Instrumentation amplifier

Instrumentation amplifier

An instrumentation (or instrumentational) 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. Additional characteristics include very low DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection ratio, and very high input impedances. Instrumentation amplifiers are used where great accuracy and stability of the circuit both short- and long-term are required.

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. These are arranged so that there is one op-amp to buffer each input (+,−), and one to produce the desired output with adequate impedance matching for the function. [R.F. Coughlin, F.F. Driscoll "Operational Amplifiers and Linear Integrated Circuits" (2nd Ed.1982. ISBN 0-13-637785-8) p.161.] [ Moore, Davis, Coplan "Building Scientific Aparatus" (2nd Ed. 1989 ISBN 0-201-13189-7)p.407. ]

The most commonly used instrumentation amplifier circuit is shown in the figure. The gain of the circuit is

:frac{V_mathrm{out{V_2 - V_1} = left (1 + {2 R_1 over R_mathrm{gain ight ) {R_3 over R_2}

The ideal common-mode gain of an instrumentation amplifier is zero. In the circuit shown, common-mode gain is caused by mismatches in the values of the equally-numbered resistors and by the non-zero common mode gains of the two input op-amps. Obtaining very closely matched resistors is a significant difficulty in fabricating these circuits, as is optimizing the common mode performance of the input op-amps. [Smither, Pugh and Woolard: ‘CMRR Analysis of the 3-op-amp instrumentation amplifier', Electronics letters, 2nd February 1989.]

Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit form from several manufacturers (including Texas Instruments, Analog Devices, Linear Technology and Maxim Integrated Products). An IC instrumentation amplifier typically contains closely matched laser-trimmed resistors, and therefore offers excellent common-mode rejection.

See also

* Differential amplifier
* Operational amplifier
* Operational amplifier applications


External links

* [ Opamp Instrumentation Amplifier]
* [ The instrumentation amplifier]
* [ Lessons In Electric Circuits — Volume III — The instrumentation amplifier]
* [ A Practical Review of Common Mode and Instrumentation Amplifiers] (PDF)
* [ The Instrumentation Amplifier]
* [ Instrumentation Amplifiers Tutorial]
* [ A Designer's Guide to Instrumentation Amplifiers (3rd Edition)]
* [ Three is a Crowd for Instrumentation Amplifiers]
* [ Shows the instrumentation amplifier compared to the differential amplifier, with images.]

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