You've already seen basic op-amp amplifiers, but what happens when you need to measure a tiny signal sitting on top of a large common-mode voltage? A single op-amp differential amplifier works in theory, but resistor mismatches destroy its common-mode rejection ratio (CMRR). The instrumentation amplifier (in-amp) solves this elegantly.

The Three Op-Amp Architecture

The classic in-amp uses three op-amps. The first two form non-inverting amplifiers on each input, connected by a single gain-setting resistor (R_G). The third op-amp is a standard difference amplifier. This topology gives you:

• Extremely high input impedance on both inputs (won't load your sensor)
• Gain set by a single resistor: G = 1 + (2R/R_G), where R is the feedback resistor in the input stage
• CMRR that doesn't depend on matched external resistors — the difference stage uses laser-trimmed internal resistors

Real-World Example: Load Cell Amplification

A strain-gauge load cell outputs about 2 mV/V. With a 5V excitation, full-scale output is only 10 mV differential, riding on a 2.5V common-mode voltage. You need a gain of 200 to bring this to a useful 2V swing for your ADC. Using an INA128, the gain equation is G = 1 + (50kΩ/R_G). Solving for G = 200: R_G = 50,000 / 199 = 251Ω. A 249Ω standard resistor gives G = 201.8 — close enough.

Practical Considerations

• Reference pin: Most in-amps have a REF pin that sets the output zero point. Tie it to your ADC's mid-scale voltage or ground, depending on your signal chain.
• Input bias current paths: Each input needs a DC path to ground. If your source is AC-coupled, add high-value resistors (1–10MΩ) from each input to ground.
• Gain bandwidth tradeoff: Higher gain means lower bandwidth. The INA128 gives 200 kHz at G=100 but 1.3 MHz at G=1. Check your datasheet.
• RFI filtering: Add matched RC filters (1kΩ + 10nF) on both inputs to reject radio-frequency interference. Mismatched filters convert common-mode RF into differential error.

Rule of thumb: For gains above 10 with microvolt-level signals, always reach for a dedicated in-amp IC (INA128, AD620, INA333) rather than building a discrete three op-amp circuit. The internal resistor matching gives you 80–120 dB CMRR that you'll never achieve with 1% discrete resistors.