Instrumentation Amplifier

Instrumentation Amplifier

An instrument amplifier is a type of differential amplifier equipped with input buffers eliminating the requirement for an input impedance matching, thus creating the amplifier highly suitable for use in test equipment and measurement. Its additional features include low drive, very low DC offset, low noise level, very high open loop gain, very high common mode rejection ratio and very high input impedances. Instrument amplifiers are used where high stability and accuracy of the circuit, both short and long term, are needed.

Although the instrument amplifier is typically equal to an ordinary op-amp, the electronic instrument amplifier is always in, consisting of 3 op amps. These are mounted so that there is an op amp for buffer each input (+, -) and one to generate the desired output with sufficient impedance matching for the function. We have a common expression of the total voltage increase in the instrumentation amplifier is Av = (1 + 2R / Rgain). We can change the differential gain of the instrumentation amplifier only by changing the value of a resistor: Rgain.

The perfect common mode enhancement of an instrument amplifier is zero. Common mode gain is obtained by improper matches in the values ​​of the uniformly numbered resistors and of the error matching in the normal mode gains of the two inputs amps. Finding very carefully matched resistors is an important difficulty in manufacturing these circuits, as well as optimizing the built-in functionalities of the usual amplitudes. An instrument amplifier can also be built with 2 op amps to reduce cost and raise CMRR, but the gain must be higher than 2 (+6 dB).

Instrument amplifiers can be built with op-amp and precision resistors, but can also be obtained in integrated circuits from many manufacturers (including national semiconductors, linear technology, analogue devices and integrated products). An IC instrument amplifier usually contains intimately matched laser trimmed resistors and therefore proposes extremely common counter-repression.

Instrument amplifiers can also be planned using indirect current feedback architecture which increases the operating change of these amplifiers to the negative power supply rail and, in many cases, the positive power supply rail. This can be predominantly useful in single supply systems, where the negative effect bar is only the ground (GND).

Feedback-free instrumentation amplifier is the high input impedance differential amplifier designed without the external feedback network. This enables reduction of the number of amplifiers, reduced noise (no thermal noise is absorbed by feedback resistors) and increased bandwidth (no frequency compensation required).

The presentation of a monolithic instrumentation amplifier utilized as an interface for a four-electrode measurement with bioimpedance is observed with a commercially available impedance meter supported on an automatic balancing bridge. Errors due to the characteristics of the impedance meters input steps when utilized without a front end were many orders of magnitude higher than the measured quantity. The review was performed on an electrical circuit replication of the skin and the electrodes over a frequency range of 20 Hz to 1 MHz

With balanced electrode impedance, the accuracy achieved for frequencies up to 100 kHz may be less than 0.1 degrees for impedance phase and 0.2 percent for impedance size within the specified ground accuracy of the LCR meter used for the measurements. At frequencies above 100 kHz the error increases and is greater than the vital accuracy of the LCR meter. This study specifies that the use of an instrument amplifier as a front end of the accurate LCR meter can significantly improve the measurement accuracy of the measurement of the electrodes bioimpedance at low frequencies.

The instrument amplifier must eliminate large interference signal which is usually the two wires (a common mode signal) and amplifies the small difference (differential) signal. The instrument amplifier is useful for changing the amplifier circuit amplification without having to change more than one resistor value, which is necessary with the previous design of the differential amplifier. The configuration of this instrument amplifier is excellent and it can be used by a large number of people who want to eliminate the need for input impedance matching and to make the amplifier particularly suitable for use in measurement and testing equipment.



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