University of Engineering & Technology

REPORT: INSTRUMENTATION AMPLIFIERZUNAIB ALIBatch-06

Objectives:

To provide detail about instrumentation amplifier. Features of instrumentation amplifier. Study and implementation of common used circuit of instrumentation amplifier. Applications of Instrumentation Amplifier. Simulation of circuit in PROTEUS. Conclusion

Instrumentation Amplifiers

An instrumentation (or instrumentational) amplifier is a type of differential amplifier that has been outfitted with input buffer amplifiers, 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 operational amplifier (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. The most commonly used instrumentation amplifier circuit is shown in the figure.

Figure 1 Instrumentation Amplifier Common Configuration

The gain of the circuit is

The rightmost amplifier, along with the resistors labelled   and   is just the standard

differential amplifier circuit, with gain =   and differential input resistance = 2· . The

two amplifiers on the left are the buffers. With  removed (open circuited), they are simple

unity gain buffers; the circuit will work in that state, with gain simply equal to    and high input impedance because of the buffers. The buffer gain could be increased by putting resistors between the buffer inverting inputs and ground to shunt away some of the negative feedback;

however, the single resistor   between the two inverting inputs is a much more elegant method: it increases the differential-mode gain of the buffer pair while leaving the common-mode gain equal to 1. This increases the common-mode rejection ratio (CMRR) of the circuit and also enables the buffers to handle much larger common-mode signals without clipping than would be the case if they were separate and had the same gain. Another benefit of the method is that it boosts the gain using a single resistor rather than a pair, thus avoiding a resistor-matching

problem (although the two  s need to be matched), and very conveniently allowing the gain of the circuit to be changed by changing the value of a single resistor. A set of switch-selectable

resistors or even a potentiometer can be used for  , providing easy changes to the gain of the circuit, without the complexity of having to switch matched pairs of resistors.

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 mis-match in 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.

An instrumentation amp can also be built with 2 op-amps to save on cost and increase CMRR, but the gain must be higher than 2 (+6 dB). 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, National Semiconductor, Analog Devices, Linear Technology and Maxim Integrated Products).

Instrumentation Amplifiers can also be designed using "Indirect Current-feedback Architecture", which extend the operating range of these amplifiers to the negative power supply rail, and in some cases the positive power supply rail. This can be particularly useful in single-supply systems, where the negative power rail is simply the circuit ground (GND). Examples of parts utilizing this architecture are MAX4208/MAX4209 and AD8129/AD8130.

Feedback-free instrumentation amplifier is the high input impedance differential amplifier designed without the external feedback network. This allows reduction in the number of amplifiers (one instead of three), reduced noise (no thermal noise is brought on by the feedback resistors) and increased bandwidth (no frequency compensation is needed). Chopper stabilized (or zero drift) instrumentation amplifiers such as the LTC2053 use a switching input front end to eliminate DC offset errors and drift.

Features Of Instrumentation Amplifier.

It is very common for sensors to require some degree of amplification. This is the commonest form of signal conditioning, to convert a low-level voltage or current into a higher level in a standardized range such as 0 to 5 volts. For experimental purposes and for short term needs this can usually be done through an op-amp (instrumentation amp). Industrial sensors will often have the instrumentation amplifiers built into the sensor module. They can also be purchased as separate modules and added to an existing system.

Instrumentation amplifiers are op-amps which are DC coupled, configured for differential high-impedance input, high common-mode-rejection-ratio (CMRR) and have a single-ended output. They often also include offset trimming and single resistor gain adjustment. Each of these features will be explained.

DC coupled: Most industrial instrumentation signals vary relatively slowly, typically changing over minutes up to varying a few hundred times a second. These low frequencies require a DC coupled amplifier. This means that there will be no capacitors in series with the input since capacitors only pass AC currents.

Differential Input: The connection(s) between the sensor and the amplifier act as antennas for any noise signals in the environment. There are usually many noise sources in industrial applications. The noise can easily be a signal of significant size compared to the sensor signal. This noise signal is induced into both the inverting and non-inverting connections of the differential input and is therefore subtracted from itself by the op-amp. This can be simply seen in the gain calculation. The gain for a differential op-amp is

R 2/ R 1(V 2−V 1) .

If we add the same noise, N, to both inputs this becomes R 2/ R 1((V 2+N )−(V 1+N ))

¿ R 2/ R 1((V 2−V 1)+(N−N ))

¿ R 2/ R 1 (V 2−V 1 )

i.e. the noise cancels out.

CMRR: (Common Mode Rejection Ratio):

Sometimes, for example in biomedical applications, the common signal (noise) applied to both inputs will be larger than the signal being measured. This common mode signal can overwhelm the amplifier. The ability of the amplifier to ignore a large common signal and amplify the small difference signal is called the common-mode rejection ratio. For example if there is a 2 volt signal on both input leads and a 50 mV difference between them then the common mode ratio is 2/0.050=40∨20 log(40)=32dB. Therefore the amplifier needs to have a CMRR>32 dB to work with these conditions.

High Impedance input:

Voltage signals such as those coming from piezo-electric sensors or thermocouples, contain very little energy. The pressure or temperature information is contained in the voltage fluctuation but we cannot treat this voltage as if it were a power supply. The sensor cannot provide energy to the amplifier so the amplifier input must have a very high input impedance to prevent it from overloading the sensor. Energy is calculated from power = V 2/ R. We can keep the power drawn from the sensor minimal by making R very large. R in this case is the input impedance of the amplifier.

Single-ended output:

A single-ended output always has the same polarity i.e. will never go negative. This is desirable as it is relatively easy to match with the computer or display that typically follows the amplifier. These are typically designed for 0−5 volt input, so the output of the instrumentation amp is designed to match..

Offset trimming:

A good instrumentation amplifier may include summing amplifier capabilities to offset the voltage signal. A variable resistor can be attached to the amplifier and used to zero out any offset signal present in the system. This allows us to calibrate the sensor for zero output when there is zero input. Alternatively if we just wish to operate the sensor over a small part of its range, for example a human-body temperature sensor that operates from 80 ° ¿120 °, we can use the offset to ensure that it is calibrated in this range. This allows us to partially correct for inherent non-linearities in the sensor.

Single Resistor Gain Adjustment:

It is possible to design a differential amplifier such that all the matched resistors are on the integrated circuit and a single external resistor is used to adjust the gain over a very wide range. This gain adjustment is not usually linear but a formula is provided by the chip manufacturer to allow you to calculate any particular gain needed.By having external variable resistors to adjust gain and offset the instrumentation amplifier can be used to calibrate the sensor, providing the sensor has a linear response in the range of interest.

Instrumentation Amplifier working principles

A differential amplifier has two types of inputs: common mode input CM and Differential mode input DM. What one needs is to amplify Dm and reject CM. Thus CM gain has to be low and DM gain has to be high. In instrumentation amplifier, two buffers are used to buffer the signal. Here the CM gain is 1. However, DM gain can be quite high, typically 10 or even 100. In next stage, the two signals get fed to a differential amplifier, whose CM gain is pretty low and can be made zero by an external pot in some cases. The DM gain of this stage can also be 1 or 5 or 10. Thus even if the CM signal has a nonzero source resistance, the CM output of first stage is true CM signal, and hence they get rejected very much in second stage.

Applications of Instrumentation Amplifier

Instrumentation 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. A variety of low-side and high-side current-sensing applications. Data acquisition from low output transducers Medical instrumentation Current/voltage monitoring Audio applications involving weak audio signals or noisy environments High-speed signal conditioning for video data acquisition and imaging High frequency signal amplification in cable rf systems

Practical Circuit:

Proteus Simulations:

In this simulation we take R4= R5 .To view output and input of this instrumentation amplifier we use oscilloscope (located at left bar of Proteus Isis)Set V1 (sine ) at 4mV and 10 hz , and run simulate , the simulation result depicted below :

Conclusion

In the midst of an unprecedented era of high-performance electronic devices, today's consumers demand not only better performance, but also more intelligent power-management schemes to enable longer battery life and energy efficiency. A transition from dual-supply analog designs to single-supply architectures is already underway that is changing the way electronics are designed and used. New, innovative architectures, such as the indirect current-feedback architecture are making the dreams of yesterday into realities of today.

References

Horowitz, Paul and Hill, Winfield,, The Art of Electronics. . New York: Cambridge University Press, 1990.

Microelectronic Circuits by Sedra Smith,5th edition

http://www.maximintegrated.com/app-notes/index.mvp/id/4034

http://ourlibro.blogspot.com/2011/07/three-op-amp-instrumentation-amplifier.html