Follow the Current (cont.)
To this point in our discussion of the basics, the model has been somewhat simplistic. We conveniently divided signals into low frequency and high frequency as if there were a well-defined boundary between the two.
The truth is that both paths are always involved. In Figure 6, at the initial transition of the IC1 output to the low state, the current comes from the bypass capacitor at IC2. This is because the output of IC1 is "demanding" a near-instantaneous current from the input pin of IC2, which pulls this current from its power pin.
We placed a bypass capacitor at IC2 with very short connections to its power and ground pins precisely to supply the fast current demands. The power source cannot provide this transient current as it is not very close to the IC and, thus, has substantial resistance and, more importantly, inductance between it and the power pin of IC2.
This is the whole reason for placing bypass capacitors at the ICs: to supply the transient (high-frequency) currents that the power supply cannot. As the transient settles out, more and more current comes from the power source and less and less comes from the bypass capacitor.
We simplify this concept further by saying that the DC current comes from the power source and the AC current comes from the bypass capacitor(s). We know, of course, that it is a bit more complex than this explanation.
As we consider more dynamic situations, we find that all the currents flow through a combination of the above four paths. The common path in either direction starts with the power pin of the sourcing component (IC1 or IC2), proceeds through that component and through the interconnect to the other component (IC2 or IC1), and then through the second component to its ground pin.
How the current completes its circuit from ground to the power pin of the sourcing component depends on the speed of the signal. The DC current will all return to the ground lead of the power source; it will flow from the power lead of the power source to the power pin of the sourcing component. High-frequency signal current will instead return to the ground lead of the sourcing component's bypass capacitor, which also supplies the current to the power pin.
In reality, both paths are always involved, with the DC path dominating for low-frequency signals. Keep in mind that even if a digital signal transitions at a slow rate (for example, a 1Hz square wave), the state transitions that cause the transient currents are just as fast as with a much higher frequency signal. They simply do not occur as often.
Of course, we are dealing with a good design here, so the bypass capacitors and the IC power and ground pins are very close. Proper bypassing like this makes a designer's job much easier. We can usually just think of the bypass capacitor and the IC as one entity when considering the flow of signal currents across a PCB.
Notice, finally, that the power current for high-speed AC signals travels a very short distance from a bypass capacitor to the IC that it is bypassing. The paths through the ICs themselves, of course, are short. The vast majority of the distance of the current loop is in the interconnect from the output of one chip to the input of the other and the ground return path.
Review Figure 4 and Figure 5 and consider what happens if the ICs are separated by a greater distance. The bypass capacitors stay close to their respective IC, and all the distance is added to the interconnect and the ground return. For higher-speed signal currents, this is where we will see problems…if they occur.