Noise appears in two forms in an electrical interconnect: as differential mode and common mode noise.
Differential mode noise is measured between two sections of an interconnect with equal and opposite polarity, while common mode noise applies to interconnects with the same phase and polarity.
Both forms of noise are induced in an interconnect via Faraday’s law from external radiation.
This drone is in an anechoic chamber for EMC testing
Break open any high speed signaling standard document, and you’ll find reference to tolerable noise limits at receivers, allowed skew on differential pairs, and loss budgets along interconnects. These standards are there to ensure components provide consistent functionality, but they don’t guarantee a system will work properly. As more high speed protocols become common in a variety of systems, noise becomes problematic in typically quiet systems. This applies to noise emitted from an interconnect, as well as noise received by an interconnect.
Because some low-level high speed protocols are noise-sensitive, and due to the use of differential pairs in modern high speed protocols, designers need to understand how different types of noise signals are induced on an interconnect. During EMC tests, common mode vs. differential mode noise can be introduced and will interfere with signal recovery. Keep reading to see exactly how this noise is induced in a system and what you can do to stop it.
Common Mode vs. Differential Mode Noise
All noise received in an electrical system is induced into the system in two ways:
Conducted EMI: This type of noise is received from some other component in the system. This noise propagates into the system as a current, either through direct conduction or by capacitive or inductive coupling.
Radiated EMI: Noise is emitted and received radiatively, meaning that radiated EMI is a form of crosstalk. Some of the strategies for reducing crosstalk will also apply to radiated EMI.
Each mechanism can put either type of noise into some portion of an electronic system. Any pair of traces or wires on a PCB, IC, or cable assembly can experience two types of noise: common mode and differential mode noise. Common mode noise will have the same magnitude and polarity on each side of the interconnect, while differential mode noise has opposite polarity. Note that we haven’t considered intrinsic random noise sources like Johnson noise, which does not need an external source.
The image below shows the difference between common mode and differential mode noise. In this image, the noise voltages on each side of the interconnect are V1 and V2. These voltages are measured with respect to the reference plane below the traces, which is assumed to be 0 V everywhere.
Common mode vs. differential mode definition
To eliminate noise from each side of the interconnect, it’s important to know how these noises are introduced via each mechanism. Let’s look at these in terms of radiated and conducted EMI.
Pairs of parallel interconnects can receive these noises from external sources, and they can emit radiation that can be received as both types of noise by some other system. For this reason, EMC tests focus on reception and emission of radiation from a product. As it turns out, both forms of noise are related to a differential interconnect’s geometry and its parasitic inductance (both self and mutual inductance), but noise is received in unique ways.
Consider the layout for two microstrip traces on the PCB shown below. In this layout, the traces are placed on a surface layer and are placed above a ground plane. Let’s assume that the traces are running parallel for some length and are connected to a receiver. Each trace has its own return path, and it has its own displacement current loop that forms in the ground plane below the trace.
Because each current path can be traced out as a closed loop within the cross section and around the traces, each loop has some inductance. These parasitic inductances allow EMI to be received from an external source or from another interconnect as inductive crosstalk, either in the common mode or differential mode. A traveling wave can induce a current to flow in one or both of these loops, depending on the direction of the magnetic field.
Loop inductance of two traces and reception of radiated EMI. There is some shared loop inductance, or mutual inductance, that can be seen from the top view, while the cross-sectional view shows self-inductance
In the cross-sectional view, a current loop can be traced out along each trace, which reflects each trace’s self-inductance. The top view also allows a loop to be traced out between the two traces running through the die on the receiver and transmitter components. Similar drawings can be constructed for ICs and cable assemblies. The astute reader will also notice that we could draw a loop at the ends of the interconnect in the cross-sectional view, although this loop is only a major contributor to EMI reception at the ends of the interconnect.
However, thanks to reciprocity, emission and reception of radiation goes both ways. This means that difficulties in radiated emissions testing can be overcome with the same solutions that address crosstalk and noise reception. These solutions are rather simple and can be applied on PCBs, cable assemblies, and IC traces. First, we have to look at how common mode and differential mode noise are received, then we can see how some simple design changes can help suppress these noises and help pass EMC testing.
Conducted EMI passes directly into the system over a conductor, and it may originate from some external source of radiated EMI. Conducted EMI can be removed with a ferrite choke, of which common mode and differential mode versions are available. As an example, power systems are one area where common or differential mode noise can be present on the input terminals, which can then propagate to load circuitry within the system.
Chokes are coupled inductors that will eliminate common mode or differential mode noise
Removing Noise Induced by EMI
Using chokes is the standard method to remove conducted EMI. Because conducted EMI in a system can originate as radiated EMI, it’s also important to suppress parasitics in the system. Of particular concern is parasitic inductance for radiated EMI, which should be minimized in interconnects to suppress the reception of EMI. Ground pour, via fences, and other periodic grounded structures are all useful for suppressing radiated noise.
For conducted EMI, the solution is to use chokes on input power lines, and to prevent capacitive coupling between return currents and other components in the system. Capacitive coupling is one reason that some Ethernet application notes will recommend total galvanic isolation on the PHY side of the system, with the goal being to prevent coupling back to magnetic components on the MAC side. Today’s advanced ECAD applications include 3D field solver utilities that can help you find and eliminate coupling due to parasitics in a PCB layout, just as one would do when addressing crosstalk.
Field solver utilities can be used to examine coupling between interconnects, reference layers, and other elements in a PCB layout
When you need to evaluate a layout for EMC, conducted mode vs. differential mode noise, and parasitics, use PCB design and analysis software with an integrated 3D EM field solver and a complete set of CAD tools. Cadence provides powerful software that helps automate many important tasks in systems analysis, including a suite of pre-layout and post-layout simulation features to evaluate your system.