There is one simple PCB design principle that all the pros know: solid ground planes are always preferable to split ground planes. Totally disconnected ground references create plenty of opportunities for EMI troubles, interface failures, SI problems, and low-frequency EMI within the PDN. Despite the wealth of expertise, knowledge, and data backing up these points, new designers still cling to this idea that separate grounds are preferable for commercial products. The fact that this continues to be a problem largely stems from EDA companies refusing to hire actual PCB design experts to run their marketing campaigns, but I digress.
Although a solid ground plane is the norm and is part of a standard design approach, there are certain types of circuits and designs where you can and probably should have physically disconnected grounds. Examples include analog and digital grounds, primary and secondary grounds, chassis ground, and different signal grounds.
Examples of PCBs with Isolated Grounds
The usage of separate ground nets, which results in physically disconnected ground planes or stitched planes in a single location, is very often applied where it is not needed. In most cases, this creates EMI disasters or unnecessary routing difficulties in the name of controlling noise. The applications outlined below rely on separate grounds or a complete lack of reference plane for various reasons.
Isolated DC-DC Converters
This is probably the most common instance where isolation between two physically disconnected ground nets is required. The main reason we have physically disconnected ground nets is for galvanic isolation; the switching regulator circuit used in these converters can often output high current on one or both sides of the isolating transformer. Implementing split grounds provides safety for the user so that they are not exposed to high voltage and/or high current.
Custom DC-DC converter designs and off-the-shelf modules enforce this same kind of ground split. The result is that you must not route traces between the primary and secondary sides of the converter, both to prevent radiated emissions and to maintain the desired galvanic isolation. To maintain isolation between each side, feedback loop sense signals can be routed back to the primary with an optocoupler. An example is our flyback converter module example project shown below.
Isolated Digital Interfaces
It's easy to forget that some digital interfaces are inherently isolated up to some voltage level, typically in the kV range. Isolated digital interfaces will naturally have separate ground nets to maintain the required level of galvanic isolation as specified in the interface standard. The most common isolated digital interface is Ethernet, which uses an isolation transformer to isolate the conductors in an Ethernet cable from the main system ground that is part of the PHY layer.
This schematic shows the implementation of an isolation transformer for an Ethernet interface. Source: Renesas.
Standard RJ45 connectors do not always have the isolation transformer and required magnetic termination circuit built into the body of the connector. However, most new equipment will use magjack connectors which include the magnetic termination circuit.
Obviously, Ethernet is a differential interface with twisted pair wiring used in the Ethernet cable. Because Ethernet is isolated, one might ask, what makes up the reference conductor on the secondary side? Because Ethernet is a differential interface, each conductor in the twisted pair cable is a reference for the other conductor. This fact applies to all differential interfaces and their cables, regardless of whether or not they are isolated.
Can other digital interfaces be isolated? The answer is yes; it just depends on how this is done. The typical approach is to use optical isolation to transfer energy across the barrier between separated ground regions. If you're doing this on a PCB, you still need to follow the standard routing guideline for digital signals: never route a trace over the gap between the two reference regions. There may also be issues with high-frequency radiation in the isolated region, which may then require an isolation capacitor (Y type) to maintain galvanic isolation while also controlling the passage of high-frequency return currents back to the main system ground and thus back to the power supply return.
Precision Analog Measurements
When most designers put an ADC on their PCB and connect it to a microcontroller, they usually think they are building a precision analog measurement system. I have found this is rarely the case, and they are probably running their ADC within standard limits with high SNR signals. Then, in order to control noise, they implement split grounds with the goal of preventing digital signals from creating crosstalk in the analog interface.
The above issue with mixed signal crosstalk is most often solved by implementing appropriate layout and routing practices, and splitting grounds into separate conductors is totally unnecessary. In addition, the DGND and AGND pins are connected internally in ADC components, so the concept of split grounds becomes essentially meaningless.
That being said, there are instances where isolation is very useful for capturing measurements of low SNR analog signals. For example, take a look at our isolated ADC example project; this module uses an isolated ADC to capture an analog input that is galvanically isolated from the digital interface on the output side.
Dual ADC module example project with an isolated ADC.
This type of isolated system also requires routing different portions of the system over their own grounds and not crossing over the split region to ensure a clear path for return currents exists for the signals. The key here is to account for the frequency of the signals you want to measure when determining your isolation strategy.
Low frequencies with disconnected grounds could use an inductor, or possibly a ferrite, to route the low-frequency return path exactly where you want it. High frequencies with disconnected grounds may require a capacitor to stitch the grounds together in order to control high-frequency radiation. If possible, consider a differential input as this takes advantage of trace-to-trace referencing to control return currents, including at very low frequencies or at DC.
The key here is keeping your digital stuff on the digital side where you have your system ground, and only keeping the low-frequency / low SNR analog stuff on the disconnected side and very carefully tracking its return path. How you do this also depends on the input interface for the analog signal: is it a single-ended trace with its own reference, is it a pair of wires but not a differential pair, or something else?
Audio
Finally, an area of design where split grounds or no ground plane is very useful is in audio systems. In the reference design example shown below, the audio lines are differential and form their own references, while a single ground plane is used as the reference for everything else.
Audio amplifier reference design from Texas Instruments, PN: TIDA-00874.
In the simplest topology, where there is no AC input, no Earth / chassis connection, and no isolated power supplies, you might get better performance in terms of noise and mixed signal crosstalk by routing ground traces alongside signal traces. Also, many components used in audio systems are differential, so they are already highly resilient against common mode noise.
Large ground conductors are more often used in audio systems when high current handling is needed, although large ground pours do aid shielding effectiveness against radiated emissions. Audio designs have to balance the need to control return current against the need for current handling, especially when there is a digital interface in the system. Other, often more important, sources of noise are related to the power input in the system, which can produce low-frequency noise that is in the audible range.
Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.
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