Market pressures are continually driving electronic designers to develop products that are smaller and higher in performance than their predecessors.
The Internet of Things (IoT) is also adding wireless functionality to practically every design. The result is a design which forces analog, digital and RF circuits to operate in close proximity, and poses problems for layout and grounding.
Designing an efficient mixed-signal grounding scheme can be both time-consuming and challenging, but following correct grounding principles can result in a system with more noise immunity, less crosstalk, and better EMI performance.
Signal Frequency and Current Return Paths
For every signal source, no matter what its type, there is a corresponding return path that completes the circuit. The conductive path formed as the current travels to and from the source constitutes a loop antenna. To minimize the loop area, the current should return to the source via the ground as directly as possible.
The path taken depends on the signal frequency. At low frequencies, current follows the path of least resistance on its way back to the source. If there are multiple possible paths, the current density along each path is proportional to the conductance of that portion relative to the whole.
For high-frequency signals, the lowest inductance return path lies directly under a signal conductor, minimizing the total loop area. Returning signal currents follow this path if available.
Characteristics of Analog and Digital Devices
Digital and analog circuits both rely on changes in voltage to transmit information, but there are important differences with consequences for the grounding system.
Analog and RF devices both transmit and receive information in the form of continuous variations in voltage or current. Any unwanted disturbance in the signal from noise or other sources is an error and reduces the signal-to-noise ratio (SNR). RF and analog circuits are similar with respect to signal propagation, but RF devices are more susceptible to noise at specific frequencies due to the extremely low amplitudes involved.
Digital circuits operate via two discrete voltage levels which define 'high' and 'low' states. A digital driver is designed to create a voltage level higher than or lower than these thresholds. As long as these threshold voltages are correctly recognized at the receiver, the digital circuit is functional.
Analog Ground vs. Digital Ground: Circuit Grounding Techniques
Digital circuits are less susceptible to noise and distortion than analog or RF circuits. Their switching activity generates high-frequency noise, though, which can cause problems in sensitive analog circuits. Mixed-signal devices such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) pose additional grounding problems, as we'll see later.
Effective grounding design must consider potential issues between analog and digital components. One standard grounding technique adds a continuous copper layer to a double-sided or multilayer printed circuit board (PCB), called a ground plane that is used as a ground reference.
The ground plane's large area of metal has the lowest resistance for low-frequency currents from analog circuits, and provides a low-inductance return path for high-frequency digital currents. It minimizes emissions from electromagnetic interference/radio-frequency interference (EMI/RFI), and also acts as a shield to reduce susceptibility to external EMI/RFI.
Figure 1: Split analog and digital ground planes can lead to large current loops (Image Source: Texas Instruments)
It is tempting to separate the analog and digital sections even further by constructing split analog and digital ground planes connected at the power supply ground, as shown in Figure 1. The noise-sensitive circuits are located in the low-noise analog section, and the noisy components, including digital logic, relays, switching DC/DC converters, etc. in the digital section. Then the noisy digital and power currents remain in their own section, isolated from the analog currents.
This design, however, poses problems for signal traces that cross the two planes as the path from signal to return through the power supply forms a large loop.
Figure 2 shows a refinement of this technique. There is only one ground plane but it is partitioned into separate analog and digital sections connected by a bridge, sometimes referred to as a star ground. All traces between the analog and digital sections pass over this partition, minimizing the loop area for these signals.
Figure 2: A single ground plane partitioned into analog and digital sections minimizes ground loops for mixed-signal traces (Image Source: Texas Instruments)
Analog and Digital Ground Separation
Correct component placement is key to effective grounding design, but what is the appropriate arrangement for ADC, DACs, and other mixed-signal components? Most such devices provide separate analog and digital ground pins and recommend that they both be connected to the analog ground, with the recognition that this will introduce some digital noise into the analog system. This is appropriate if the digital noise is small, but some mixed-signal devices such as sigma-delta ADCs have large digital sections and might introduce considerable noise.
In such a situation, the grounding scheme in Figure 3 is recommended. This layout separates the analog and digital grounds; they each connect to their respective ground plane partition mixed-signal device which straddles the two partitions adjacent to the bridge.
If the design contains several data converters, this approach is problematic because the star points at each converter create multiple ground loops between the digital and analog partitions.
In this case, the partitions should be connected together under all the ADCs and DACs. The analog and digital grounds should both be connected to the analog ground plane.
The power supply should be connected to the board at the digital partition and power the noisy digital circuitry directly. Then it should be filtered or regulated to power the sensitive analog circuitry.
Figure 3: Grounding scheme for high-current mixed-signal devices routes each ground to the appropriate partition and locates the connecting bridge or star ground (Image Source: Analog Devices)
Conclusion
Designing an effective grounding scheme for a mixed-signal design is a complex undertaking, and there is no single best solution for all situations. Following the recommendations above, however, will maximize the likelihood of success.