--- title: "3 op-amp IA advantages; electromagnetic isolation" subject: "Advanced Electronics" chapter: 4 key: "semester_4/advanced_electronics:three-opamp-ia-electromagnetic-isolation" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/three-opamp-ia-electromagnetic-isolation site: BCT Engineering --- # 3 op-amp IA advantages; electromagnetic isolation Advanced Electronics · Chapter 4: Instrumentation and Isolation Amplifiers - Asked in: 2071 Chaitra, 2+2+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/three-opamp-ia-electromagnetic-isolation - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-4 ## Question What are the advantages of 3 op-amp Instrumentation Amplifier (IA) over 1 op-amp and 2 op-amp IA? List out the application of Isolation amplifier. Explain the block diagram of electromagnetically coupled isolation amplifier. ## Short answer Three-opamp IA offers high balanced input impedance, gain set by one RG with no CMRR compromise, higher CMRR, and symmetric signal paths compared to 1- and 2-opamp designs. Applications: medical, high-voltage, ground loops, industrial. Electromagnetic isolation: oscillator powers input side and modulates signal; transformer couples modulated carrier; demodulator recovers Vo, with kV isolation. ## Answer #### Advantages of the 3 op-amp IA over the 1 and 2 op-amp IA - **Very high input impedance on both inputs.** Both inputs go to non-inverting terminals of buffer op-amps. The 1 op-amp IA has an input impedance of only R1 or R3 + R4. - **Gain set by a single resistor RG** without changing CMRR. In the 1 op-amp IA, changing the gain requires changing two matched resistors. - **Higher CMRR.** The first stage has a common-mode gain of 1 but a differential gain of (1 + 2R1/RG). This raises the CMRR before the difference stage. - **Symmetric signal paths** (unlike the 2 op-amp IA, where V1 passes through two op-amps and V2 through one). This gives better CMRR at high frequency. - Low output impedance and wide gain range. #### Applications of isolation amplifiers - Medical equipment such as ECG and EEG, for patient safety - Measuring high voltages and currents in power electronics and motor drives - Breaking ground loops in industrial instrumentation - Industrial process control in noisy, high common-mode environments - Protecting computers and ADCs from transients and high voltage #### Electromagnetically (transformer) coupled isolation amplifier ```text INPUT SIDE (floating) || OUTPUT SIDE (grounded) || Vin->[Input amp]->[MOD]=T1=>[DEMOD]->[LPF]->[Out amp]->Vo ^ || ^ | || | carrier [Rect+Reg]<=====T2=====||==[OSCILLATOR ~ 100 kHz] (isolated +/-V) || ^ || +/-Vs (output side) ``` **Operation:** 1. An **oscillator** on the output side (for example about 100 kHz) does two jobs: - Through transformer T2 it sends power across the barrier. A rectifier and regulator then make the isolated ±V supply for the input side. - It also provides the carrier for the modulator and demodulator. 2. The **input amplifier** amplifies Vin (it is often an IA). 3. The **modulator** varies the carrier amplitude (or pulse width) with the signal. This turns DC or low-frequency signals into AC, which a transformer can pass. 4. Transformer **T1** carries the modulated signal across the isolation barrier, with no ohmic contact (isolation of 1–5 kV). 5. A **synchronous demodulator** and **low-pass filter** recover the original signal. The output amplifier buffers it: ```text Vo = G · Vin ``` It gives high isolation voltage, high CMRR and good accuracy. Example ICs: AD202 and AD210. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does the 3 op-amp IA have a higher input impedance than the 1 op-amp IA? A. Both inputs go to non-inverting buffer inputs B. It uses larger resistors C. It uses a transformer D. It has no feedback Answer: A. The 1 op-amp IA input impedance is only R1 or R3 + R4. (Key point: Input impedance advantage) 2. How is gain changed in the 3 op-amp IA without hurting CMRR? A. Changing all four resistors B. Changing the single resistor RG C. Changing the supply D. Changing R3 only Answer: B. In the 1 op-amp IA two matched resistors must be changed. (Key point: Gain adjustment) 3. Why does the transformer-coupled isolation amplifier modulate a carrier? A. A transformer cannot pass DC, so the signal modulates a high-frequency carrier B. To reduce gain C. To store energy D. To cancel noise only Answer: A. Modulation turns DC or low-frequency signals into AC. (Key point: Why modulate) 4. What two jobs does the oscillator perform? A. Power the input side via T2 and supply the carrier B. Filter the output and amplify C. Rectify and invert D. Only supply the carrier Answer: A. Through T2 and a rectifier it makes the isolated supply, and it gives the carrier. (Key point: Oscillator role) ## Related in Advanced Electronics - [Electromagnetic isolation; IA CMRR numerical](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aelectromagnetic-isolation-cmrr-numerical) — Advanced Electronics, chapter 4 - [Ideal IA characteristics; optically coupled isolation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aia-ideal-optical-isolation) — Advanced Electronics, chapter 4 - [Isolation amplifier applications; optically coupled circuit](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aisolation-applications-optical) — Advanced Electronics, chapter 4 - [Practical IA characteristics; optical isolation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aia-practical-characteristics-optical) — Advanced Electronics, chapter 4 - [Design three op-amp IA with gain 20](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adesign-3opamp-ia-gain-20) — Advanced Electronics, chapter 4, asked 3× - [Design three op-amp IA with gain 50](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adesign-3opamp-ia-gain-50) — Advanced Electronics, chapter 4, asked 3× ## Related in other subjects - [Op-amp properties and summing amplifier](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-summer-v1-2v2-3v3) — Basic Electronics Engineering, chapter 4 - [Summing amplifier using op-amp](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Asumming-amplifier-op-amp) — Fundamental of Electrical and Electronics Engineering, chapter 6 - [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3× - [Op-amp properties and integrator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-integrator) — Basic Electronics Engineering, chapter 4