---
title: "Ground bounce, decoupling and crosstalk"
subject: "Instrumentation II"
chapter: 7
key: "semester_5/instrumentation_ii:bounce-decoupling-crosstalk"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/bounce-decoupling-crosstalk
site: BCT Engineering
---

# Ground bounce, decoupling and crosstalk

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2078 Kartik, 6 marks
- Video: [EELE 461-561 Lecture - Ground Bounce (4/6/20)](https://www.youtube.com/watch?v=dflJjQs_43o) — Digital Logic & Programming
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/bounce-decoupling-crosstalk
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Explain ground bounce, decoupling and crosstalk in the context of circuit design.

## Short answer

Ground bounce: simultaneous switching causes transient voltage L·di/dt across ground pin inductance, temporarily lifting chip ground, causing glitches. Decoupling: a capacitor (0.1 uF) placed close to IC acts as local charge source, reducing supply noise and bounce. Crosstalk: unwanted coupling between traces via mutual C and L, causing noise; reduced by spacing, ground plane, or guard trace.

## Answer

#### 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).

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the visible effect of ground bounce on a quiet output?
   A. Output becomes permanently high
   B. A false pulse (glitch) and possible misreading of inputs
   C. Output frequency doubles
   D. No effect
   Answer: B. The local ground rises by L·di/dt, so quiet outputs show glitches. (Key point: Ground bounce effect)

2. What is the role of bulk (10 uF) capacitors alongside 0.1 uF ones?
   A. They block high-frequency noise only
   B. They replace the need for ground
   C. They supply lower-frequency demands, while using different values in parallel covers a wide frequency range
   D. They increase ground bounce
   Answer: C. Small caps handle high-frequency transients, bulk caps handle low-frequency demand. (Key point: Decoupling values)

3. In crosstalk, the trace that induces noise and the trace that receives it are called
   A. Victim and aggressor respectively
   B. Aggressor and victim respectively
   C. Source and sink
   D. Master and slave
   Answer: B. The aggressor couples signal into the victim. (Key point: Crosstalk terms)

4. Which reduces crosstalk?
   A. Fast edges and close spacing
   B. Longer parallel runs
   C. Wider spacing (3W rule), ground planes or guard traces and slower edges
   D. Removing the ground plane
   Answer: C. These measures lower mutual capacitance and inductance coupling. (Key point: Crosstalk reduction)

## Related in Instrumentation II

- [Ground bounce and crosstalk with reduction ways](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-crosstalk-reduce) — Instrumentation II, chapter 7
- [Ground bounce, crosstalk, impedance matching, skew](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-crosstalk-skew) — Instrumentation II, chapter 7
- [Ground, returns and shields in circuit layout](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aground-returns-shields) — Instrumentation II, chapter 8
- [Decoupling capacitor and transient current](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adecoupling-capacitor-large) — Instrumentation II, chapter 7

## Related in other subjects

- [Virtual ground and summer design](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Avirtual-ground-summer) — Basic Electronics Engineering, chapter 4, asked 2×