Module 3 - Industrial Instrumentation and Automation

H330 Automotive Engineering MODULE 3 SIGNAL CONDITIONING CIRCUITS: INSTRUMENTATION AMPLIFIERS UNBALANCED BRIDGE, BRIDGE LINEARIZATION USING OP AMP PRECISION RECTIFIERS, LOG AMPLIFIERS, CHARGE AMPLIFIERS, ISOLATION AMPLIFIER, SWITCHED CAPACITOR CIRCUITS, PHASE SENSITIVE DETECTORS, NOISE PROBLEM IN INSTRUMENTATION AND ITS MINIMISATION SIGNAL CONDITIONING CIRCUITS - Signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing. - In control engineering applications, it is common to have a sensing stage (which consists of a sensor), a signal conditioning stage (where usually amplification of the signal is done) and a processing stage (normally carried out by an ADC and a micro-controller). - Operational amplifiers (op-amps) are commonly employed to carry out the amplification of the signal in the signal conditioning - The Signal Conditioning System may be required to perform linear processes such as amplification, attenuation, integration, differentiation, addition or subtraction. - They are also required to do non-linear processes such as modulation, demodulation, sampling, filtering, clipping and clamping, squaring and linearizing or multiplication by another function. - These functions require proper selection of components and faithful reproduction of the final output for the presentation stage. - The Signal Conditioning System or data acquisition equipment is in many cases an excitation and amplification system for passive transducer, or an amplification system for active transducer. 2 - In both cases, the transducer’s output is brought to the required level to make it useful for conversion, processing, indicating and recording. - Excitation is needed only for passive transducers, because they do not generate their own voltage or current. - It is essential for passive transducers like strain gauge, potentiometers, resistance thermometers, and inductive or capacitive transducers to be excited from an external source. - Active transducers such as thermocouples, piezo-electric crystal and inductive pickups etc. do not require an external excitation because they produce their own voltages only by the application of physical quantities. - However these signals are at a low level, and require amplification. - The excitation source may be AC or DC. DC signal conditioning system -A simple DC signal conditioning system is shown in figure -The strain gauge (resistance transducer) constitutes one or more arm of the Wheatstone’s bridge, which is excited by the DC source. 3 - The bridge can be balanced by using a potentiometer and can also be calibrated to indicate the unbalanced condition. - The DC amplifier should have the following characteristics. Its input stage may be a balanced differential inputs giving a high CMRR. It should have extremely good thermal and long term stability. Easy to calibrate at low frequency. Able to recover from an over load condition, unlike an AC system. The main disadvantage of a DC amplifier is the problem of drift. Hence low frequency spurious signals are available as data at the output and to avoid this low drift of a DC amplifier, special low drift DC amplifiers are used. The DC amplifiers is followed by a low-pass filter, which is used to eliminate high frequency components or noise from the data signals. AC signal conditioning system - In order to overcome the problem of drift in the DC systems, AC systems are used. - An AC signal conditioning system is shown in figure (next slide) 4 - The output of a transducer is applied to the bridge circuit, whose output is an amplitude modulated carrier signal. - This waveform is amplified by an AC amplifier. - This amplified modulated output is then applied to a phase sensitive demodulator, the carrier signal. - This produces a DC output that indicates the direction of the parameter change in the bridge output. INSTRUMENTATION AMPLIFIER - Instrumentation amplifier is a kind of differential amplifier with additional input buffer stages. - The addition of input buffer stages makes it easy to match (impedance matching) the amplifier with the preceding stage. - Instrumentation amplifiers are commonly used in industrial test and measurement application. - The instrumentation amplifier also has some useful features like low offset voltage, high CMRR (Common mode rejection ratio), highEEEinput MITS resistance, high gain etc. 5 - The circuit diagram of a typical instrumentation amplifier using op amp is shown in figure. - A circuit providing an output based on the difference between two inputs (times a scale factor) is given in the figure. - In the circuit diagram, op amps labelled A1 and A2 are the input buffers. - Anyway the gain of these buffer stages are not unity because of the presence of R1 and Rg. - Op amp labelled A3 is wired as a standard differential amplifier. - R3 connected from the output of A3 to its non inverting input is the feedback resistor. R2 is the input resistor. - The voltage gain of the instrumentation amplifier can be expressed by using the equation 6 below. • 7 - Here the resistances labelled R1 are shorted and Rg is removed. - This results in a full series negative feedback path and the gain of A1 and A2 will be unity. - The removal of R1 and Rg simplifies the equation to Av = R3/R2. Features/characteristics of instrumentation amplifier - Low DC offset (The differential voltage which must be applied to the input of an op amp to produce zero output is called DC offset voltage) - Low drift - Low noise - Very high open-loop gain : For the amplification of the low-level input signals - Very high common-mode rejection ratio : High CMRR is the most important consideration because the input of this amplifier is from the output of the transducer - Very high input impedance & Low output impedance : To protect the circuit from the effect of loading. 8 PRECISION RECTIFIERS - The precision rectifier, also known as a super diode, is a configuration obtained with an operational amplifier in order to have a circuit behave like an ideal diode and rectifier. - It is useful for high-precision signal processing. - Precision rectifier is a small signal rectifier which is capable of rectifying signals of very small peaks - The basic circuit (half wave rectifier) implementing such a feature is shown in the figure, where RL can be any load. - When the input voltage is negative, there is a negative voltage on the diode, so it works like an open circuit, no current flows through the load, and the output voltage is zero. - When the input is positive, it is amplified by the operational amplifier, which switches the diode on. - Current flows through the load and, because of the feedback, the output voltage is equal to the input voltage. 9 - The actual threshold of the super diode is very close to zero, but is not zero. It equals the actual threshold of the diode, divided by the gain of the operational amplifier. - This basic configuration has a problem, so it is not commonly used. - When the input becomes (even slightly) negative, the operational amplifier runs open-loop, as there is no feedback signal through the diode. - For a typical operational amplifier with high open-loop gain, the output saturates. - If the input then becomes positive again, the op-amp has to get out of the saturated state before positive amplification can take place again. - This change generates some ringing and takes some time, greatly reducing the frequency response of the circuit. - An alternative version is given the figure - In this case, when the input is greater than zero, D1 is off, and D2 is on, so the output is zero because the other end of R2 is connected to the virtual ground and there is no current through R2. 10 - When the input is less than zero, D1 is on, and D2 is off, so the output is like the input with an amplification of (-R2/R1). - Its input–output relationship is shown in figure. - This circuit has the benefit that the op-amp never goes into saturation, but its output must change by two diode voltage drops (about 1.2 V) each time the input signal crosses zero. - Hence, the slew rate of the operational amplifier and its frequency response (gain–bandwidth product) will limit high-frequency performance, especially for low signal levels, although an error of less than 1% at 100 kHz is possible. LOG AMPLIFIER - A log amplifier is an amplifier for which the output voltage VOUT is K times the natural log of the input voltage VIN - Log amplifier is a linear circuit in which the output voltage will be a constant times the natural logarithm of the input 11 - The basic output equation of a log amplifier is Vout = K ln (Vin/Vref) where Vref is the constant of normalization, and K is the scale factor. - Log amplifier finds a lot of application in electronic fields like multiplication or division (they can be performed by the addition and subtraction of the logs of the operand), signal processing, computerized process control, compression, decompression, RMS value detection etc. - Basically there are two log amp configurations: Op amp diode log amplifier and Op amp transistor log amplifier Op amp diode log amplifier - The schematic of a simple Op amp diode log amplifier is shown in figure. - This is nothing but an op amp wired in closed loop inverting configuration with a diode in the feedback path. - The voltage across the diode will be always proportional to the log of the current through it and when a diode is placed in the feedback path of an op amp in inverting mode, the output voltage will be proportional to the negative log of the input current. - Since the input current is proportional to the input voltage, we can say that the output voltage will be proportional to the negativeEEElog MITS of the input voltage. 12 - According to the PN junction diode equation, the relationship between current and voltage for a diode is Id = Is (e(Vd/Vt) -1)…………(1) Where Id is the diode current, Is is the saturation current, Vd is the voltage across the diode and Vt is the thermal voltage. - Since Vd , the voltage across the diode is positive here and Vt the thermal voltage is a small quantity, the equation (1) can be approximated as Id = Is e(Vd/Vt) …………………(2) - Since an ideal op amp has infinite input resistance, the input current I r has only one path, that is through the diode. - That means the input current is equal to the diode current Id. Ir = Id ………………….(3) - Since the inverting input pin of the op amp is virtually grounded, we can say that, I r = Vin/R - Since Ir = Id (from equation (3) ), Vin/R = Id …………………..(4) 13 - Comparing equation (4) and (2) we have Vin/R = Is e(Vd/Vt) i.e. Vin = Is R e(Vd/Vt) ……………(5) - Considering that the negative of the voltage across diode is the output voltage V out (see the circuit diagram), we can rearrange the equation (5) to get Vout = -Vt In[Vin/(IsR)] Op amp transistor log amplifier - In this configuration, a transistor is placed in the feedback path of an op amp wired in inverting mode. - Collector of the transistor is connected to the inverting input of the op amp, emitter to output and base is grounded. - The necessary condition for a log amp to work is that the input voltage must be always positive. - Circuit diagram of an Op amp-transistor log amplifier is shown in figure 14 Charge amplifier - A charge amplifier is an electronic current integrator that produces a voltage output proportional to the integrated value of the input current, or the total charge injected. - The amplifier offsets the input current using a feedback reference capacitor, and produces an output voltage inversely proportional to the value of the reference capacitor but proportional to the total input charge flowing during the specified time period. - The circuit therefore acts as a charge-to-voltage converter. - The gain of the circuit depends on the values of the feedback capacitor. - A simple charge amplifier circuit is shown in figure - Here, the current source represents the sensor - The capacitor C represents the capacitance of sensor & cable - Cf is the feedback capacitor - The charge amplifier circuit is similar to a transimpedance amplifier (current to voltage converter) but with a capacitor instead of a resistor in the feedback path. • 16 • 17 Practical charge amplifier - In an ideal charge amplifier, the op-amp’s input bias current will charge up the capacitor and cause the amplifier to saturate. - We can remedy this situation by adding a resistor (which provides a discharge path) in parallel with the capacitor as shown in figure Advantages The output voltage is proportional to the charge produced by the piezo electric transducer The amount of charge present is not affected by the cable capacitance The sensitivity s well as the time constant are independent of the capacitance of the crystal and also that of connecting cables Disadvantages The signal to noise ratio tends to be small The natural frequency of the transducer is red 18 Isolation amplifier - An isolation amplifier (also called a unity-gain amplifier) is an op-amp circuit which provides isolation of one part of a circuit from another, so that power is not used, drawn, or wasted in a part of the circuit. - The purpose of an isolation amplifier isn't to amplify the signal. - The same signal that is input into the op amp gets passed out exactly the same. - This means that output voltage is the same exact as the input voltage, meaning if 10V AC is input into a circuit, 10V AC is output. - An isolation amplifier circuit is shown in figure Working - An op amp is a device with a very high input impedance. This high input impedance is how it causes isolation. - When a circuit has a very high input impedance, very little current is drawn from the circuit. 19 - According to Ohm's law, the current, I=V/R. - Thus, the greater the resistance, the less current is drawn from a power source. - Thus, an op amp, being of very high impedance, does not cause any significant amount of current to be drawn from the power. - It draws very little current; thus, practically no current is drawn and transferred from the first part of the circuit to the second. - The high-impedance load of the op amp ensures this. Thus, the op amp serves as an isolation device from one part of a circuit to the next or of different circuits. 20 Switched capacitor circuits - A switched capacitor (SC) is an electronic circuit element implementing a filter. - It works by moving charges into and out of capacitors when switches are opened and closed. - Usually, non-overlapping signals are used to control the switches, so that not all switches are closed simultaneously. - Filters implemented with these elements are termed "switched-capacitor filters" and depend only on the ratios between capacitances. - This makes them much more suitable for use within integrated circuits, where accurately specified resistors and capacitors are not economical to construct. - SC circuits are typically implemented using metal–oxide–semiconductor (MOS) technology, with MOS capacitors and MOS field-effect transistor (MOSFET) switches - The simplest switched-capacitor (SC) circuit is the switchedcapacitor resistor, made of one capacitor C and two switches S1 and S2 which connect the capacitor with a given frequency alternately to the input and output of the SC as shown in figure. 21 - Each switching cycle transfers a charge ‘q’ from the input to the output at the switching frequency ‘f’. - The charge ‘q’ on a capacitor ‘C’ with a voltage ‘V’ between the plates is given by: q = CV, where ‘V’ is the voltage across the capacitor. - Therefore, when S1 is closed while S2 is open, the charge stored in the capacitor CS is: qin = CsVin - When S2 is closed (S1 is open - they are never both closed at the same time), some of that charge is transferred out of the capacitor, after which the charge that remains in capacitor CS is: qout = CSVout - Thus, the charge moved out of the capacitor to the output is: q = qin – qout = CS (Vin – Vout) - Because this charge q is transferred at a rate f, the rate of transfer of charge per unit time is: 22 I = qf (A continuous transfer of charge from one node to another is equivalent to a current, so I (the symbol for electric current) is used.) - Substituting for q in the above, we have: I = CS (Vin – Vout)f - Let V be the voltage across the SC from input to output. So: V = Vin – Vout - So the equivalent resistance R (i.e., the voltage–current relationship) is: R = V/I = 1/(CSf) Advantages of Switched capacitor circuits - Very high value of resistors can be easily simulated using small value capacitors of order of 10pf - High accuracy - Excellent temperature stability - Low system cost - The switched capacitor filters requires no external reactive components like inductors and capacitors Disadvantages - They generate more noise than standard active filter circuits 23 Phase Sensitive Detector - A phase sensitive detector is used to measure the amplitude & phase of a periodic signal w. r. to another reference signal of the same frequency - Figure shows a simplified block schematic of basic phase sensitive detector - It is composed of 2 basic circuits : a synchronous switch & an amplifier/filter 24 - Consider first the action of the switch - It is designed so that it spends an equal amount of time in each position corresponding to the period of a reference signal, Tm = 1/fm - In the upper position, it passes the signal unchanged, in the lower position it passes the signal inverted - If we apply a sine wave to the input which has the same frequency as the reference, V(t) = V0Sin(2πfmt+Ф), the signal at the output of the switch (at point A in the figure) will depend upon the phase angle Ф between the reference signal & the input signal. - If the phase angle Ф = 0, the 2 signals are in phase & the result at point A looks like the lower trace of figure (a) shown in next slide : the sine wave is rectified - If the result is then passed to the amplifier/filter stage, with a time constant τ of the filter sufficiently long, the output signal will be constant (DC) signal whose value will be proportional to the amplitude V0 - However, if Ф = 90, as in figure(b), the signal at point A will be symmetric about the zero volt axis & the output from the amplifier/filter would be zero. - Likewise it is easy to see that if Ф =180, the output would be proportional to –V 0 25 - This dependance on the phase between the input and reference signals is why this method is called phase sensitive detection 26 - If one were to apply a signal to the input at a different frequency f than the reference frequency fm, the phase Ф would be constantly changing & the net effect would be to produce a signal that would average to zero - The ability of the phase sensitive detector to reject signals such as random noise which differs in frequency from that of the reference signal depends on the averaging time of the smoothening filter - The longer averaging time, the greater is its rejection capability Noise problem in instrumentation & its minimization - Noise is any kind of unwanted signal that is not related to the input - Error sources that arise during the measurement process of sensing the value of a physical variable & generating an output signal - Errors are often created in measurement systems when electrical signals from measurement sensors & transducers are corrupted by induced noise - The induced noise arises both within the measurement circuit itself & also during the transmission of measurement signals to remote points - When designing measurement systems it is important to reduce such induced noise voltage levels as far as possible - Noise can be generated from sources both external & internal to the measurement system 27 Internal noise includes - Thermoelectric potentials - Shot noise - Potentials due to electrochemical action Induced noise from external sources arises in measurement system for a number of reasons that include their proximity to mains powered equipment & cables - Proximity to fluorescent lighting circuits - Proximity to equipment operating at audio & radio frequencies - Switching of nearby DC & AC circuits - Corona discharge Inductive coupling - The primary mechanism by which external devices such as mains cables & equipment like fluorescent lighting & circuits operating at audio or radio frequencies generate noise is through inductive coupling - If signal carrying cables are close to such external cables or equipment, a significant mutual inductance M can exist between them can generate a series mode noise voltage of 28 several millivolts Capacitive coupling - It is also known as electrostatic coupling - It can occur between the signal wires in a measurement circuit & a near by mains current carrying conductor - The magnitude of the capacitance between each signal wire & the main conductor is represented by C1 & C2 - In addition to these capacitances, a capacitance can also exist between the signal wires & earth, represented by C3 & C4 - It can be shown that the series mode noise voltage Vn is zero if coupling capacitances are perfectly balanced, i.e, if C1 = C2 & C3 = C4 - However exact balance is unlikely in practice, since the signal wires are not perfectly straight, causing the distances & thus the capacitance to the mains cable & earth to vary - Thus, series mode noise voltage induced by capacitive coupling Noise due to multiple earths - Measurement signal circuits are isolated from earth - Leakage paths often exist between measurement circuit signal wire & earth at both the source (sensor) end of the circuit & also the load end EEE MITS - Multiple earths can cause a series mode noise voltage in the measurement circuit 29 Noise in the form of voltage transients - When motor & other electrical equipment (both AC & DC) are switched on & off, large change of power consumption suddenly occur in the electricity supply system - This can cause voltage transients in measurement circuits connected to the same power supply - Such noise voltages are of large magnitude but short time duration - Corona discharge can also cause voltage transients on the mains power supply Shot noise - Shot noise occurs in transistors, ICs & other semiconductor devices - It consists of random fluctuations in the rate of transfer of carriers across the junctions within such devices Electrochemical potentials - These are potentials that arise within measurement systems due to electrochemical action - Poorly soldered joints are a common source 30 Techniques for reducing measurement noise 1. Location & design of signal wires - Both mutual inductance & capacitance between signal wires & other cables are inversely proportional to the square of the distance between the wires & the cable - Thus noise due to inductive & capacitive coupling can be minimized by ensuring that signal wires are positioned as far as away from such noise sources. This design is known as twisted pair. - In the first loop, wire A is closest to the noise source & has a voltage V1 induced in it, while wire B has an induced noise voltage V2 - For loop 2, wire B is closest to the noise source and has an induced voltage V1 while wire A has an induced voltage V2. - Thus the total voltage induced in wire A is V1 +V2 & wire B is V2 + V1 over these two loops - This pattern continues for all the loops & hence the 2 wires have an identical voltage induced in them 31 2. Earthing - Power earth: provides a path for fault currents due to power faults - Logic earth: provides a common line for all logic circuit potentials - Analog earth (Ground): provides a common reference for all analogue signals - Safety earth: connected to all metal parts of equipment to protect personnel (the power lines may come into contact with the metallic enclosure of equipment) 3. Shielding - Shielding consist of enclosing the signal wires in an earthed metal shield that is itself isolated electrically from the signal wires - The shield should be earthed at only one point, preferably the signal source end - A shield consisting of braided metal eliminates 85% of noise due to capacitive coupling - The wires inside such a shield are normally formed as a twisted pair so that protection is also provided against induced noise due to nearby electromagnetic fields 4. Other techniques - The phase locked loop is often used as a signal processing element to clean up poor quality 32 signals - It is useful for noise removal because its output waveform is pure square wave at the same frequency as the input signal, irrespective of the amount of noise, modulation or distortion on the input signal Previous University Questions 1. 2. What is an instrumentation amplifier and discuss its role in instrumentation systems? With the circuit diagram of charge amplifier show how it enables measurement of electrical charge. 3. 4. 5. Explain the purpose of signal conditioning in instrumentation systems. What is an isolation amplifier? Discuss its application in instrumentation. List any five important features of instrumentation amplifiers. 6. 7. 8. Explain the importance of signal conditioning in industrial instrumentation systems. Explain the principle of operation of phase sensitive detector. Derive an expression for the output voltage of a logarithmic amplifier and show that it is proportional to logarithm of input voltage. 33