In the past, the term instrumentation amplifier (INA) was often misused, referring to the application rather than the device’s architecture. INAs are related to op amps, in that they’re based on the same architecture, but an INA is a specialized version of an op amp. INAs are specifically designed and used for their high differential gain to amplify microvolt-level sensor signals while simultaneously rejecting high-common-mode signals that can be several volts. This is important since some sensors produce a relatively small change in voltage or current, and this small change must be accurately captured.
Let’s consider a few applications that benefit from INAs. For example, a medical instrument that uses sensors to align laser stepper motors for vision-correction eye surgery. High accuracy is crucial, and other equipment in the operating room can’t be allowed to compromise the sensor signals and cause unexpected results.
Another example is a factory press. These machines apply thousands of pounds of force to bend metal into shapes. Using sensors, these machines are designed to stop if it detects a human hand. In this example, it’s critical that electrical noise from other factory equipment doesn’t cause interference that could lead to a malfunction.
In both cases above, the first step in the journey of the sensor signal is through an instrumentation amplifier. The tiny sensor signals must be accurately amplified in all environments. Instrumentation amplifiers are specially designed to do exactly that—to accurately amplify small signals resulting in high gain accuracy in an electrically noisy environment.
Other considerations further enhance the performance of an INA. Low power consumption is important to extend battery life. A low operating voltage allows the battery to be used over more of its depletion curve, extending battery life. A wide input voltage range allows compatibility with more sensors. And impedance matching at the input contributes to the seamless interface to sensors.
How INA Designs Have Evolved
With an endless number of consumer, medical, and industrial applications, designs have evolved over the years to take advantage of the performance benefits offered by INAs. Let’s look at the evolution of INA designs, from the original approaches to the instrumentation amplifiers available today. By reviewing these architectures and their associated strengths and limitations, this article shows the performance improvements seen in present-day instrumentation amplifiers along with real-life applications.