Instrumentation Amplifier

An instrumentation amplifier amplifies the difference between two signals with very high input impedance on both channels and excellent common-mode rejection, all in one package. The INA128 is the go-to IC for ECG front-ends, Wheatstone bridge readouts, and thermocouple amplifiers.

Core Concept

The standard instrumentation amplifier uses three op-amps. The first two are non-inverting buffers with their inverting inputs cross-connected through a single gain-setting resistor Rg. This arrangement provides very high input impedance on both inputs and amplifies the differential signal without loading the source.

The first stage provides differential gain of (1 + 2R/Rg) while passing common-mode signals at unity gain. The third op-amp is a difference amplifier that subtracts the two first-stage outputs, rejecting the common-mode component. The total gain is set entirely by Rg, which is convenient: you change one resistor to change gain.

The INA128 implements this topology in a single 8-pin package. It achieves CMRR of 120 dB at DC with gain of 1000, input impedance of 10 GΩ, and input offset voltage of only 25 µV. The gain is set by connecting a single external resistor between pins 1 and 8, using the formula G = 1 + 50 kΩ/Rg.

Key Equations

Gain for three op-amp IA (general form): G = 1 + 2R/Rg where R is the internal resistor in each first-stage arm.

INA128 gain formula: G = 1 + 50 kΩ / Rg. Set Rg = 50 Ω for G = 1001 ≈ 1000.

INA126 gain formula: G = 5 + 80 kΩ / Rg. Minimum gain is 5 (no Rg connected).

CMRR in dB: CMRR = 20 × log10(Adiff / Acm). INA128 gives 120 dB at G = 1000.

Output referred noise: Vnoise_out = Vn_input × G. Minimise input-referred noise by choosing a low-noise input op-amp stage.

Given:
  INA128 instrumentation amplifier
  Internal resistor R = 25 kΩ per arm (so 2R = 50 kΩ)
  Required gain G = 200
  Differential input voltage Vdiff = 5 mV
  Find: Rg value and Vout

Why this formula:
  INA128 gain equation: G = 1 + 50kΩ / Rg

Formula:
  G = 1 + 50,000 / Rg
  Rg = 50,000 / (G - 1)
  Vout = G × Vdiff

Substitution:
  Rg = 50,000 / (200 - 1) = 50,000 / 199
  Vout = 200 × 5×10^-3

Calculation:
  Rg = 50,000 / 199 = 251.3 Ω → use 249 Ω (nearest standard E96 value)
  Vout = 200 × 0.005 = 1.0 V

Final Answer:
  Rg ≈ 249 Ω (1% tolerance)
  Vout = 1.0 V

Exam Tip: GATE often asks to calculate Rg for a required gain using the INA128 formula G = 1 + 50k/Rg. A common error is using G = 2R/Rg and forgetting the +1 term, especially at low gain values. Also, in the three op-amp topology, changing Rg does not affect common-mode rejection because CMRR is determined by matching of the difference stage resistors, not by Rg.

Key Properties

• Input impedance is typically 10 GΩ for the INA128, making it compatible with high-impedance biomedical sensors and pH electrodes.
• Gain is set by a single external resistor Rg; no matched pairs are needed, unlike the single op-amp difference amplifier.
• CMRR of INA128 is 120 dB minimum at G = 1000, allowing it to reject 1 V of 50 Hz mains interference while amplifying a 1 mV ECG signal.
• Input offset voltage is 25 µV (max) for INA128, about 80× better than the LM741's 6 mV maximum.
• The INA126 has a minimum gain of 5 due to its internal architecture; it cannot operate at unity gain without external modification.
• Power supply range is ±1.35 V to ±18 V for INA128, enabling battery-powered portable instruments.
• Bandwidth reduces as gain increases; INA128 has a GBP of approximately 1.3 MHz, giving 1.3 kHz at G = 1000.

Quick Revision

• Three op-amp topology: two input buffers + one difference amplifier stage.
• Gain set by single Rg: G = 1 + 2R/Rg (general) or G = 1 + 50k/Rg (INA128).
• CMRR > 120 dB at high gain; determined by matching in difference stage, not by Rg.
• Very high input impedance (~10 GΩ) on both inputs; does not load the source.
• INA128 Vos = 25 µV max; suitable for millivolt-level sensor signals.
• Bandwidth = GBP / G; at G = 1000, INA128 gives ~1.3 kHz bandwidth.
• Reference pin (pin 5 on INA128) sets the output zero level; connect to ground for 0 V reference.
• Exam trap: students confuse the INA128 gain formula with the general three op-amp formula and use G = 2R/Rg, dropping the +1 term, which gives significant error at low gains like G = 2 or G = 5.