Shielding bias nets

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Munna107

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Hi,

Normally we will do shielding for nets with ground. But particularly we will do shielding for PBIAS with VDD/VDDH and NBIAS with VSS, infact in the design we will add cap on PBIAS net intentionally with VDD. What are the advantages by doing like that.?
 

Hi,

I'm not sure, but maybe you talk about guarding.
The main goal is to reduce leakage currents.

Klaus
 

Pmos Vgs is defined wrt Vdd, while nmos wrt to gnd. This might be a reason for such shielding connection.

The reason for shield is high impedance of current source and not necessarily low impedance of it load. So, any small capacitive coupling with noisy line can strongly affect current.
 

I like to route biases under / alongside their reference rail.
This produces "free decoupling". Routing against the other
rail, makes HF PSRR degrade.
 

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