12 Grounding
Grounding has to be considered from the very beginning of a hardware development project. This webpage presents the basic grounding concepts and how to apply them:
Types Of Ground.
In EMC, a distinction is made between these types of ground:
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Safety ground. Safety ground means the ground of an electrically conductive chassis or electric circuit, which protects humans against shock hazards and the electric circuit from ESD pulses.
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Functional ground. A ground for purposes other than electrical safety, e.g., for mitigating EMC and EMI issues.
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Chassis ground. A chassis ground refers to the connection that establishes an electrical link to a metallic enclosure. For example, cable shields are usually connected to the chassis ground.
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Signal ground. Signal ground means the return current path of a signal current back to its source.
Ground Symbols.
The standard IEC 60417 [12.1] defines the symbols for safety, functional, and chassis ground.
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No. 5017. Earth, ground. To identify an earth (ground) terminal in cases where neither the symbol 5018 nor 5019 is explicitly required:
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No. 5018. Functional earthing/grounding (US). To identify a functional earthing (grounding) terminal, for example, of a specially designed earthing (grounding) system to avoid causing malfunction of the equipment:
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No. 5019. Protective earth/ground (US). To identify any terminal which is intended for connection to an external conductor for protection against electric shock in case of a fault, or the terminal of a protective earth (ground) electrode:
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No. 5020. Frame or chassis. To identify the frame or chassis terminal:
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No. 5021. Equipotentiality. To identify the terminals which, when connected together, bring the various parts of equipment or of a system to the same potential, not necessarily being the earth (ground) potential, e.g. for local bonding:
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No. 6032. Do not connect to protective earth/ground (US). To indicate that conductive parts shall not be connected to protective earth (ground), e.g. on electrical equipment with conductive parts inside an insulating enclosure:
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No. 6092. Class II equipment with functional earthing/grounding (US). To identify class II equipment (appliances with double insulation and no protective conductor) with functional earthing (grounding):
where Rg [Ω] is the ground resistance, Lg [H] is ground inductance, and ω=2𝜋f [rad/sec] is the angular frequency of the sinusoidal signal.
Another point to remember is that the radiated emission due to differential-mode current is proportional to the current loop area A [m2] and the frequency f [Hz] squared! Thus, care must be taken that the current loop area A of high-frequency signals, formed by the signal current flowing along the signal conductor and through the ground back to the signal source, is kept small.
The EMC grounding philosophy includes the following points:
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Ground is a return current path. Do always consider the signal ground as the return current path, which closes the signal current loop.
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Minimize circuit loop area. With the return current in mind, minimize the current loop area A [m2] for high-frequency signals, because the radiated emissions of differential-mode current loops are proportional to the current loop area A [m2] and the frequency f [Hz] squared.
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Consider common impedances. When multiple signals share a common ground conductor path, common impedance coupling occurs. This coupling can be minimized by minimizing the common impedance Zg [Ω] or by forcing the return currents to flow through different paths.
Return Current Path On PCB Ground Planes.
Current does always take the path of the least impedance Z [Ω]. Therefore, in the case of a PCB signal trace above a solid ground plane (shown in the figure below), we can state the following:
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At low frequencies (around f<100kHz), the signal return current path in the ground plane flows more or less straight from via 2 back to via 1. This is because at low frequencies, the resistance Rg [Ω] dominates the impedance Zg=Rg+jωLg of the return current path and the lowest resistance from via 2 to via 1 is a direct line.
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At high frequencies (around f >1MHz), the signal return current in the ground plane flows directly below the respective PCB trace. This is because the inductive reactance X=2𝜋fLg starts to dominate the impedance Zg=Rg+jωLg of the return current path at high frequencies and the inductive reactance is lowest when the return current flows directly underneath the forward current PCB trace.
The points above lead to the conclusion that a solid ground plane should not be split up underneath a high-speed data line because in such a case, the signal return current would have to flow around that gap in the ground plane. This would lead to increased differential-current loop area A [m2] and therefore unintended emissions (for high-frequency signals). Generally speaking, a solid ground plane without any gaps does often lead to the lowest unintended emissions.
Bear in mind that a digital signal - like a clock or data signal - consists of many harmonics where the amplitudes of the high-frequency harmonics primarily depend on the signal’s rise- and fall-time. Given this, the return current path of a digital signal on a PCB is not identical for all its harmonics: the higher the frequency, the closer the harmonic signal follows the line underneath the forward current path of the digital signal.
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Single-point ground systems.
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Multi-point ground systems.
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Hybrid ground systems.
Single-Point Ground Systems.
Single-point ground systems are only useful for low frequencies (<100kHz). It is nearly impossible to implement a single-point ground system at high frequencies (>1MHz) because high-frequency currents start to flow through the stray capacitances Cstray [F], thus converting a single-ground system into an unintended and, therefore, not optimal multi-ground system.
The figures below show two types of single-point ground systems:
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Daisy-chain ground system. The daisy-chain ground system is a simple way to implement a single-point ground system, but it is not recommended because of the common-impedance coupling.
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Star ground system. A star ground system is an improved way to implement a single-point ground system than the daisy-chain variant because it reduces the common-impedance noise coupling.
Multi-Point Ground Systems.
The concept of a multi-ground system - as shown in the figure below - is usually applied to high-frequency systems. At high frequencies (>1MHz), the ground impedance Zg=Rg+jωLg is primarily dominated by the ground inductance Lg [H]. This means that the ground connections of the circuits to the ground should be as short as possible with low impedance (e.g., with multiple vias). For PCBs, a multi-point ground system is best achieved with one or several solid and uninterrupted ground plane(s).
Hybrid Ground Systems.
Hybrid ground systems provide different paths to ground for low-frequency currents ILF [A] and high-frequency currents IHF [A]. The figure below shows a typical hybrid ground system, which provides a single-point ground system for low-frequency signals and a multi-point ground system for high-frequency signals.
Hybrid grounding can also be applied to cable shields, where one end of the cable shield is connected to ground with low impedance and the other end is connected via a capacitor. A hybrid grounded cable shield could provide reasonable protection against inductive coupling of HF magnetic fields and at the same time prevent LF currents from flowing along the cable shield.
A ground loop is a current loop formed by ground conductors and the ground itself, as shown in the figure below.
Ground loops could lead to the following interference problems:
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Interference caused by ground voltage potential difference. A voltage potential difference between the ground connection points of two circuits, which are interconnected via a ground conductor, could cause an unintended noise current Isg [A] along the signal ground wire between the two circuits. As a consequence, a noise voltage Vn=IsgZsg is introduced in the signal connection between the circuits, where Zsg [Ω] is the impedance of the signal ground wire of the interconnection between the two circuits.
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Interference caused by magnetic field coupling to ground loop. A voltage Vinduced [V] could be induced into the ground loop by a magnetic field. Vinduced [V] causes a noise current Isg [A] through the signal ground wire, which introduces a noise voltage Vn=IsgZsg to the signal interconnection.
Well-balanced differential interfaces (e.g., LVDS) are more robust against ground loop coupling than single-ended interfaces (e.g., CMOS) because the noise voltage Vn [V] affects both signal lines of a differential interface and is canceled out. The picture below shows how a ground loop noise voltage Vn [V] interferes with the single-ended (unbalanced) interface data signal.
Ground loop noise coupling in the case of a differential (balanced) interface does add to both differential signal lines likewise, and the noise voltage is canceled out (see figure below).
Ground loop noise coupling in the case of a differential (balanced) interface does add to both differential signal lines likewise, and the noise voltage is canceled out (see figure below).
In many cases, ground loops do not lead to EMC problems. Meaning, the EMC design engineers should not avoid ground loops at any cost. However, EMC design engineers should be aware of ground loops and how to deal with them if they lead to an EMC issue. Ground loops are primarily a problem for low-frequency applications (f<100kHz) because the size of ground loops is usually large. Thus, they have a large inductance, which means that they represent a high impedance for HF signals. A typical example of an EMC issue due to a ground loop is the 50/60Hz hum coupled into audio systems.
If ground loops are a problem, the following are options for minimizing their impact:
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Use balanced transmission lines. As mentioned above because differential interfaces with a balanced transmission line are very robust against interference caused by ground loops.
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Applying single-point or hybrid grounding. Single-point or hybrid ground systems (see grounding of systems) help avoid ground current loops caused by voltage potential differences in the ground system. However, they do not help to avoid ground loop currents induced by magnetic fields.
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Reducing the ground impedance. Reducing the ground impedance Zsg [Ω] in the signal ground conductor (see figures above) reduces the noise voltage Vn [V] caused by the unintended ground loop current Isg [A] through the signal ground conductor.
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Breaking the ground current loop:
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Transformers, photocouplers.
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Common-mode chokes.
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Fiberglass.
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