Fig 8 combines the architectures of Figs 5 and 7 . This different supply voltage influences how much current Device A draws. Similarly, Device A's bias current varies the power-supply voltage available to Device B, influencing how much current Device B draws. If the loops are small enough, no opportunity arises to add up in any circuit branch, and, therefore, no crosstalk develops. The objective is to confine signal currents to the devices that use them.
Electrons always prefer the path of lowest impedance.
The bypass-element impedance is Z BP . Z BP must be lower than Z PS at the operating frequency of the current. In other words, the impedance of the bypass element to the spike current must be lower than the apparent impedance of the power supply. But, the impedance of the power supply is already low. To see how difficult this scenario can be, consider a pc board using both power and ground planes.