Skip to main content

Ideal IA characteristics; optically coupled isolation

Short answer

Ideal IA: infinite input impedance, zero output impedance, infinite CMRR, finite accurate gain set by one resistor, zero offset/drift, infinite bandwidth, no noise. Optically coupled isolation amplifier: input amp drives an LED; light hits matched PD1 (feedback to linearize) and PD2; output amp converts PD2 current to voltage, giving Vo = (Rf2/R1)Vin, with kV isolation.

List out the ideal characteristics of Instrumentation Amplifier. Explain the operation of optically coupled isolation amplifier.

Ideal characteristics of an instrumentation amplifier

  • Infinite input impedance on both inputs (no loading of the source)
  • Zero output impedance
  • Infinite CMRR (zero common-mode gain)
  • Finite, accurate and stable differential gain, set by one resistor
  • Zero input offset voltage, offset current and drift
  • Infinite bandwidth and slew rate, and no noise
  • Gain independent of temperature and supply voltage

Optically coupled isolation amplifier

An isolation amplifier passes the signal from input to output with no electrical (galvanic) connection between them. In the optical type, the signal crosses the barrier as light, from an LED to photodiodes. The input and output sides have separate grounds and power supplies.

       INPUT SIDE (GND1)    |   OUTPUT SIDE (GND2)
                            |
 Vin -[R1]-> A1 --> LED ~~~~|~~~~> PD2 --> A2 --> Vo
             ^       ~      |      (I2)   (I to V,
             |       ~      |              Rf2)
             +----- PD1     |
           (feedback, I1)   |
                    isolation barrier

Operation:

  1. Vin drives a current Vin/R1 into the inverting input of A1. A1 drives the LED.
  2. The LED light falls equally on two matched photodiodes, PD1 on the input side and PD2 on the output side.
  3. Linearizing feedback: PD1's current I1 goes back to A1's inverting input. A1 adjusts the LED drive until I1 = Vin/R1. The light output is therefore forced to be proportional to Vin, and the LED's non-linear and temperature-dependent light-current curve is removed from the transfer function.
  4. PD2 receives the same light, so I2 = I1 = Vin/R1 (matched diodes).
  5. A2 works as a current-to-voltage converter: Vo = I2·Rf2. So:
Vo = (Rf2 / R1) · Vin

Features:

  • Isolation of several kV, with very high isolation resistance and low coupling capacitance
  • Good linearity due to the matched-photodiode feedback
  • Wide bandwidth (DC to tens of kHz)
  • Needs isolated power for the input side

Uses: medical (ECG) instruments, high-voltage sensing and breaking ground loops.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Advanced Electronics chapter 4 questions →