Due to the specialized nature of instrumentation amplifiers, there are additional specifications that are not typically found in standard operational-amplifier datasheets, including gain error and a non-linearity specification. Gain error is typically specified as a maximum percentage, and represents the maximum deviation from the ideal gain equation for that particular amplifier. Variations in resistor values and temperature gradients among the resistor networks can all contribute to gain error. The non-linearity specification also describes the gain characteristic of the amplifier. This specification defines the maximum variation from an ideal straight-line transfer function, when comparing output versus input. For example, if an instrumentation amplifier is configured for a gain of ten, then a DC input of 100 mV should produce 1V at the output. If the input is taken up to 500 mV, then the output should be at 5V. These two points represent the straight-line input to output transfer function for the amplifier. Any deviation from this straight line is highlighted by the non-linearity specification.
Application Example: Wheatstone Bridge
As noted previously, instrumentation amplifiers are designed to provide differential gain and good rejection of common-mode signals. These characteristics make INAs very popular for sensors (such as strain gauges) arranged in the classic Wheatstone-bridge configuration. A Wheatstone bridge for a strain-gauge application consists of four elements arranged in a diamond pattern, with each side consisting of a resistive element (either a strain gauge or a fixed resistor). An excitation voltage is then applied to the bridge, and the output voltage across the middle of the bridge is measured. A quarter bridge consists of only one variable-resistor element—the strain gauge. A half bridge has two variable-resistor elements, and a full bridge has all four elements as variable-resistor elements—in this case, strain gauges. The advantage of having more strain gauges is an increase in sensitivity. All else being equal, a half-bridge configuration will have twice the sensitivity as a quarter bridge, while the full bridge will have four times the sensitivity as the quarter bridge.
In this example, the Wheatstone bridge is excited by a DC source. Assuming VDD is set to 5V, this creates a DC common mode of approximately 2.5V at the center taps of the bridge. A force applied to the strain gauges will cause a change in their respective resistances, creating a small voltage differential across the center taps. This voltage change is very small relative to the common-mode voltage—typically on the order of 10 millivolts—hence the need for amplification of this small differential voltage. An instrumentation amplifier is ideally suited for this task, not only providing the needed amplification, but also rejecting the relatively high common-mode signal (and any additional noise that is common to both input signals). Keep in mind that an operational amplifier configured as a simple gain stage will still pass the common-mode signal (at unity gain) to the output, reducing the dynamic range of the output signal.
In the world of system design, the term “instrumentation” can take several meanings. Historically, the term has been used to describe the application, usually a physical phenomenon that is being measured or recorded. Hence, any operational amplifiers that were designed for use in such applications became known as “instrumentation amplifiers”. Adding to the confusion is the fact that actual instrumentation amplifiers can be constructed using operational amplifiers.
In reality, operational amplifiers and instrumentation amplifiers are very different devices, designed to do different functions. Instrumentation amplifiers can be thought of as a specialized amplifier, used specifically for its differential-gain and common-mode-rejection capabilities. As we have seen throughout this article, circuits implementing traditional operational amplifiers can be created to perform these same functions. However, in most cases, a monolithic instrumentation amplifier will provide a substantially higher level of performance and reliability.