Key methods for controlling EMC
Hardware engineers should address EMC issues during the pc board design phase to ensure a system free of EMC faults.
A well-grounded design
A low-inductance ground system is the most vital element for minimizing EMC problems. Maximizing the ground area on a pc board reduces the inductance of ground in the system, which in turn reduces electromagnetic emissions and crosstalk. Crosstalk can exist between any two traces on a board and is a function of mutual inductance and mutual capacitance proportional to the distance between the traces, the edge rate and the impedance of the traces.
In digital systems, crosstalk caused by mutual inductance is typically larger than the crosstalk caused by mutual capacitance. Mutual inductance can be reduced by increasing the spacing between the traces or by reducing the distance from the ground plane.
Signals can be connected to ground using various methods. A board design in which components are connected randomly to ground points generates high ground inductance and leads to unavoidable EMC issues. Use of a full ground plane is recommended because it minimizes impedance as the current returns back to its source, but the ground plane requires a dedicated pc board layer, which may not be feasible for two-layer boards.
Designers thus are advised to use ground grids, as shown in Figure 1a. The inductance of ground in this case will depend on the spacing between the grids.
The way a signal returns to system ground is also important. When a signal takes a longer path, it creates a ground loop, which in turn forms an antenna and radiates energy. Therefore, every trace carrying current back to the source should follow the shortest path and must go directly to the ground plane.
It is not advisable to connect all the individual grounds and then connect them to the ground plane; doing so not only increases the size of current loop but also increases the probability of ground bounce. Figure 1b shows the recommended method of connecting components to the ground plane.
Another good mechanism for reducing EMC-related problems is a Faraday cage, formed by stitching the ground along the complete periphery of the board and not routing any signal outside that boundary (Figure 1c). The mechanism restricts board emissions to the area defined by the boundary, while preventing external missions from interfering with signals on the board.
Proper arrangement of the layers is also vital from an EMC point of view. If more than two layers are used, then one complete layer should be used as a ground plane. In the case of a four-layer board, the layer below the ground layer should be used as a power plane. Care must be taken to locate the ground layer between high-frequency signal traces and the power plane. If a two-layer board is used and a complete layer of ground is not possible, then ground grids should be used. If a separate power plane is not used, then ground traces should run in parallel with power traces to keep the supply clean.
Layout guidelines
For a design free of EMC complications, components on the board must be grouped according to their functionality (analog, digital, power supply sections, low-speed circuits, high-speed circuits and so on). The tracks for each group should stay in their designated area. Filters should be used at subsystem boundaries.
When dealing with digital circuits, extra attention must be given to clocks and other high-speed signals. Traces connecting such signals should be kept as short as possible and should be adjacent to the ground plane to keep radiation and crosstalk under control.
With such signals, engineers should avoid using vias or routing traces on the board edge or near connectors. The signals must also be kept away from the power plane since they are capable of inducing power plane noise. Traces carrying differential signals should run close to each other to make the most effective use of magnetic field cancellation.
Traces carrying clock signals from a source to a device must have matching terminations because whenever there is an impedance mismatch, a part of the signal gets reflected. If proper care is not provided to handle the reflected signal, the largest amount of energy will be radiated. The various forms of effective termination include such methods as source, end and ac termination.
While routing traces for an oscillator, no other trace apart from ground should run in parallel or below the oscillator or its traces. The crystal should also be kept close to the appropriate chips.
And since return current always follows the path of least reactance, current-carrying ground traces should be kept close to the trace carrying the associated signal, in order to keep the current loop as short as possible.
Traces carrying analog signals should be kept away from high-speed or switching signals and must always be guarded with a ground signal. A low-pass filter should always be used to get rid of high-frequency noise coupled from surrounding analog traces.
In addition, it is important that the ground plane of analog and digital subsystems not be shared.
Off-board considerations
Any noise on the power supply tends to alter the functionality of a device under operation. Generally, noise coupled on the power supply is of a high frequency; thus a bypass capacitor or decoupling capacitor is required to filter it out.
A decoupling capacitor provides a low-impedance path for high-frequency current on the power plane to ground. The path followed by the current as it travels toward ground forms a ground loop.
This path should be kept to the minimum possible level by placing a decoupling capacitor very close to the IC.
A large ground loop increases the radiation and can act as a potential source of EMC failure. The reactance of an ideal capacitor approaches zero with increasing frequency, but there is no such thing as an ideal capacitor available on the market.
The lead and the IC package add inductance as well. Multiple capacitors with low equivalent series inductance should be used to improve the decoupling effect.
Many EMC-related problems are caused by cables carrying digital signals that effectively act as an efficient antenna. Ideally, the current entering a cable leaves it at the other end. In reality, parasitic capacitance and inductance emit radiation.
Using a twisted-pair cable helps to minimize coupling by canceling any induced magnetic fields. When a ribbon cable is used, multiple ground return paths must be provided. For high-frequency signals, shielded cable must be used, with the shielding connected to ground both at the beginning and at the end of the cable.
Finally, shielding is not an electrical solution but a mechanical approach to reducing EMC. Metallic packages (conductive and/or magnetic materials) are used to prevent EMI from escaping the system. A shield may be used to cover either the whole system or a part of it, depending on the requirements.
The shield is a form of closed conductive container, connected to ground, that effectively reduces the size of loop antennas by absorbing and reflecting
a part of their radiation. In this way, a shield also acts as a partition between two regions by attenuating the radiated EM energy from one region to another.
Shields reduce EMI by attenuating both the E-field and H-field components of a radiating wave.
About the authors
Ashish Kumar is senior product engineer at Cypress Semiconductor India Pvt. Ltd. (Bangalore). He holds a B.Tech in electronics and communications from Uttar Pradesh Technical University (Lucknow, India) and a postgraduate diploma in microelectronics design from Vedant Chandigarh (India).
Pushek Madaan is senior application engineer at Cypress Semiconductor India. He holds a B.Tech. in electronics and communications from Bharati Vidyapeeth College of Engineering (New Delhi).
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