Understanding instumentation amplifiers

An instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment. The ideal common-mode gain of an instrumentation amplifier is zero, which means that it is a very good at rejecting common mode voltages (typically noise from various sources including ground loops).

Although the instrumentation amplifier is usually shown schematically identical to a standard op-amp, the electronic instrumentation amp is almost always internally composed of 3 op-amps. The three-op-amp instrumentation amplifier is seemingly a simple configuration in that it uses a basic, decades-old operational amplifier to gain a differential input signal. Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit form from several manufacturers.

EDN Magazine article Understanding CMR and instrumentation amplifiers tells that from the CMR (common-mode rejection) perspective, instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. his situation is not so mysterious when you think about the inside of this device. And the inside operation of the instrumentation amplifier is well presented on this article.



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    Three is a Crowd for Instrumentation Amplifiers

    nstrumentation amplifiers amplify small differential voltages in the presence of large common-mode voltages, while offering a high input impedance. This characteristic has made them attractive to a variety of applications, such as strain-gauge bridge interfaces for pressure and temperature sensing, thermocouple temperature sensing, and a variety of low-side and high-side current-sensing applications.

    The classic three-op-amp instrumentation amplifier offers excellent common-mode rejection and accurate differential gain programmable by a single resistor. The architecture is based on a two-stage configuration: the first stage provides unity common-mode gain and all (or most) of the differential gain, while the second stage provides unity (or small) differential-mode gain and all of the common-mode rejection

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    Amplifier cancels common-mode voltage

    Since the dawn of time—or at least since the dawn of precision electronics—a major headache for analog designers has been CMV (common-mode-voltage)-induced errors, also known as the dreaded ground loop. Although almost mystical is the fear it strikes in the hearts of engineers, there’s nothing particularly mysterious about CMV. CMV errors occur for a simple reason: The common voltage references—that is, ground—of circuitry in different places, such as sensors in one chassis and an ADC in another, are apt to differ. Therefore, when you route signals between remotely located circuits, the CMV differential appears as additive noise and offset, corrupting the desired signals.

    Many approaches exist for eliminating CMV errors. These methods include the brute-force approach of using massive amounts of copper in ground interconnections, fully differential instrumentation-amplifier signal conditioners, and galvanic isolators. Each has its place, depending on such factors as the severity of the CMV problem and the number of signal channels needing CMV remediation. One of the most popular and effective CMV remedies is differential amplification,

    The downside of this method is that it requires a dedicated amplifier for every signal channel. The circuit in Figure 1 is a variation on that same differential-amplifier idea, but it combines two shared CMV amplifiers with simple passive-resistor pairs among eight multiplexed channels to provide CMV cancellation for a large number of analog channels at minimum component count.

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