# Lecture – 40 Signal Conditioning Integrated Circuits

that is our ability. So, I put a excitation here and I will get 2 secondary voltages here, This is the basic diagram of LVDT right. I put a displacement here, please note LVDT is electromechanical sensors and it basically, a displacement sensors directly, it is measuring displacement. But it can measure, so many other indirectly, it can measure pressure it can measure many other parameters right. So, the basic it is displacement, now, I am giving an excitation. So, all things, so all those things are I mean very, I mean deeply discussed or very widely discussed in the case of LVDT in the lesson of LVDT, but here. Actually I mean, I am making brief discussion to have a look re-capsulations of the LVDT Now, if I you see, if I this move this core up and down say there will be a linkage. If I put this LVDT up this core up there is more linkage, between primary and secondary. If I put it down, there will be more linkage between and this secondary right. So, 2 voltage I will find the voltage will be different now, if I put this in opposition in series opposition like this 1. So, what I will get the output voltage of a function of output voltage of a function of the displacement x at width positions, geometric width positions. I will get a null voltage I will get a 0 voltage rather as you get a 0 voltage, because these 2 out secondary this 2 voltage will cancel out each other. So, I will suppose to get a 0 voltage, but due to mismatch of the secondaries always We will get some nonzero voltage right due to mismatch of the secondary, if I look at the two secondary I mean, looks like this Suppose this is 1, 1 current is I s 1 secondary current is I s 1 and 1 secondary current is 2 right. But you will find that the 2 current will exactly, not be the, it is very difficult to make a 2 identical magnetic circuits right So, 2 voltages will never cancel out each other So, there will be a certain residual voltages, that voltage now, if I plot this LVDT as we know, this all things, we have discussed as I told, you several times x this is output voltage e naught. So, the response will be look like this one, it should be a straight line, because it is a linear straight line like this one. This is the output voltage, so if I make a displacement, if is measure the magnitude it will look like this one, because it is a AC voltage. So, I will get a amplitude, so this will be plotted clear? So, this will give you our basic LVDT. Now, you see there is always, some null voltage, which will appear, we have also shown there is an input output voltage Sorry this will be ex I am sorry, this will be our, this will be our, so this will be actually ex excitation right Now, for this movement, you will always you will find there is a output voltage. And this excitation voltage, there is a phase difference between input and output, which is undesirable properties. Now, all these things, we will find the null voltage cancellation and always wherever you are using a LVDT I need a separate excitation, voltage right separate, because it is like a bridge as you know, we use a Wheatstone bridge Wheatstone bridge also need an excitation either DC or AC it does not matter, but in the case of LVDT since we are using transformer So, always there is an excitation, so this can be easily avoided, if I use a particular chip available right. So, this excitation also inherently, you can vary the frequency of excitations right and I am getting output voltage, which is free from all this problems let us go back Now, the input to LVDT, which is linear variable differential transformer, is the displacement of the core. And it is output is a pair of ac voltages proportional to the core position, as we know this is basic LVDT It is an electromechanical transducer consisting of a primary winding, energized by an external sine wave reference sources, external sine wave reference sources, please note always we need this source outside. And two secondary windings are usually, connected in the series opposition, this we have discussed in details. When we covered the LVDT the moveable core change the flux linkage between the primary and secondary windings,

which is giving a non zero output voltage e naught Now, LVDT signal conditioner circuit is AD598 it is highlights of the chip, what are those. So, the chip number AD598 developed by the analog devices. AD598 is the monolithic IC to be used with an LVDT for signal conditioning purpose a complete transformation of displacement. Or mechanical position to dc voltage both unipolar and bipolar is possible directly, because you we have seen that to make it sensitive phase sensitive I need a phase sensitive demodulator So, those thing also can be, I mean eliminated, if I use this AD598 So, this 3 most 3 most important things, which we need to use an LVDT, I need a first of all I need a source excitation voltage is necessary. I need is a null voltage I mean, null voltage reduction circuit that will make the null voltage 0. That means, at the geometric position 0 position there, should be the 0 null voltage thirdly. I need a phase sensitive demodulation circuits; that means, sensitive modulation circuit is something like, that on which side of the null position of the core lies. It will be deducted by the phase sensitive demodulation circuit; that means, if I look at If I use an LVDT, you see that we have seen that, if I put a displacement x I will get a I mean circuit, I mean output like this one so on. You see on both this is a null position, this is a null position, this is a output voltage here on both side of the null, I am getting a non zero output voltage. So, it is very difficult by looking at a voltage, it is very difficult to tell on which side of the null actually the core lies. So, what I can do? I can do like this one. So, if I use a phase sensitive demodulation circuit, this is the circuit with output voltage e naught with phase sensitive demodulator phase sensitive, demodulator right if the output will look like this one clear So, for looking at the voltages or the polarity of the voltages, I can tell whether which side of the null the voltage lies. So, all these is incorporate or incorporated at AD598 So, it is a great I mean, so many external components will be saved by using a single chip So, AD598 is the monolithic IC to be used with an LVDT for signal conditioning purpose a complete transformation of displacement or mechanical position or dc voltage Both unipolar and bipolar is possible with this, it can used also for the RVDT rotational velocity displacement transducers, this also possible with this, I mean type of device with this type of chip AD598 accepts a wide range of input and output voltages, these are all the features what it can and frequencies. It can drive on LVDT primary with up to 24 volt rms and accepts secondary input levels as low as hundred millivolt rms right. Hence, it can be used with almost all types of LVDT; that means, you please you see that LVDT and the primary can go up to 24 volt rms The excitation frequency is determined by a single external capacitor by using a single external. Which is outside the chip by excitation frequency of the LVDT can be I mean, determined by a single external capacitor the range is 20 Hertz to 20 kilo hertz Moreover the input signal need not be synchronous with the LVDT, primary drive thus an external primary excitation of any desirable frequency, can also be used right this is another most important thing The ratiometric decoding scheme used by AD598 is such that, the ill effects caused by the primary to secondary phase shift. That I told you will be eliminated and transducer nonzero null voltage, do not influence the overall circuit performance. So, it will be independent of the non zero null voltage, which is obvious in all LVDT, because you cannot make 2 LVDT secondaries, exactly the same right. So, say a little I mean, I mean dissimilarities of the 2 secondaries will give us a non zero null voltage. This can be measured very easily in LVDT, if we use without any signal conditioning circuitry AD598 can be used in telemetry to the interface electronics, when it is far away from the LVDT. And it can be drive it can drive an LVDT at the end of the 100 meters of capable and the position output can drive as much as 30 meter 300 meter of cable. I think it will be a cable if I am not wrong, so it will be a cable right

The excitation output of AD598 is thermally protected and AD598 can drive the multiple LVDT in either series or parallel as long as power dissipation limits are not exceeded This is a great advantage you see the is same LVDT same chip can be used for several I mean, LVDT excitation. So, that is a great saving cost, it can be used as a loop integrator in the design of the electromechanical servo loops Theory of operation, let us look at a functional block diagram of AD is shown in the figure 1 and AD598 energizes the LVDT primary senses. The LVDT secondary output voltage and produces a dc output voltage, proportional to the core position, that is actually, ultimate result; that means, for different core positions; I need a dc output voltage. Why dc? Because I need a dc, because I have to make the phase sensitive on which side of the core, the on which side of the null position, the core lies that, I must know. So, that only can tell by the polarity of the dc voltage right To drive the LVDT primary AD598 uses a sine wave oscillator and a power amplifier. It also consists of a decoder, which determines the ratio of the difference between the LVDT secondary voltage divided by their sum and a filter and an output amplifier. These are used to produce the dc output voltage proportional to the core position You see this is the block diagrams on the LVDT right LVDT our AD598, this is actually inside the chip. You can see this portion, this portion is inside the chip, this portion is inside the chip right and there are pin numbers 2 3. So, 2 and 3 will be connected to the primary of the LVDT and 10 11 and 17 will be connected to the 2 secondaries of the LVDT 10 and 11 are 2 extreme point of the secondaries. And one will be connected 2 secondaries are will be connected like this, as it is shown and it is grounded and these 2 signals are coming to an signal conditioning circuits which will make 2 signals. That means, A minus B divided by A plus B means; that means that is; that means, VA actually, it is VA minus VB divided by VA plus VB ratiometric principles as I told you So, this will make when the 2 voltages are equal; obviously, this should be 0 right; that means, output voltage will be 0. So, output voltage will come out as a 0 here a filters and amplifiers and this is an oscillator and amplifier to have a stable oscillation. What they do actually, in the case of LVDT they make first make a triangular generate a triangular wave. They generate a triangular wave then with the signal processing with they convert the triangular wave approximate sine wave Because the triangular wave can be converted to a sine wave, then it is fit to the amplifier and it is fitting through the primary of the, this is the primary, this is the primary of the LVDT clear. Now, how you do it? You see that, if you can look at, it looks like this one So, triangular waves are like this. So, with diode, I can what I can I can round of this value. So, it will look like this one like this one though it is not exactly, sine wave, but for most of the purposes, it will suffice It is a continues form, there is error, if I look at there is a distortions and all these does not matter, it hardly matters, because nobody say that exactly use. We have to use the sine wave there right and this frequency of this is determined of this triangular wave is determine or the sine wave approximated sine wave is determined by an external capacitor clear right. So, this voltage will be divided by a A plus B, so it will give you the filter, this is the block diagram, but inside there is a much detail circuit This is the chip pin diagrams, we can see it is a 20 pin DIP, there will be line packages So, we have all the excitation lines 2 3 we have filter lines 8 9, we have a output filter then we have signal output. So, this basically, to excitations will go to primary and we have a dual supply V plus Vs and minus Vs either single ended. If you need single ended this is to be grounded; that means, if I need only unipolar. So, this is to be grounded and this output is signal output will come out, you can take the output filter also and signal differences right this is our pin diagram The oscillator consists of multivibrator, because multivibrator as you know multivibrator frequency. I can use wind bridge oscillators and all these things, but multivibrator frequency will be more stable. So, that is the reason they use multivibrator, which