---
title: "Energy coupling; capacitive coupling and remedies"
subject: "Instrumentation II"
chapter: 6
key: "semester_5/instrumentation_ii:capacitive-coupling-remedies"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/capacitive-coupling-remedies
site: BCT Engineering
---

# Energy coupling; capacitive coupling and remedies

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2078 Bhadra, 6 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/capacitive-coupling-remedies
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

Explain the principle of energy coupling. Describe about capacitive coupling with remedies.

## Short answer

Energy coupling transfers noise from source to receiver via a path. Capacitive coupling occurs through stray capacitance C; noise current is I = C dV/dt, causing noise voltage Vn = R C dV/dt. Remedies: increase spacing, use grounded electrostatic shields, reduce receiver impedance, slow source edge rates, use differential lines, and add guard traces.

## Answer

#### Principle of energy coupling
Every noise problem needs a **source**, a **coupling path** and a **receiver**. **Energy coupling** is the transfer of electrical energy from the noise source to the receiver through the coupling path. The path can be a conductor (conductive), an electric field (capacitive), a magnetic field (inductive) or a radiated wave (electromagnetic). The noise can be eliminated by reducing the source, breaking the path, or making the receiver immune.

#### Capacitive coupling
Any two conductors at different potentials with a dielectric (air) between them form a stray capacitance C. A changing voltage on the source conductor passes a displacement current into the victim conductor:

I_noise = C · dV/dt

The noise voltage across the victim impedance R is V_n = R · C · dV/dt (for low frequencies). It is more serious when dV/dt is high (fast edges, high voltages), the receiver impedance is high, conductors run parallel for a long way and the spacing is small.

```text
  Source ----+----------
             | C (stray)
  Victim ----+----[R]--- GND
```

#### Remedies
1. **Increase spacing** and reduce parallel run length; cross traces at right angles.
2. **Electrostatic (Faraday) shield** grounded, around the victim or the source; use shielded cables.
3. **Reduce the receiver impedance** (lower R, e.g. a pull-down resistor or buffer).
4. **Reduce dV/dt** of the source: slow the edges using series resistors, snubbers or RC filters.
5. Use **balanced/differential lines** so the induced noise appears as common-mode.
6. Place a **ground trace/guard** between the lines on a PCB; use ground planes.

## Check yourself

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

1. Which three elements are needed for any noise problem?
   A. Voltage, current and resistance
   B. Shield, ground and filter
   C. Source, coupling path and receiver
   D. Input, output and supply
   Answer: C. Noise can be removed by reducing the source, breaking the path or making the receiver immune. (Key point: Energy coupling elements)

2. What is the noise current in capacitive coupling?
   A. I = M dI/dt
   B. I = C dV/dt
   C. I = V/R of the receiver only
   D. I = L dV/dt
   Answer: B. A changing source voltage drives displacement current through the stray capacitance. (Key point: Capacitive formula)

3. When is capacitive noise more serious?
   A. High dV/dt, high receiver impedance, long parallel runs and small spacing
   B. Slow edges and low receiver impedance
   C. Large spacing and short runs
   D. Low voltage with grounded shield
   Answer: A. V_n = R C dV/dt grows with R, C and dV/dt. (Key point: Worsening factors)

4. Which is a remedy for capacitive coupling?
   A. Higher receiver impedance
   B. Faster edges
   C. Parallel, closer routing
   D. Grounded electrostatic shield, more spacing, lower receiver impedance and slower edges
   Answer: D. Each reduces C, dV/dt or R in V_n = R C dV/dt. (Key point: Remedy)

## Related in Instrumentation II

- [Inductive noise and capacitive coupling shielding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ainductive-noise-capacitive-shield) — Instrumentation II, chapter 6
- [Remedial strategies for energy coupling](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aenergy-coupling-remedies) — Instrumentation II, chapter 6

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