Table 1 shows why the above four capacitors are necessary. Here a single 0.1µF ceramic capacitor is not effective in reducing interfering noise and spurious components below 33kHz or above 15MHz. Power line and switching power-supply ripple below 33kHz would not be attenuated sufficiently. A larger value electrolytic would be better. Noise between two and fifteen Megahertz are shunted to ground properly by the 0.1µF capacitor. Observe that at 15.8MHz, the 0.1µF capacitor becomes self-resonant. This means that the unavoidable parasitic inductor in series with the capacitor dominates. Above that frequency the capacitor looks like an inductor and this noise and garbage is not able to pass through the capacitor to ground.
The Importance of the Ground Plane
So far we have spoken of ground as if it is a low-impedance sink for everything, and that the interfering signals are single frequencies. Neither is true. Remember that the fast edges of switching power supplies and digital logic circuits create high-frequency harmonics which must be suppressed. Ground is also problematic; seldom is it anywhere near perfect.
It is incorrect to envision ground alone without the complementary power planes or buses. The current must make a complete circle to complete the circuit. However in this note, we will consider only the ground plane and ignore the power planes for simplicity.
It sounds uncharacteristic, even funny, but thinking like an electron does help in this discussion (Figure 4). Electrons by nature are lazy. They always take the easiest path. Water is also a useful analogy. When we observe floodwaters, we see the water spread across the flood plain if it is not constrained in some way. Current behaves the same way in a voltage or ground plane.
Figure 4. A caricature of an electron on a ground plane.
Figure 5a imagines the electrons playing on the ground plane. A close look reveals the "good guys" (the returning signal currents) in the upper left and "bad guys" at the lower right. Some electrons know their place and keep to it. Some are confused, and others can be bought (or co-opted) by the highest bidder or most persuasive force.
Figure 5a. Electrons abound without clear guidelines.
These electrons are free to move over the ground plane. As they travel toward the star point, lower left, these good and bad electron currents can, and do, mix together. (Yes, we know electrons flow the opposite way, but we want the analogy to be simple and to match the concept of water flow.) Noise and interference are the unwanted contamination of one current into another. As the noisy digital electrons flow past the analog circuits, they will induce noise into the linear analog circuit, and vice versa. It is thus quite clear that all circuits are degraded by bad power and grounds. Maxim parts are no exception.
The MAX5400/MAX5401 digital potentiometers can have one side connected to the power bus or ground. Noise on the power or ground would be directly added to the signal. Voltage references like the MAX6133 have a line regulation specification, which indicates the amount of input voltage change that will be reflected in the output voltage. The MAX5532–MAX5535 family of 12-bit DACs has a power-supply rejection ratio (PSRR) of more than 20dB at 1kHz. This means that noise on the power bus will be reduced by a ratio of more than ten times. Nonetheless, having a part that reduces power-supply noise is no excuse for poor power design management on the ground. Good design demands good power.
One can now ask why the quiet analog signals do not harm the digital circuits? Oh, but they do damage the digital signals, just as the noise from one digital circuit interferes with other digital circuits. As we have discussed, analog and digital circuits are actually the same in this regard. The only difference is the presence of thresholds in digital. Really all noise in digital circuits is bad. Even if it is below the threshold, the noise contributes to errors and jitter. In analog every noise is bad, because there is no threshold to hide behind.
Figure 5b compounds the problem by adding a brushed motor with arcing brushes. These sparks are large enough in amplitude to cause errors in the digital circuits despite the thresholds.
Figure 5b. Adding an arcing , sparking brushed motor to the ground plane.
In both Figures 5a and 5b the ground areas are labeled as analog and digital. While one hopes that the signals stay in their regions, nothing keeps them separate. So in Figure 6a we have erected a fence.
Figure 6a. A fence is intended to corral the electrons (signals).
The fence is really a slot cut into the ground plane, shown here as a dotted line under the fence. The slot separates the ground plane into two sections only allowing a connection near the star ground point. Now the electrons are forced to remain only on their side of the fence.
Return now to arcing sparking motor in Figure 5b. Figure 6b addresses this challenge with two fences.
Figure 6b. Two fences isolate the circuits.
In the case of the brushed motor, the garbage created by the motor would be suppressed as close to the motor as possible. This is why it is common to find a capacitor soldered directly on the motor as well as diodes arranged to short out the back electromotive force (EMF). The EMF is the reverse voltage spike created by the motor's collapsing magnetic field. In a brushed motor that happens as the brushes bounce and as they commutate (switch) the motor current.