Types of grounding
Single-point grounding. Here, each subsystem/module has its own ground. These individual grounds are connected by the shortest route to a single system ground point by simple wires. Such a system is advantageous for low-frequency and analog circuits because no common impedances exist.
At high frequencies (above 1MHz), however, grounding wires start to exhibit high inductance and, consequently, start offering high impedance to ground currents. At the same time, capacitive reactance of stray capacitance between the modules starts reducing. Ground currents no longer follow the high-impedance path offered by ground wires to ground, but are rather invited to follow the low impedance offered by the parasitic capacitance to other modules. This causes common-mode coupling, which can be reduced by reducing the inductive reactance of the ground wires. This is achieved by multi-point grounding.
Multi-point grounding. In multi-point grounding (Fig. 19), each subsystem/module is bonded as directly as possible to a common low-impedance equi-potential ground plane (essentially a continuous sheet of metal). A metal sheet offers far less impedance at high frequencies (above 1MHz) compared to wires. This is because sheets have more surface area, which reduces resistance to high-frequency currents that tend to flow on the surface due to skin effect.
The sheet also provides multiple parallel paths for ground currents, which reduce inductance. In such a scenario (when modules are mounted on a ground plane), ground currents are invited to follow the low impedance of the ground plane to ground, rather than going to another module.
Hybrid grounding. Hybrid grounding is used in situations where systems involve both high-frequency (digital) circuits and low-frequency (analogue) circuits.
Reducing ground impedance coupling
Due to improper grounding and bonding, or due to improper grounding practices, ground impedance tends to increase. This is shown as lumped impedance in the Fig. 20 (a).
EMI currents flowing through this impedance cause voltage drop Vcm across it, which forces common-mode EMI currents I1 and I2 through the circuit. The first way is to eliminate this common impedance completely by grounding the modules at a single point as shown in Fig. 20 (b).
The second option is to open the ground loop by grounding only one of the modules as shown in Fig. 20 (c). But these hold good only at low frequencies up to a few tens of kilohertz, where this impedance behaves only as resistance.
At higher frequencies, stray capacitance (shown as dotted lines) begins to appear between grounding cables or between the module and the ground, causing common-mode currents to circulate again.
Also, at higher frequencies, ground wires themselves start offering high impedance since their inductive reactance and resistance due to skin effect start to increase, causing potential gradients along the wire.
Therefore at high frequencies, efforts should be directed towards reducing ground impedance, which, in turn, can be reduced first by reducing inductance, that is, replacing the wires by a metal sheet (a mounting plate) and then to reduce the value of ground resistance by proper bonding practices.
Also, interconnecting cables should be shielded so that common-mode EMI currents now flow as shown in Fig. 20 (d) on the outside of the shield, reducing common-mode coupling.
This article is an extract from a book by the author. The next part will cover bonding.
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Chetan Kathalay is working as scientist in Electronics Test and Development Centre, Pune. He is BE in electronics from Nagpur University