If you don't add local sources for charge to lower the supply impedance,
that's right. You can't just add some Z to the supply path and call it
"fixed".
> The voltage drop in the VSS and GND planes is given by Leff * di/dt where L is the effective inductance for a particular switching pattern. If you add L in series, your effective L is going to increase and so is the noise.
Again, you're using a shunt C/series L to make a low pass network
onto supply. With the proper C between the L and the part, you've
maintained a local low impedance source while attenuating the high
frequencies getting onto the supply.
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