What is Discontinuous Grounding in an Electrical System?

May 7, 2020 Cadence PCB Solutions

Key Takeaways

  • Discontinuous grounding commonly refers to the use of different grounds for different circuits, or the use of multiple connections to the same ground with some high impedance between them.

  • Discontinuous grounding is one source of ground loops (at DC and low frequencies) and noise coupling between ground points (at ~kHz to MHz frequencies).

  • These noise problems can interfere with low-level signals and

 

Electronics schematic with discontinuous grounding

Can you spot how discontinuous grounding points in this schematic couple to each other?

 

Grounding is often overlooked in electronics design, both in PCB design and in larger power system designs. For a 4-layer PCB with a single power plane, grounding is simple enough, and you’ll need to worry more about your layout strategies to suppress noise coupling and reduce EMI susceptibility. In more complex systems with multiple grounding points, your ground strategy may not be so simple, creating some annoying noise problems in power electronics.

Power electronics systems might make intentional use of discontinuous grounding, but be careful when implementing multiple grounding points in your system. Low-level signals can be masked in many systems due to simple ground loops, which can arise from discontinuous grounding. There are some important things to calculate and simulate when examining your system’s behavior. In particular, you need to examine leakage and noise coupling between grounding points, including the potential for ground loops. 

Discontinuous vs. Continuous Grounding

The term “discontinuous grounding” could mean a few different things:

  • A system of multiple ground points that are not directly connected to each other

  • A system of multiple ground planes, each connected to different power supply return points

  • A system with multiple types of grounds that are not connected at the board or power supply levels

Contrast this with a typical ground plane: as long as the ground plane is uniform and is not physically split into multiple sections, then the ground in your PCB can be considered continuous. This is the standard method for defining grounding for modern PCBs on a 4-layer board: you’ll normally have two plane layers (power and ground) and 2 signal layers (top and bottom). This helps prevent grounding problems that can occur in other systems, such as multiboard systems and power electronics.

Multiboard Systems

One major challenge in multiboard systems, whether they run at low or high frequencies, is maintaining consistent grounding and defining return paths. There can be a temptation to create discontinuous grounding by connecting circuits to different ground points (e.g., net 1 to the power supply ground, net 2 to earth or chassis), which can produce ground loops or allow other sources of conducted EMI to move between different board sections. It can also cause you to mistakenly create a ground return path with very large loop inductance, creating susceptibility to external radiated EMI.

Read more about grounding strategies and return path planning in this article.

Power Electronics and Low-Level Systems

High voltage/high current power electronics will sometimes be placed on 2-layer PCBs with extremely heavy copper to prevent excess temperature rise. To examine coupling between different grounding points, you need to consider how each point will be isolated against DC and AC currents. For DC power electronics, isolation between DC grounds is pretty simple; if things are arranged on a PCB, the DC sheet resistance between two ground points on FR4 is on the order of 108 Ohms/sq. In other words, the DC sheet resistance is large enough that any DC leakage current directly between grounding points (i.e., across the substrate) is too low to be detected.

Discontinuous grounding points on a PCB

The ground points on this PCB will have high DC isolation, but they also have some stray capacitance.

 

This is not the case with a switching power supply on a board with discontinuous grounding. There is some stray capacitance between grounding points in the absence of a ground plane. There is also some stray capacitance between grounding points, and this stray capacitance will start to dominate impedance between grounding points if there is no ground plane.

You can roughly calculate the capacitance between two points with the standard formula with the dimensions of the ground contacts and dielectric constant of the substrate (usually FR4). This stray capacitance can induce some noise coupling between the two points, just like capacitive crosstalk in high speed/high frequency boards. You can quantify this level of noise and any settling in the initial potential difference between grounding points with some basic circuit simulations.

At the opposite end of the power spectrum, devices that need to collect low-level analog signals at low frequencies are not normally built on high layer count boards. If there is a discontinuous ground, noise circulating in a ground loop and power supply noise, in general, can mask a desired signal unless some filtering and/or lock-in amplification techniques are used. In these systems, you’ll also need to examine how noise couples between different grounding points and whether a large ground loop will mask desired signals.

Here’s how you can use a SPICE-based simulator to examine discontinuous grounding in your power systems with some simple analysis steps.

SPICE Simulations for Discontinuous Grounding

When defining two ground points in your schematic, you can simulate the capacitive leakage current between two ground points using transient simulations and a frequency sweep. You can model the coupling between two ground points with a parallel RC circuit between the two grounds (see the schematic below). The resistor in this circuit is just the DC resistance (a very large value) and a small capacitor for the AC capacitance. Together, this can create an RC time constant on the order of microseconds.

Discontinuous grounding in an electronics schematic

The circuit in the green box can be used to examine resistive and capacitive coupling in a system with discontinuous grounding.

 

The circuit in the green box above shows a simple method to examine coupling between the two closest grounding points in a pre-layout simulation. With a transient simulation, you can examine how long a ground level fluctuation on one net takes to propagate to the other net. This also allows you to examine how a noisy ground point induces noise back into a quiet ground (i.e., ground loops), and how the potential difference between the two ground points varies over time.

In the case of a ground loop, you could try to simulate the potential difference directly by simply placing a positive voltage source plus some noise on one of the ground nets. This would qualitatively model the effect of a constant potential difference (usually on the order of mV) between the two grounding points. Judiciously-placed probes would then provide the measurements you need to see how noise propagates back into upstream components. In this type of simulation, you’ll likely be able to directly see the benefits of a continuous ground plane or star grounding for your circuits.

When you have a system with discontinuous grounding, you can examine potential problems like noise coupling as part of PCB design and analysis if you use the right pre-layout simulation features. The front-end design features from Cadence integrate with the powerful PSpice Simulator, creating a complete system for designing electrical systems and simulating signal behavior. Once a design is ready for signoff, you can use the SI/PI Analysis Point Tools for post-layout verification and simulation.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

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