VSS (Ground) is used for shielding because it provides a more stable and low impedance path compared to VDD (Power Supply).
Here's a breakdown of the reasons:
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Stability: VSS is typically designed to be a stable reference point in a circuit. It's less susceptible to voltage fluctuations and noise that might be present on the VDD rail. Shielding with a stable reference like VSS minimizes the introduction of noise into the shielded signal.
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Low Impedance: VSS often has a lower impedance than VDD. A low impedance path to ground allows any noise coupled into the shield to be effectively shunted to ground, preventing it from affecting the signal being protected. This is crucial for effective shielding.
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Clock Signal Considerations: While shielding is often applied to critical signals, clock signals are sometimes shielded too. However, doing so increases the capacitive load on the clock signal, which can degrade its performance. Therefore, the decision to shield a clock signal is made on a case-by-case basis depending on the specific noise environment and signal integrity requirements.
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Noise Reduction: The primary goal of shielding is to reduce electromagnetic interference (EMI) and noise. Connecting the shield to VSS helps to accomplish this by creating a Faraday cage effect, which redirects external noise to ground instead of allowing it to couple into the signal line.
In summary, VSS is chosen for shielding due to its stability and low impedance, which effectively grounds any induced noise and provides a stable reference, enhancing signal integrity.
