Instrumentation Amplifier

To Study Instrumentation Amplifier Kamal Preet Singh Lovely Professional University B.Tech-M.Tech ECE R170A24 3060071012

Abstract: 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. 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. 1.1 INTRODUCTION: One of the most useful analog subsystems is the true instrumentationamplifier. It can faithfullyamplify low level signals in the presence of high common mode noise. This aspect of its performance makes it especially useful as the input amplifierof a signal processing system. Other features of theinstrumentation amplifier are high input impedance, low input current, and good linearity.

It has never been easy to design a high performance instrumentation amplifier; however, the availability of high performance IC's considerably simplifies the problem. IC op amps are available today that can give very low drifts as well as low bias currents; however, most of the circuits have some drawbacks.The most commonly used instrumentation amplifier designs utilize either 2 or 3 op amps and several precision resistors. These are capable of excellent performance; however, for high performance they require very precisely matched resistors. The common mode rejection of these designs depends on resistor matching and overall gain. Since op amps arenow available with exceedingly high CMRR, this is no longer a problem. The CMRR of the instrumentation amplifier is approximately equal to half resistor mismatch plus the gain. For a 1% resistor mismatch the CMRR is limited to 46 dB plus the gainÐreferred to the input. Each of the three basic Instrumentation Amplifier architectures that have been already discussed have been implemented in standard integrated circuit packages. To achieve a hig CMRR,extensiveresistor trimming is required with lasers or other suitable techniques. While each of these devices provide adequate specifications for a precision Instrumentation Amplifier, each device has its own compromise based on operating voltage range, supply current, common mode operating range, input impedance, etc. These instrumentation amplifiers use one external resistor to set the gain; while this may seem to be an advantage, there are considerations which make the single resistor configuration undesirable from a design viewpoint. The temperature coefficient (TC) of the external resistor will be a direct gain drift. Also, an external filter can not be applied to the feedback network because it is internal to the device. 1.2 Operational Amplifier: An operational amplifier, which is often called an op-amp, is a DC-coupled high-gain electronic voltage amplifier with differential inputs[1] and, usually, a single output. Typically the output of

the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation. High input impedance at the input terminals and low output impedance are important typical characteristics.Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Modern designs are electronically more rugged than earlier implementations and some can sustain direct shortcircuits on their outputs without damage. The op-amp is one type of differential amplifier. Other types of differential amplifier include the fully differential amplifier (similar to the op-amp, but with 2 outputs), the instrumentation amplifier (usually built from 3 op-amps), the isolation amplifier (similar to the instrumentation amplifier, but which works fine with common-mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from 1 or more op-amps and a resistive feedback network). Shown on the right is an example of an ideal operational amplifier. The main part in an amplifier is the dependent voltage source that increases in relation to the voltage drop across Rin, thus amplifying the voltage difference between V + and V − . Many uses have been found for operational amplifiers and an ideal op-amp seeks to characterize the physical phenomena that make op-amps useful. V + and V − are not connected to the circuit within the op-amp because they power the dependent voltage source's circuit (not shown). These are notable, however, because they determine the maximum voltage the dependent voltage source can output. For any input voltages the ideal op-amp has       

infinite open-loop gain, infinite bandwidth, infinite input impedances (resulting in zero input currents), zero offset voltage, infinite slew rate, zero output impedance, and zero noise.

The operational amplifier is an extremely efficient and versatile device. Its applications span the broad electronic industry filling requirements for signal conditioning, special transfer functions, analog instrumentation, analog computation, and special systems design. The analog assets of simplicity and precision characterize circuits utilizing operational amplifiers. Originally, the term, “Operational Amplifier,” was used in the computing field to describe amplifiers that performed various mathematical operations. It was found that the application of negative feedback around a high gain DC amplifier would produce a circuit with a precise gain characteristic that depended only on the feedback used. By the proper selection of feedback components, operational amplifier circuits could be used to add, subtract, average, integrate, and differentiate.As practical operational amplifier techniques became more widely known, it was apparent thatthese feedback techniques could be useful in many control and instrumentation applications.Today, the general use of operational amplifiers has been extended to include such applicationsas DC Amplifiers, AC Amplifiers, Comparators, Servo Valve Drivers, Deflection Yoke Drivers,Low Distortion Oscillators, AC to DC Converters, Multivibrators, and a host of others.What the operational amplifier can do is limited only by the imagination and ingenuity of the user.With a good working knowledge of their characteristics, the user

will be able to exploit more fully the useful properties of operational amplifiers. 1.3 The Feedback Technique The precision and flexibility of the operational amplifier is a direct result of the use of negative feedback. Generally speaking, amplifiers employing feedback will have superior operating characteristics at a sacrifice of gain.

the“openloop” amplifier. Thus, amplification almost any degree of precision can be achieved with ease1.4 Notation and Terminology Texas Instruments employs the industry standard operational amplifier symbols shown in figure. Power pins are often omitted from the schematic symbol when the power supply voltages are explicit elsewhere in the schematic. Some op amp symbols also include offset nulling pins, enable / disable pins, output voltage threshold inputs, and other specialized functions.

Fig. Feedback circuit. With enough feedback, the closed loop amplifier characteristics become a function of thefeedback elements. In the typical feedback circuit, figure 1, the feedback elements are two resistors. The precision of the “closed loop” gain is set by the ratio of the two resistors and is practically independent of Input Terminals In figures 2b and 2c, the “-“ pin is the “inverting input” or “summing point,” meaning a positive voltage produces a negative voltage at the output on symbol (b), and the top (non-inverting) output on symbol (c). When only one input or output terminal exists, its voltage is measured with respect to ground. This is indicated by the term, “single ended.” It is a popular ambiguity not to explain if a circuit, earth, or chassis ground is meant by this, so the use of a common line is preferred with the ground symbol used to indicate which line is the common.When there is an inverting input, such as in figures 2b and 2c, the voltage at the inverting input may be measured with respect to the non-inverting input. In use, such an amplifier responds to the difference between the voltages at the inverting and noninverting inputs, i.e., a “differential input.In many circuits, the non-inverting input is connected to ground. Due to the high gain of operational amplifiers, only a very small input voltage then appears at the output and the output is virtually at

ground potential. For purposes of circuit analysis, it can be assumed to be ground -a “virtual ground.” Output Terminals The relation between the inverting and non-inverting inputs and the output was stated above. For the symbol of (c) the second output voltage is approximately equal and opposite in polarity to the other output voltage, each measured with respect to ground. When the two outputs are used as the output terminals without ground reference, they are known as “differential outputs”.

Fig OP AM IC pacakages Power Connections: Power is supplied to each of these units at connections as shown in figure 4. Such a connection is implied in all operational amplifier circuits. The dual supply presents the same absolute value of voltage to ground from either side, while the center connection ultimately defines the common line and ground potential. The exceptions to this are AC amplifier circuits that may use a single power supply. This is accomplished by creating a floating AC ground with DC blocking capacitors. In such circuits, a source of “half-supply” creates a “virtual ground” exactly half way between the positive supply and ground potentials.

Simulinking:

Fig power connections. Defining the Ideal Operational Amplifier Gain: The primary function of an amplifier is to amplify, so the more gain the better. It can always be reduced with external circuitry, so we assume gain to be infinite. Input Impedance: Input impedance is assumed to be infinite. This is so the driving source won’t be affected by power being drawn by the ideal operational amplifier. Output Impedance: The output impedance of the ideal operational amplifier is assumed to be zero. It then can supply as much current as necessary to the load being driven. Response Time: The output must occur at the same time as the inverting input so the response time is assumed to be zero. Phase shift will be 180
. Frequency response will be flat and bandwidth infinite because AC will be simply a rapidly varying DC level to the ideal amplifier. Offset: The amplifier output will be zero when a zero signal appears between the inverting and non-inverting inputs.

Simunlink Using Tina software. 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. The most commonly used instrumentation amplifier circuit is shown in the figure. The gain of the circuit isThe ideal common-mode gain of an instrumentation amplifier is zero.

packages. To achieve a hig CMRR,extensiveresistor trimming is required with lasers or other suitable techniques An IC instrumentation amplifier typically contains closely matched laser-trimmed resistors, and therefore offers excellent common-mode rejection.

Input Signal Amplitude=1v F=50 Hz. In the circuit shown, common-mode gain is caused by mismatches in the values of the equallynumbered 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 opamps.[3]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. The CMRR of the instrumentation amplifier is approximately equal to half resistor mismatch plus the gain. For a 1% resistor mismatch the CMRR is limited to 46 dB plus the gainÐreferred to the input. Each of the three basic Instrumentation Amplifier architectures that have been already discussed have been implemented in standard integrated circuit

Output Of Instrumentation amp. Our input voltage differential is still zero volts, yet the output voltage changes significantly as the common-mode voltage is changed. This is indicative of a common-mode gain, something we're trying to avoid. More than that, its a common-mode gain of our own making, having nothing to do with imperfections in the op-amps themselves. With a much-tempered differential gain (actually equal to 3 in this particular circuit) and no negative feedback outside the circuit, this common-mode gain will go unchecked in an instrument signal application. There is only one way to correct this common-mode gain, and that is to balance all the resistor values. When designing an instrumentation amplifier from discrete components (rather than purchasing one in an integrated package), it is wise to provide some means of making fine adjustments to at least one of the four resistors connected to the final op-amp to be able to "trim away" any such common-

mode gain. Providing the means to "trim" the resistor network has additional benefits as well. Suppose that all resistor values are exactly as they should be, but a common-mode gain exists due to an imperfection in one of the op-amps. With the adjustment provision, the resistance could be trimmed to compensate for this unwanted gain. One quirk of some op-amp models is that of output latch-up, usually caused by the common-mode input voltage exceeding allowable limits. If the common-mode voltage falls outside of the manufacturer's specified limits, the output may suddenly "latch" in the high mode (saturate at full output voltage). In JFET-input operational amplifiers, latch-up may occur if the common-mode input voltage approaches too closely to the negative power supply rail voltage. On the TL082 op-amp, for example, this occurs when the common-mode input voltage comes within about 0.7 volts of the negative power supply rail voltage. Such a situation may easily occur in a single-supply circuit, where the negative power supply rail is ground (0 volts), and the input signal is free to swing to 0 volts. Latch-up may also be triggered by the common-mode input voltage exceeding power supply rail voltages, negative or positive. As a rule, you should never allow either input voltage to rise above the positive power supply rail voltage, or sink below the negative power supply rail voltage, even if the op-amp in question is protected against latch-up (as are the 741 and 1458 op-amp models). At the very least, the op-amp's behavior may become unpredictable. At worst, the kind of latch-up triggered by input voltages exceeding power supply voltages may be destructive to the op-amp.

Simple circuit of Instrumentational Amplifier.

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REVIEW: Op-amp inputs usually conduct very small currents, called bias currents, needed to properly bias the first transistor amplifier stage internal to the op-amps' circuitry. Bias currents are small (in the microamp range), but large enough to cause problems in some applications. Bias currents in both inputs must have paths to flow to either one of the power supply "rails" or to ground. It is not enough to just have a conductive path from one input to the other. To cancel any offset voltages caused by bias current flowing through resistances, just add an equivalent resistance in series with the other op-amp input (called a compensating resistor). This corrective measure is based on the assumption that the two input bias currents will be equal. Any inequality between bias currents in an op-amp constitutes what is called an input offset current. It is essential for proper op-amp operation that there be a ground reference on some terminal of the power supply, to form complete paths for bias currents, feedback current(s), and load current.

References:

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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 0-201-131897)p.407. Smither, Pugh and Woolard: ‘CMRR Analysis of the 3-op-amp instrumentation amplifier', Electronics letters, 2nd February 1989. 4.Jung, Walter G. (2004). "Chapter 8: Op Amp History". Op Amp Applications Handbook, Newnes. 5. Jung, Walter G. (2004). "Chapter 8: Op Amp History". Op Amp Applications Handbook, Newnes.. 6. A.P. Malvino, Electronic Principles (2nd Ed.1979. ISBN 0-07-039867-4) p.476.