In many systems, it is common to build impedance matching circuits from discrete components, usually SMD passives. Sometimes, when ground is cleared below an impedance matching network, we are not completely eliminating ground below these circuits. This is because the feedlines coming into and out of the circuit need to be a target impedance (usually 50 Ohms), so ground is still needed in the device.
In some cases, the ground plane may be intentionally removed in impedance matched areas of a high-speed electronic system. This practice is known as "split ground planes" or "island grounding," and it is done to address specific design considerations and improve signal integrity.
Signal isolation: In some cases, it may be necessary to isolate certain signals or signal paths from the common ground. This can be done to prevent ground loops, reduce noise coupling, or maintain signal integrity. By removing the ground reference in impedance matched areas, the signals can be effectively isolated from the rest of the system, minimizing unwanted interactions.
Differential signaling: Impedance matching is often crucial for high-speed differential signals, such as those used in USB, Ethernet, or HDMI interfaces. These signals consist of a pair of complementary signals (positive and negative) that have equal and opposite voltages with respect to a reference point. In differential signaling, the impedance matching is performed between the two signal lines, while the reference point or ground is usually common to both lines. However, in certain cases, such as in some high-speed serial communication protocols, it may be advantageous to remove the ground reference and allow each line to have its own local reference, enhancing signal integrity and reducing crosstalk.
It's important to note that removing the ground reference in impedance matched areas requires careful consideration and analysis. It should only be done when it aligns with the specific requirements of the circuit or system design. Proper signal integrity analysis, simulation, and testing are necessary to ensure that the removal of the ground does not introduce other undesirable effects or compromise the overall performance and reliability of the system
Therefore, in some cases, it could make sense to modify where ground is placed around these components. We want to ensure that the sizes of the pads and traces on the components do not create excessive additional parasitic capacitance that modifies the impedance of the capacitors and inductors in the impedance matching network.
Noise considerations: In some scenarios, the presence of a ground plane or a common ground reference can exacerbate noise-related issues. For example, if there are significant ground potential differences or ground loops in the system, the shared ground could introduce noise or interfere with signal quality. By removing the ground in impedance matched areas, the impact of such noise sources can be minimized.
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