Return current path in high-speed circuits

In low-frequency circuits we assume that "ground" is a single point at zero volts. In high-speed circuits this is false: every conductor has resistance and inductance (impedance), so ground points are not at equal potential, and current must always return to its source, forming a loop. The signal quality depends on the return path that the current takes.

Reasons

1. Return current follows the path of least impedance, not least resistance. At low frequency (below about 10 kHz) the current takes the lowest-resistance path. At high frequency, inductance dominates (X_L = 2πfL), so the return current flows directly under the signal trace, where the loop area and inductance are minimal.
2. The current loop radiates and picks up noise. The loop area (signal + return) decides inductance, crosstalk and EMI. A broken or detoured return increases the loop area.
3. Ground is not equipotential. Ground bounce V = L·di/dt appears across the ground impedance when many outputs switch, so the "ground" of different chips differs by hundreds of mV.
4. Common impedance coupling occurs when two signals share a return path.

Examples

• A trace crossing a slot or split in the ground plane forces the return to go around it, producing a large loop, EMI and crosstalk.
• A signal changing layers needs a nearby via (stitching via) for the return current to follow.
• With a connector where several signals share one ground pin, the shared inductance causes ground bounce.

So a designer must ask what path the return current will take and make that path short, direct and continuous, usually via a solid ground plane under the trace.