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

# Ground bounce, crosstalk, impedance matching, skew

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2071 Shrawan, 4 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-crosstalk-skew
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Write about ground bounce, cross talk, impedance matching and timing skew.

## Short answer

Ground bounce: transient voltage from simultaneous switching currents across package inductance (V = L·di/dt) causing false logic. Crosstalk: unwanted capacitive/inductive coupling between adjacent traces, reduced by spacing and ground plane. Impedance matching: equalizing source, line, and load impedances to avoid reflections. Timing skew: difference in signal arrival times due to unequal trace lengths; match lengths.

## Answer

#### Ground bounce
When many outputs of a chip switch together, a large transient current flows through the inductance of the ground (and power) lead and pin. The voltage developed, V = L·di/dt, raises the local ground above the system ground for a short time, which can cause false logic levels on quiet outputs. Reduced by decoupling capacitors, short and wide ground paths, ground planes and by limiting simultaneous switching.

#### Crosstalk
Unwanted coupling of signals between neighbouring traces by mutual capacitance (I = C·dV/dt) and mutual inductance (V = M·dI/dt). Reduced by larger spacing (3W rule), shorter parallel length, ground plane or guard trace.

#### Impedance matching
Making source, line and load impedances equal (Z_s = Z₀ = Z_L) to avoid reflections; done by terminating with resistors and controlled-impedance routing.

#### Timing skew
The difference in arrival times of a signal (e.g. clock) at different receivers, or between related signals, caused by unequal trace lengths, loads and propagation delays. Reduced by matching trace lengths, balanced loading and clock distribution buffers.

## Check yourself

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

1. What produces the voltage that causes ground bounce?
   A. Resistance of the ground plane carrying DC current
   B. Transient current di/dt through the inductance of the ground lead and pin, V = L·di/dt
   C. Mutual capacitance between adjacent traces
   D. Reflection from an unmatched load
   Answer: B. Many outputs switching together cause a large di/dt through the ground lead inductance, raising the local ground by L·di/dt. (Key point: Cause of ground bounce)

2. Crosstalk between neighbouring traces is caused by which pair of couplings?
   A. Mutual capacitance (I = C·dV/dt) and mutual inductance (V = M·dI/dt)
   B. Skin effect and dielectric loss
   C. Ground bounce and supply droop
   D. Reflection and ringing
   Answer: A. Adjacent traces couple through mutual capacitance and mutual inductance, injecting noise into the victim. (Key point: Crosstalk mechanism)

3. What condition describes impedance matching to avoid reflections?
   A. Z_s > Z0 > Z_L
   B. Z_L much larger than Z0
   C. Z_s = Z0 = Z_L
   D. Z_s = 0 and Z_L = infinity
   Answer: C. Source, line and load impedances are made equal so no wave is reflected. (Key point: Impedance matching condition)

4. Timing skew is best described as
   A. The difference in arrival times of a signal at different receivers, due to unequal lengths, loads and delays
   B. Voltage rise of the ground above system ground
   C. Coupling between two traces
   D. A reflection caused by an open stub
   Answer: A. Skew is the arrival-time difference of clock or related signals, reduced by matched trace lengths and balanced loading. (Key point: Timing skew)

## Related in Instrumentation II

- [Ground bounce, decoupling and crosstalk](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-decoupling-crosstalk) — Instrumentation II, chapter 7
- [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
- [Routing rules for crosstalk and impedance matching](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Arules-mismatch-crosstalk) — Instrumentation II, chapter 8, asked 3×
- [Impedance matching and discontinuities](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aimpedance-matching-discontinuity) — Instrumentation II, chapter 7
- [Stub discontinuity and crosstalk causes](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Astub-discontinuity-crosstalk) — Instrumentation II, chapter 7

## Related in other subjects

- [Impedance matching techniques](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Aimpedance-matching-quarter-wave-stub) — Electromagnetism, chapter 5, asked 3×