Skip to main content

3 op-amp IA advantages; electromagnetic isolation

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.

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.

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

 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:
Vo = G · Vin

It gives high isolation voltage, high CMRR and good accuracy. Example ICs: AD202 and AD210.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Advanced Electronics chapter 4 questions →