What Is an Instrumentation Amplifier?
An instrumentation amplifier (in-amp) is a special differential amplifier that amplifies the small difference signal between two inputs with high accuracy and high input impedance.
Why a Special Circuit?
The signal produced by sensors (e.g. a load cell, a thermocouple) is very small and usually buried in noise. An in-amp amplifies this weak difference without distorting it and rejects the common noise.
High CMRR
Its most important property is a high CMRR (common-mode rejection ratio): it rejects the noise that appears on both inputs together, while amplifying only the difference between them. This is critical for long-cable sensors.
| Property | Value |
|---|---|
| Input impedance | Very high |
| CMRR | Very high |
| Gain setting | With a single resistor |
Difference From an Op-Amp Circuit
A simple op-amp difference circuit suffers from low input impedance and resistor-matching problems. An in-amp (usually with 3 op-amps) solves these; the gain is set with a single external resistor.
Where Is It Used?
Load cells/scales, biomedical measurement (ECG), bridge sensors, temperature and pressure measurement, industrial data acquisition.
Setting the Gain With a Single Resistor
The practical advantage of an in-amp is that in most models the entire gain is set by a single external resistor (Rg). This removes the matching problem of multi-resistor op-amp circuits and makes the gain precise and repeatable; the desired gain is easily chosen with the datasheet's formula.
Common Mistakes
- Reference terminal: if the output reference isn't connected correctly, the measurement drifts.
- Gain resistor: a wrong value gives an unexpected gain.
- Input protection: sensitive inputs must be protected against overvoltage.
Frequently Asked Questions
What's the difference between an in-amp and an op-amp?+
Why does CMRR matter?+
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