---
title: "Working principle of decoupling capacitor"
subject: "Instrumentation II"
chapter: 7
key: "semester_5/instrumentation_ii:decoupling-capacitor"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/decoupling-capacitor
site: BCT Engineering
---

# Working principle of decoupling capacitor

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2070 Chaitra, 2 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/decoupling-capacitor
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Describe the working principle of decoupling capacitor in short.

## Short answer

A decoupling capacitor (typically 0.1 uF) placed close to IC power pins acts as a local charge reservoir, supplying transient switching currents and bypassing high-frequency noise, preventing supply dips and ground bounce caused by trace inductance.

## Answer

A decoupling (bypass) capacitor, usually 0.1 uF ceramic, is connected between the supply and ground pins of an IC, as close to it as possible. When the IC switches, it needs a sudden burst of current. The distant supply cannot provide it quickly because of trace inductance (V = L·di/dt), which would cause supply dips and ground bounce. The capacitor acts as a **local charge reservoir**, supplying the transient current, and also provides a low-impedance path to ground for high-frequency noise on the supply, keeping the supply voltage stable.

## Check yourself

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

1. Where is a decoupling capacitor connected?
   A. In series with the signal output
   B. Across the input signal line and ground at the connector
   C. Between the supply and ground pins of the IC, as close as possible
   D. Across the crystal
   Answer: C. It sits right across the IC's supply and ground pins so it can supply current locally. (Key point: Placement)

2. Why can the distant supply not meet the IC's sudden current burst?
   A. The IC draws no current when switching
   B. Trace inductance opposes fast current change (V = L·di/dt)
   C. The supply voltage is too low
   D. Ceramic capacitors have high resistance
   Answer: B. Trace inductance prevents the supply from delivering fast current, so the local capacitor supplies it. (Key point: Local charge reservoir)

3. What does the decoupling capacitor do besides supplying transient current?
   A. Provides a low-impedance path to ground for high-frequency supply noise
   B. Raises the supply voltage
   C. Blocks DC from reaching the IC
   D. Increases trace inductance
   Answer: A. It shunts high-frequency noise to ground, keeping the supply stable. (Key point: Role of capacitor)

## Related in Instrumentation II

- [Decoupling capacitor and transient current](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adecoupling-capacitor-large) — Instrumentation II, chapter 7
- [Importance and working of RS-232C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-importance-signals) — Instrumentation II, chapter 3

## Related in other subjects

- [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2×
- [Capacitor start and run motor principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-run-principle) — Electric Machines II, chapter 3, asked 2×
- [GPS, components and working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-components-working-principle) — Surveying II, chapter 8, asked 19×
- [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4×