--- title: "Practical IA characteristics; optical isolation" subject: "Advanced Electronics" chapter: 4 key: "semester_4/advanced_electronics:ia-practical-characteristics-optical" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/ia-practical-characteristics-optical site: BCT Engineering --- # Practical IA characteristics; optical isolation Advanced Electronics · Chapter 4: Instrumentation and Isolation Amplifiers - Asked in: 2069 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/ia-practical-characteristics-optical - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-4 ## Question List out the practical characteristics of instrumentation amplifier. What are the advantages of 3 op-amp instrumentation amplifier over one op-amp and two op-amp instrumentation amplifiers? Explain the operation of optically coupled isolation amplifier. ## Short answer Instrumentation amplifier features adjustable gain (1-1000) via one resistor, very high input impedance, high CMRR (100-130 dB), low offset and drift. The three-op-amp IA has buffered inputs for high equal impedance, single-resistor gain without CMRR degradation, and symmetric paths for better high-frequency CMRR than two-op-amp designs. An optically coupled isolation amplifier uses an LED and matched photodiodes with feedback to cancel non-linearity, providing Vo=(Rf2/R1)*Vin and kV isolation. ## Answer #### Practical characteristics of an instrumentation amplifier - Differential gain from 1 to 1000, set accurately (about 0.1 %) by one external resistor RG - Very high input impedance, about 10⁹ Ω or more, on both inputs - Low output impedance, below 1 Ω - High CMRR of 100–130 dB, which falls at higher frequency - Low input offset voltage (µV range) and low drift (µV/°C) - Low input bias current and low noise - Moderate bandwidth (tens to hundreds of kHz, falling at high gain) and a good slew rate - Good gain linearity and stable gain with temperature #### Advantages of the 3 op-amp IA over 1 and 2 op-amp IAs - Both inputs are buffered, so both have very high and equal input impedance. The 1 op-amp IA has only a few kΩ. - The gain is adjusted by a single resistor RG without affecting CMRR. - The input stage has unity common-mode gain but high differential gain, so CMRR is higher. - The signal paths are symmetric, which gives good CMRR at high frequency. In the 2 op-amp IA one input passes through two op-amps. #### Optically coupled isolation amplifier ```text INPUT SIDE (GND1) | OUTPUT SIDE (GND2) | Vin -[R1]-> A1 --> LED ~~~~|~~~~> PD2 --> A2 --> Vo ^ ~ | (I2) (I to V, | ~ | Rf2) +----- PD1 | (feedback, I1) | isolation barrier ``` 1. A1 drives an LED with a current that depends on Vin. 2. The light falls on two matched photodiodes, PD1 (input side) and PD2 (output side). 3. PD1's current I1 is fed back to A1's inverting input. A1 sets the LED drive so that I1 = Vin/R1. This feedback cancels the LED's non-linearity and temperature drift. 4. PD2 gets the same light, so I2 = I1. A2 converts it into voltage: ```text Vo = I2 · Rf2 = (Rf2/R1) · Vin ``` There is no conductive path across the barrier, so isolation of several kV is obtained. It is used in medical and high-voltage measurement. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What CMRR does a practical IA typically have? A. 20 to 40 dB B. 100 to 130 dB, falling at higher frequency C. Exactly infinite D. 10 dB Answer: B. Practical CMRR is very high but frequency dependent. (Key point: Practical CMRR) 2. Why does the 3 op-amp IA beat the 2 op-amp IA at high-frequency CMRR? A. Its signal paths are symmetric, whereas in the 2 op-amp IA one input goes through two op-amps B. It has fewer op-amps C. It has lower gain D. It uses RG Answer: A. Unequal delays degrade high frequency CMRR. (Key point: Symmetric paths) 3. How does the first stage raise CMRR? A. Common-mode gain 1 but differential gain 1 + 2R1/RG B. It attenuates the differential signal C. It adds common-mode gain D. It removes RG Answer: A. The ratio of differential to common-mode gain is increased before A3. (Key point: First stage CMRR) 4. In the optical isolation amplifier, Vo equals: A. (R1/Rf2) Vin B. (Rf2/R1) Vin C. Vin/2 D. Rf2 R1 Vin Answer: B. I2 = Vin/R1 and Vo = I2 Rf2. (Key point: Optical amplifier output) ## Related in Advanced Electronics - [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 - [3 op-amp IA advantages; electromagnetic isolation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Athree-opamp-ia-electromagnetic-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 - [Output voltage of two op-amp IA](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atwo-opamp-ia-output) — Advanced Electronics, chapter 4 - [Single op-amp IA with mismatched resistances](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Asingle-opamp-ia-mismatched) — 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× ## 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