instrumentation amplifier is properly used to describe a category of true differential-input amplifiers that emphasize high common mode rejection (CMR) and accuracy. Although both instrumentation amplifiers and difference amplifiers use op amps as basic architectural “building blocks”, they are distinctly different from their op amp cousins.
Op amps are “single-ended” and they are usually intended to operate in a variety of applications with their feedback determining their functions. Instrumentation amplifiers and difference amplifiers are used primarily to provide differential gain and common mode rejection. Employing feedback from output to input is not intended.
In some instances this term has been widely misused and this has created confusion as to the correct definition of an instrumentation amplifier (IA). In the early days of monolithic operational amplifiers, one well-known vendor referred to their new precision op amp as an instrumentation amplifier. What they meant to say was that it was an “instrumentation-grade” op amp.
In addition, large laboratory bench-top amplifiers and even traveling- wave tube (microwave) amplifiers have been called instrumentation amplifiers. It is not surprising, then, that so much confusion exists about what an IA really is and what it does.
Most common IAs are one of three types: the simple “Difference Amplifier”, the “Two Op Amp Instrumentation Amplifier”, and the “Classical Three Op Amp Instrumentation Amplifier” architecture. As we shall see, these three architectures are interrelated but their performance differs in certain important aspects. For now, let’s just think of the IA as a “blackbox” differential amplifier.
Op amps are “single-ended” and they are usually intended to operate in a variety of applications with their feedback determining their functions. Instrumentation amplifiers and difference amplifiers are used primarily to provide differential gain and common mode rejection. Employing feedback from output to input is not intended.
In some instances this term has been widely misused and this has created confusion as to the correct definition of an instrumentation amplifier (IA). In the early days of monolithic operational amplifiers, one well-known vendor referred to their new precision op amp as an instrumentation amplifier. What they meant to say was that it was an “instrumentation-grade” op amp.
In addition, large laboratory bench-top amplifiers and even traveling- wave tube (microwave) amplifiers have been called instrumentation amplifiers. It is not surprising, then, that so much confusion exists about what an IA really is and what it does.
Most common IAs are one of three types: the simple “Difference Amplifier”, the “Two Op Amp Instrumentation Amplifier”, and the “Classical Three Op Amp Instrumentation Amplifier” architecture. As we shall see, these three architectures are interrelated but their performance differs in certain important aspects. For now, let’s just think of the IA as a “blackbox” differential amplifier.
The Wheat Stone Bridge Sensor
To better understand the instrumentation amplifier and why its high common- mode rejection is so important, let’s take a look at one of the most common transducers in use today the Wheatstone Bridge. While the usual way of depicting the bridge circuit is shown the diagram of Figure 2a, it can be redrawn (Figure 2b) to show that the bridge is nothing more than two voltage dividers driven by a single voltage (Vex) or current (Iex) excitation source.
Conventional & Redrawn Bridge Circuit
Let’s look at an example of a Wheatstone Bridge sensor with zero input stimulus (pressure, temperature, force, etc.):
Balanced Bridge Generates CMV But No Differential Output Voltage
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