instrumentation amplifier

(electronics) An amplifier that accepts a voltage signal as an input and produces a linearly scaled version of this signal at the output; it is a closed-loop fixed-gain amplifier, usually differential, and has high input impedance, low drift, and high common-mode rejection over a wide range of frequencies.

A special-purpose linear amplifier, used for the accurate amplification of the difference between two (often small) voltages, often in the presence of much larger common-mode voltages, and having a pair of differential (usually high-impedance) input terminals, connected to sources V_in1 and V_in2; a well-defined differential-mode gain A_DM; and a voltage output V_out, satisfying the relationship given in the equation below. It differs from an operational amplifier (op-amp), which ideally has infinite open-loop gain and must be used in conjunction with external elements to define the closed-loop transfer function. At one time built in discrete or hybrid form using operational amplifier and resistor networks, instrumentation amplifiers are readily available as inexpensive monolithic integrated circuits. Typical commercial amplifiers provide present gains of 1, 10, 100, and 1000. In some cases, the gain may be set to a special value by one or more external resistors. The frequency response invariably is flat, extending from 0 (dc) to an upper frequency of about 1 kHz to 1 MHz. See also Integrated circuits; Operational amplifier; Resistor.

Instrumentation amplifiers are used to interface low-level devices, such as strain gages, pressure transducers, and Hall-effect magnetic sensors, into a subsequent high-level process, such as analog-to-digital conversion. See also Amplifier; Differential amplifier; Hall effect; Pressure transducer; Strain gage.

You may be looking for instrument amplifier, as used for amplifying musical instruments. This page is about the electronic device.

Typical instrumentation amplifier schematic

An instrumentation amplifier is a type of differential amplifier that has been specifically designed to have characteristics suitable for use in measurement and test equipment. These 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. They are used where great accuracy and stability of the circuit both short- and long-term are required.

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

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. See - 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

• 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)

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