System ground is the most critical area and foundation related to noise and EMI problems on the board. The most practical way to minimize these problems is to have a separate ground plane.

What is Ground Noise?

Each signal originating from a circuit (for example, a driver) has a return current flow to the source by ground path. As the frequency increases, or even for simple but high-current switching like relays, there is a voltage drop due to line impedance generating interference in the grounding scheme. The return path is always through the least resistance. For DC signals, that is the lowest resistive path. For high frequency signals, that is the lowest impedance path. This explains how a ground plane simplifies the issue and is the key to making sure of signal integrity.

TI does not recommend that the digital return signals propagate inside the analog return (ground) area; therefore, the designer must split the ground plane to keep all the digital signal return loops within the ground area. Splitting needs to be done carefully. Many designs use a single (common) voltage regulator to generate a digital and analog supply of the same voltage level (for example, 3.3V). Users need to isolate the analog rail and digital supply rails and the respective grounds from each other. Be careful while isolating the ground, as both grounds have to be shorted somewhere. Figure 9-2 shows how possible return paths for digital signals are not allowed to form a loop passing through the analog ground. On each design, decide the common point considering the component placements and so forth. Do not add any inductors (ferrite bead) or resistors (not even zero Ω) in the series with any ground trace. The impedance increases due to associated inductance at a high frequency, causing a voltage differential. Do not route a signal referenced to digital ground over analog ground or the other direction.