“ In the previous article (How to Control Common Impedance Interference on PCBA? – Reducing Common Impedance), we mainly discussed how to reduce interference from the perspective of lowering common impedance. However, in the design of PCBA, there are many limitations to reducing common impedance, such as the non-zero impedance characteristics of conductors. If we can distinguish the grounding in PCBA according to the characteristics of the signals, it can greatly reduce interference to sensitive circuits. This article will first explain the necessity of using different grounding topologies, and subsequent articles will provide detailed introductions to each grounding topology.”
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The Purpose and Necessity of Grounding
As mentioned in the article (Summary of the Purpose of System Grounding), there are three purposes of grounding: Safety grounding, Signal functional grounding, and Control of electromagnetic interference grounding. Therefore, to ensure system performance, the grounding design of PCBA must provide a low impedance path for return currents while minimizing common impedance interference coupling. To achieve this goal, it is essential to identify the characteristics of signals and circuits to select different grounding topologies.
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Common Signal Characteristics in PCBA
Low-frequency analog signals: Low-frequency analog signals are typically narrowband low-level signals, often as low as µV or mV. The gain devices in analog circuits not only amplify the signals but also amplify the interference. Analog circuits must be equipped with dedicated low-noise loop paths that should not be shared with any other circuits to avoid coupling of high-level noise signals on common impedance.
High-frequency analog signals: High-frequency analog signals (RF, video, etc.) have a wide frequency and level range. Similar to low-frequency analog signals, high-frequency analog signals are very sensitive to noise, especially at the receiver. To achieve optimal performance (especially considering the high-frequency characteristics of certain signals), these circuits must have low-impedance, noise-free loop paths, typically implemented in the form of planes or their extensions (e.g., coaxial cables).
Digital signals: Digital signals are typically signals with a characteristic bandwidth and moderate levels, usually at 5V or lower. The loop paths for digital signals (especially high-speed digital) need to maintain low impedance across the entire operating bandwidth (the operating bandwidth is determined by the “rise time” rather than the pulse repetition frequency, see the secrets of pulse signals – understanding the inductive signal spectrum characteristics).
Power load signals: Signals associated with power loads (such as motors and relays) will exhibit broadband characteristics and have extremely high levels (potentially up to several kV), and the loop paths for these signals should not be shared with any other signal paths.
Note: In special cases, such as electric detonators and ignition circuits, isolation and dedicated loops may be required.
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Common Grounding Topologies
Common grounding topologies include: floating ground topology, single-point grounding topology, multi-point grounding topology, mixed grounding topology, tree grounding topology, nested grounding topology, and frequency-selective grounding topology. In subsequent articles, we will introduce each one in detail.
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