Instrumentation Amplifiers

Clarus Goldsmith, June 2024

What is an instrumentation amp?

The internal schematic for the INA821D instrumentation amplifier

Design considerations

Setting the gain

Instrumentation amps take in signals and amplify them by a gain value set using a gain resistor put between two of the ports. The instrumentation amplifier manual will include an equation for calculating the resistor value for a chosen gain. Then you just wire the resistor between the two ports.

Input orientation matters

Instrumentation amps have two inputs, typically marked as IN+ and IN-. Which of your two signals goes into which pin matters, as it sets the polarity of the signal. Using the strain gauge as an example, connecting the inputs one way will result in tension producing positive strain, while flipping them will result in compression producing positive strain. It's recommended you prototype your circuit that uses an instrumentation amp on a breadboard first to ensure your inputs are oriented correctly.