Shielding should be done with VDD or VSS and on which factors it depends?

Hello Karri , Its very nice question,

Generally, the decision to use VDD or VSS for shielding depends on the nature of the signals you're dealing with. If we want to shield digital signals, it's often recommended to use VSS (ground) since most of the noise is referenced to ground. For sensitive analog signals, using VDD might make more sense because it can prevent coupling with high-frequency noise from other parts of the circuit. So, I think it’s important to analyze the specific application before deciding.

If your signal is highly sensitive to noise, it's generally better to use VSS for shielding. This is because the ground plane (VSS) provides a more stable reference and is less likely to pick up or couple noise.

For high-frequency signals, sometimes VDD is used for shielding, especially when there’s a need to isolate different power domains or when the noise margin is lower on the VDD side.

If the noise margin on the VSS is higher (i.e., VSS is already noisy), then VDD might be a better option for shielding.

So in short, if you want to minimize capacitive coupling to a sensitive signal, use VSS as the shield, and if the noise is coming from the VSS itself, using VDD for shielding can be a viable option.