Many of the problems that occur when transforming a design from the mythical world of textbooks and SPICE to the real world emanate from the non-ideal power supply. Real power supplies can cause noise and spurious oscillations that can force the designer into a frustrating glitch hunt. Rules of thumb can usually be applied successfully to simple problems, but a little understanding and forethought will usually provide clean solutions to even the more obscure problems. This paper will provide the understanding of the dynamics of power distribution, but the forethought is up to you.
Bypassing and decoupling are often poorly understood and poorly applied. Many designers believe bypassing and decoupling are synonymous. They are not; they are distinct concepts and each is a solution to a different problem (see Figure 1).
Figure 1 Bypassing and decoupling
Bypassing is the reduction of high frequency current flow in a high impedance path by shunting that path with a bypass, usually a capacitor (in this case, Cbyp). Bypassing is used to reduce the noise current on power supply lines.
Decoupling is the isolation of two circuits on a common line. The decoupling network is usually a low pass filter and the isolation is rarely equal in both directions. Decoupling is used to prevent transmission of noise from one circuit to another. In the figure a bypass capacitor, Cbyp, is shown along with the decoupling circuit, Ldec and Cdec. This is because in practice bypassing is always used when decoupling.
Most circuits require bypassing, not decoupling. Using decoupling techniques to accomplish bypassing will give disappointing, if not disastrous, results. Complete understanding of both concepts is vital. We begin with bypassing.