Digital Circuit Boards: Mach 1 GHz

5.8 The Decoupling Capacitor

In Chapter 3, it was pointed out that a decoupling capacitor and its connection can be considered as short sections of transmission lines. The connections that are made inside and outside of the IC are also transmission lines. There are usually several parallel connections to logic traces in an IC driver, as shown in Figure 5.2.

Figure 5.2 A set of connections at a logic structure and a decoupling capacitor.

When the logic switch closes, the process of obtaining energy involves transmissions and reflections along all the connected transmission lines. It is important to note that the initial energy will come from the electric field associated with the die and connected logic and power traces. The next nearest source of energy is the ground/power plane capacitance and finally energy will come from the decoupling capacitor. If the energy in this geometry is adequate and if there is sufficient time to gather energy through multiple reflections, a logic signal of sufficient amplitude will propagate over the connected transmission lines and arrive at the logic gates within the time permitted by the error envelope. If the energy used in this transmission is not immediately replaced then the transmission at the next clock time will have a lower voltage.

The path that fields must take to supply energy to the decoupling capacitor is worth discussion. The energy ...