the logical location of the analog to digital converter or converters used in the PCB design. It is not unheard of to see analog to digital converters straddle isolated analog and digital ground planes, but as discussed, a very good solution is to simply place the analog to digital converters along the dividing line of the digital and analog portions of the board, where the board has a single continuous ground plane.
Finally, it is worth mentioning other approaches to isolate the analogue from the digital portion of the board. It is not uncommon to optically couple the digital portion of the board with the analog side by way of using optical isolators. In this way the analog and digital portions of the board can in fact have their own electrically isolated ground planes. This sort of configuration also works by isolating the two portions of a PCB using a transformer, where the two sides of the board are coupled magnetically. While both approaches are valid, they are typically reserved for specialty applications.
Here is a summary of the general rules to laying out a mixed signal PCB:
• Begin by defining the analog and digital portions of your design.
• Partition your PCB into an analog and a digital portion.
• Make sure that digital components and analog components are assigned to their respective partitions.
• Never route digital signals through the analog portion of the board and never route analogue signals through the digital portion of the board.
• Place analog to digital converters in such a way that they straddle the dividing line between the analog and digital partitions of the board.
• Using a single solid ground plane will produce the best results, with the added benefit of being the easiest approach.
• If a signal trace must be routed from the analogue to digital partition, make sure that it is located entirely over the ground plane of the board.
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