Ground bounce, decoupling and crosstalk

Ground bounce

When several gate outputs switch at the same time, the sudden current change through the inductance (L) of the package pin and ground trace produces a voltage V = L·di/dt. This lifts the chip's local ground relative to the board ground for a short time, so a quiet output may show a false pulse (glitch) and inputs may be misread. Similar supply droop is called power bounce (rail collapse).

 chip GND pin --L--+-- board GND
 many outputs switch -> di/dt -> V = L di/dt

Decoupling

A decoupling (bypass) capacitor, such as a 0.1 uF ceramic, is placed across the supply and ground pins of each IC, as close as possible. It acts as a local reservoir of charge that supplies the transient current of switching, and offers a low-impedance path for high-frequency noise, which reduces ground bounce and supply noise. Bulk capacitors (10 uF) supply lower-frequency demands. Use of different values in parallel covers a wide frequency range.

Crosstalk

Unwanted coupling of signal from one trace (aggressor) to a nearby trace (victim) through mutual capacitance (I = C·dV/dt) and mutual inductance (V = M·dI/dt), producing noise or false transitions on the victim. It increases with fast edges, small spacing and long parallel runs, and is reduced by wider spacing (3W rule), ground planes or guard traces, shorter parallel length and slower edges.