--- title: "Inductive noise and capacitive coupling shielding" subject: "Instrumentation II" chapter: 6 key: "semester_5/instrumentation_ii:inductive-noise-capacitive-shield" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/inductive-noise-capacitive-shield site: BCT Engineering --- # Inductive noise and capacitive coupling shielding Instrumentation II · Chapter 6: Grounding And Shielding - Asked in: 2073 Shrawan, 3+3 marks - Video: [Stop EMI! How to Identify Capacitive vs Inductive Coupling & Eliminate Noise in Your PCB Design.](https://www.youtube.com/watch?v=pW48TkiQw2c) — Technologies Discussion - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/inductive-noise-capacitive-shield - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6 ## Question How is inductive noise introduced in electronic system? Discuss the shielding mechanism for capacitive coupling. ## Short answer Inductive noise is introduced when a changing current in one circuit induces voltage in another via mutual inductance, increasing with loop area and dI/dt. Capacitive coupling is countered by placing a grounded conductive shield between source and receiver, diverting the noise current to ground and minimizing stray capacitance. ## Answer #### How inductive noise is introduced Inductive (magnetic) coupling occurs when a changing current in one circuit produces a changing magnetic flux that links with another loop. A noise voltage is induced in the victim loop: V_noise = M · dI/dt (M = mutual inductance, in henry) M increases with larger loop areas, long parallel conductors and closer spacing. The induced voltage also increases with frequency, because a higher frequency means a larger dI/dt. Sources are motors, relays, transformers, switching supplies and power cables carrying high di/dt. The induced voltage appears in series with the victim loop and is independent of the loop's impedance. ```text Loop 1 (noisy) Loop 2 (victim) ___ I(t) ___ | | <-- flux --> | | V = M dI/dt |___| |___| ``` #### Shielding for capacitive coupling Capacitive coupling passes a current I = C·dV/dt through a stray capacitance C between the source and the receiver. An **electrostatic (Faraday) shield** works as follows: 1. Place a conductive shield (metal foil, braid or enclosure) around the receiver or between the two circuits. 2. **Connect the shield to ground** (the reference of the circuit) at a proper point. The stray capacitance is split into source-to-shield and shield-to-receiver capacitances. 3. The noise current goes from the source to the shield, then to ground, and does not reach the receiver. The remaining coupling capacitance (source to receiver) becomes very small. ```text Source --Cs1--[ SHIELD ]--Cs2-- Receiver | GND ``` An ungrounded or badly grounded shield does not help (it may even increase the coupling). Keep the shield leads short and cover the receiver conductor completely. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which expression gives the noise voltage induced in a victim loop by inductive coupling? A. I = C dV/dt, with C the stray capacitance B. V = I R of the shared ground wire C. V = L dI/dt of the victim's own inductance only D. V = M dI/dt, with M the mutual inductance Answer: D. A changing current in one loop changes the flux linking the victim loop, inducing V = M dI/dt. (Key point: Inductive noise formula) 2. How does the induced inductive noise voltage relate to the victim loop's impedance? A. It grows in proportion to the loop impedance B. It falls as the loop impedance rises C. It appears in series with the loop and is independent of the loop impedance D. It exists only if the victim impedance is high Answer: C. The induced emf is a series voltage source set by M and dI/dt, not by the victim's impedance. (Key point: Impedance dependence) 3. How does a grounded electrostatic shield reduce capacitive coupling? A. It absorbs the magnetic flux with high-permeability material B. It splits the stray capacitance so noise current flows from source to shield and to ground C. It raises the receiver impedance so less current flows D. It increases the source-to-receiver capacitance Answer: B. The shield intercepts the displacement current and diverts it to ground, leaving very little source-to-receiver capacitance. (Key point: Capacitive shield action) 4. What is the effect of an ungrounded or badly grounded shield against capacitive coupling? A. It does not help and may even increase coupling B. It works just as well as a grounded one C. It works only at low frequency D. It converts capacitive coupling into inductive coupling Answer: A. Without a proper ground path the noise current cannot be diverted, so the shield may act as a coupling bridge. (Key point: Ungrounded shield) ## Related in Instrumentation II - [Grounding and shielding; inductive and capacitive shielding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Agrounding-shielding-inductive-capacitive) — Instrumentation II, chapter 6 - [Identify noise coupling mechanism by calculation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aswitching-noise-calc) — Instrumentation II, chapter 6 - [Grounding and shielding to reduce PCB noise](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Apcb-grounding-shielding-noise) — Instrumentation II, chapter 8 - [Energy coupling; capacitive coupling and remedies](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acapacitive-coupling-remedies) — Instrumentation II, chapter 6 - [Noise coupling types and checking predominance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-predominance) — Instrumentation II, chapter 6, asked 2× - [Noise coupling mechanisms with pseudo impedance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-pseudo-impedance) — Instrumentation II, chapter 6 ## Related in other subjects - [Transformer at resistive, inductive, capacitive loads](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atransformer-loading-conditions-phasor) — Electrical Machines, chapter 2 - [Features introduced from 2G to 5G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afeatures-2g-to-5g) — Wireless Communication, chapter 1