Amplifier of instrumentation

See also: Amplifying

An amplifying of instrumentation is an electronic device intended for the treatment of weak electrical signals. The typical application is the signal processing resulting from Capteur S of measurement. Its operation is based on the principle of the differential amplification.

The amplifier of instrumentation is generally produced from one or of several amplifier operational (AOP), in such a way that it improves their intrinsic characteristics: offset, drift, noise of amplification, profit in open loop, Rate of rejection of the common mode, impedance of entry.

Principle

The measurement of signals coming from sensors presents an main issue: these signals are generally of very low amplitude, about a few millivolts. This report involves two consequences:

1. These signals are strongly prone to the disturbances, because the Rapport signal on noise is very weak.
2. It is necessary to amplify these signals with large a profit, while avoiding amplifying the noise.

The recourse to amplifying of instrumentation is done for these two reasons: they allow an amplification with an important profit by rejecting most of the disturbances.

One can classify the disturbances according to three categories

them disturbances of common mode

them disturbances of differential mode

it noise of amplification

Disturbances of common mode

For a differential signal, the common mode is written as being the average of the two signals:

$V\_\; \backslash \; mathrm\; \{mc\}\; =\; \backslash \; frac\; \{(V\_\; \backslash \; mathrm\; \{e+\}\; +\; V\_\; \backslash \; mathrm\; \{E\})\}\{2\}$

A disturbance of common mode will be thus a disturbance on the two lines simultaneously. This disturbance is present in the lines of transmission by electric coupling, or in the feeders (fluctuations of the electric ground, or the supply voltage of the sensor). One defines the Taux of rejection of the common mode as being the capacity of an amplifier to propagate and amplify this mode. In an amplifier of instrumentation, this rate will be minimized.

Disturbances of differential mode

the differential mode is written as being the difference of the two signals:

$V\_\; \backslash \; mathrm\; \{D\}\; =\; V\_\; \backslash \; mathrm\; \{e+\}\; -\; V\_\; \backslash \; mathrm\; \{E\}$

It corresponds to the superposition of the useful signal and the disturbance of differential mode: a disturbance of this type will be thus difficult to remove. The realization of the system can make it possible to decrease it to a significant degree (shielding, insulation of the masses, establishment of the printed circuit with a copper screen). One can also carry out a filtering of the noise during or after amplification.

Noise of amplification

The important profits during amplification increase the influence of the harmful effects generated by the amplifier itself. The use of a symmetrical assembly such as that of the amplifier of instrumentation makes it possible to cancel part of these harmful effects.

Operation

The amplifier of instrumentation is produced starting from amplifier operational (AOP). The AOP is an amplifier with a very important differential profit. The ideal transfer transfer function is written $G=\; \backslash \; frac\; \{V\_s\}\; \{(e\_+\; -\; e\_-)\}$.
Actually the AOP presents defects: currents of offset and tension of offset at the entry, TRMC, output impedance, variation in frequency of the profit.

Here a table giving the characteristics of a AOP:

The goal of the amplifier of instrumentation is to reduce these defects.

Assembly with 1 AOP

The profit of this assembly is

$G\_d\; =\; \{R\_2\; \backslash \; over\; R\_1\}$

This assembly is the least complex to realize, but it presents certain disadvantages: the TRMC of the assembly corresponds to the TRMC of the AOP, the impedance of entry is equal to R1 + R2, therefore relatively weak.

The profit of the assembly can be regulated by a resistance placed between the common points of both R2 divided beforehand into two chacunes.

Assembly with 2 AOP

The profit of this assembly is

$G\_d\; =\; \backslash \; left\; (1+\; \{R\_2\; \backslash \; over\; R\_1\}\; \backslash \; right)$ (without R3)

$G\_d\; =\; \backslash \; left\; (1+\; \{R\_2\; \backslash \; over\; R\_1\}\; +2*\; \{R\_2\; \backslash \; over\; R\_3\}\; \backslash \; right)$ (with R3, which makes it possible to control the profit with only one variable resistor)

Assembly with 3 AOP

This assembly is most used. Its profit is

$\backslash \; frac\; \{V\_\; \backslash \; mathrm\; \{out\}\}\; \{V\_2\; -\; V\_1\}\; =\; \backslash \; left\; (1\; +\; \{2\; R\; \_1\; \backslash \; over\; R\_\; \backslash \; mathrm\; \{profit\}\}\; \backslash \; right)\; \{R\_3\; \backslash \; over\; R\_2\}$

The ideal profit in common mode (TRMC) of the amplifier of instrumentation is zero. In the circuit opposite, the value of this profit is caused by the tolerances of the values of resistances which make the diagram asymmetrical, and by the profit of mode common not no one of the 2 AOP S used. The realization of resistances appairées in value is the principal constraint of manufacturing of the circuits of instrumentation

Achievements

The amplifiers of instrumentation can be realized with individual AOP and precision resistors, but they are also available in integrated circuits in many manufacturers (Texas Instruments, Analog Devices, Linear Technology, etc). Generally these circuits offer very good TRMC, because of very precise manufacture of integrated resistances (cutting with the laser).

Some very current circuits of AOP to a circuit

• AD620
• INA111

References

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