Preparation
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Examination
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Created it, 06/10/19
Update it, 06/11/02
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All physical sizes (pressure, temperature, light intensity, speed…) can be converted into electric quantities (running, tension) by means of specific components called transducers.
A microphone is an example of transducer which converts a variation of pressure of the ambient air into an electric tension, whose amplitude is a function of the variation of pressure.
The microphone delivers an analogical tension, i.e. a tension being able to take all the possible values on a given interval.
An analogical signal thus has a minimal value and a maximum value.
A numerical signal, on the other hand, takes only two logical values (1 and 0).
Consequently, all the digital circuits seen until now cannot generate or treat an analogical signal.
To fulfill this function, it is necessary to insert into the entry of the digital circuit, a device which converts the tension or the analogical current into a numerical signal: this device is called analogical / digital converter.
Opposite conversion, i.e. of a numerical signal in an analogical signal, takes place thanks to a digitizer / analogical.
In the theoretical lesson “14” digital electronics corresponding to this practice, you learned which are the characteristics of the converters and on which principles their operation is based.
In this practice, you will try out these circuits and will see some applications.
You point out that the principal characteristics of a digital/analogical converter are :
Its resolution :
it is the number of analogical levels which one collects on his exit.
This resolution is a function of the number of bits on which the converter functions.
The larger the number of bits is, the more the resolution is important. With 8 bits, the converter delivers 256 analogical levels of 0 volt to the maximum tension ; with 9 bits, there are 512 levels and so on.
Its precision :
this parameter indicates the variation which can exist between the real tension
and the theoretical tension. The precision is not related to the resolution. a
converter can have a weak resolution, but a high degree of accuracy.
Its linearity :
a converter is linear if its output voltage is proportional to the binary number
present on its entry.
End of scale (full scale) :
it is the maximum value of the analogical tension that the converter can provide
in exit.
Its offset : it
is the value of the output voltage when all the logical entries are to zero. An
ideal converter has a tension of offset (shift) null.
1. - PREPARATION
OF THE MATERIAL
Prepare the following material to carry out the whole of the experiments which will follow :
1 integrated circuit ADC 0804
1 integrated circuit LM 335
1 integrated circuit LM 747
1 integrated circuit MM 74C00
1 integrated circuit MM 74C74
1 integrated circuit MM 74C154
1 integrated circuit MM 74C193
1 resistance of 2,7 kW - 1 / 4 W - tolerance 5 %
1 resistance of 4,7 kW - 1 / 4 W - tolerance 5 %
1 resistance of 5,6 kW - 1 / 4 W - tolerance 5 %
3 resistances of 10 kW - 1 / 4 W - tolerance 5 %
1 resistance of 22 kW - 1 / 4 W - tolerance 5 %
1 resistance of 39 kW - 1 / 4 W - tolerance 5 %
1 resistance of 82 kW - 1 / 4 W - tolerance 5 %
1 resistance of 2,2 kW - 1 / 4 W - tolerance 5%
1 trimmer potentiometric of 1 kW
1 trimmer potentiometric of 10 kW
1 condenser of 150 pF
2 condensers of 0,1 µF
1 electrolytique capacitor with the tantalum of 10 µF - 10 Volts
1 braid of flex-wire (red and black) of 0,25 mm².
2. - FIRST EXPERIMENT : EXAMINATION OF AN OPERATIONAL AMPLIFIER
It is necessary to have some concepts on the operational amplifiers, which are analogical circuits, to approach the examination of the converters.
The latter are at the same time analogical components and numerical components.
The operational amplifiers are integrated circuits of universal use, from which one can design an analogical circuit.
The experiment which follows will enable you to check the operation and the principal characteristics of an operational amplifier.
On figure 1 is represented the stitching of the integrated circuit LM 747 which will be used in the experiments which will follow. This circuit includes/understands two independent operational amplifiers.
Each operational amplifier has a noted inverseuse entry (-) and a noted noninverseuse entry (+). If one applies a signal to the entry (-), the output signal east in opposition of phase compared to this input signal. On the other hand, if one applies the signal to the entry (+), the output signal east in phase with that of entry.
Operational amplifiers A and B of circuit LM 747 can function with a symmetrical food.
The positive food is applied to pins 13 and 9, while the common negative food is applied to pin 4.
In the experiment which follows, the negative food will be provided by a pile of 4,5 volts.
Take care that the pile is in good condition (new pile).
2. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix the connections and the components relating to the preceding experiment.
b) Introduce the integrated circuit LM 747 on the matrix as indicated figure 2-a.
c) Insert the other components and carry out the connections indicated on this same figure 2-a.
The figure 2-b represents the electric diagram of the assembly carried out.
You should now carry out a cord to connect the pile of 4,5 volts to the matrix.
d) For that, cut two pieces of flex-wire, one red and the other black and weld two crocodile clips as indicated figure 2-a.
At the other end of two wire, approximately weld a piece of naked galvanized rigid wire of 1 cm length.
Insert the two caps red and black as indicated figure 3 and twist two wire between them.
e) Connect the pile of 4,5 volts using the cord as represented figure 2-a.
Note that it is the red wire which is connected to the mass of the digilab. Do not connect, in the immediate future, the black crocodile clip with the pole (-) of the pile.
First test :
a) Connect the black crocodile clip to the pole (-) of the pile and put the digilab under tension.
b) Prepare the controller for the measurement of tension D.C. on the gauge 10 volts and measure the Ve tension applied to the entry of the amplifier. This Ve tension is located between the item X which correspond on a terminal of R1 and the mass of the digilab which is also the pole (+) pile.
This tension can be positive, null or negative according to the position of the cursor of the trimmer.
To take the measurement of Ve, pose the black test probe of the controller on the mass and the red test probe on item X.
If the needle deviates on the left of graduation 0, reverse the two test probes bus Ve is negative.
c) Defer the value of Ve in the left-hand column of the table of figure 4.
| Tension of entry (Ve) | Output voltage (Vs) |
d) Measure the Vs tension in the same way corresponding to Ve by connecting the test probes of the controller to the point Y and the mass.
e) Defer this value in the second column (Vs) of the table of figure 4. Each time, do not omit to indicate the sign of the measured tension.
f) Carry out a series of measurements for Ve and Vs while playing on the trimmer P. You can thus traverse all the possible values in entry of - 4,5 volts to + 5 volts for example.
Defer some values (Ve and Vs) in table (figure4).
Observe this table, you note that Ve and Vs are equal but of contrary sign, if you measure + 1,5 volt for Ve, Vs will be worth - 1,5 volt and so on.
g) We will calculate the profit G of the amplifier. We have the relation :
For example, with Vs = - 1,5 volt and Ve = + 1,5 volt, profit = - 1,5 / + 1,5 = - 1
This circuit with thus a unit profit (G = - 1) and it reverses the input signal.
You could also notice that there is a beach of tension for the Ve tension, for which the amplifier has a profit of - 1, but that beyond this beach, this profit is not respected any more.
Indeed, the maximum output voltage is at approximately + 4,5 volts and the minimal tension with - 2,5 volts.
h) Put the digilab not under tension and disconnect the pile.
Second test :
a) Withdraw R2 resistance of 10 kW (figure 2), connected between pins 1 and 12 of the LM 747 and replace-there by a resistance of 22 kW.
b) Connect the pile and put the digilab under tension.
c) Effectuez the same series of measurements that previously and defer the whole of the values in a table identical to that of figure 4.
You observe this time that, for a Ve tension equalizes to + 1 volt, the output voltage Vs reaches already - 2,5 volts, i.e. the lower limit for the exit of the operational amplifier. In the same way, when the Ve tension reached - 2 volts, the Vs tension reaches the higher limit which is + 4,5 volts.
When the tension of Ve entry is located outside the beach - 2 volts at + 1 volt, the amplifier is saturated.
The two limiting values at exit of the amplifier are a function of the values of the food positive and negative of the integrated circuit.
In general, an operational amplifier is supplied by tensions of values higher than those of the experiment presents (for example : + 15 volts and - 15 volts).
d) For this last test, the profit G is worth : G = Vs / Ve = - 2,2
This formula is valid only if the amplifier and unsaturated.
One obtains Vs = - 2,2 x Ve
e) Replace R2 by another resistance of 39 kW and made a series of measurements like previously. Take care to remain in a beach of tension of entry such as the amplifier is not in saturation.
Calculate the profit G :
You must find approximately - 3,9.
Note :
Because of the tolerance on the value of resistances R1 and R2 as well as precision of the controller, the tensions raised during the calculated tests, and consequently profits, can be appreciably different from the values given in this practice.
By convenience, the value of R3 resistance was not modified with each time one changed the value of R2. However, it would have been necessary to adapt for each series of measurements the value of R3.
We will reconsider this problem.
These various tests enabled you to check the formula already seen in theory:
R2 is the resistance of negative feedback and R1 is the resistance of entry.
In the first test, R1 = R2 = 10 kW and the profit G is worth - R2 / R1 = - 104 / 104 = - 1
In the second test G = - (2,2 x 104) / 104 = - 2,2
For the last test G = (3,9 x 104) / 104 = - 3,9
In conclusion, one can give the following formula :
f) This experiment is finished, also put the digilab not under tension.
In short, this experiment enabled you to see the operation of an operational amplifier assembled out of amplifier of tension reverser.
This operational amplifier requires two supply voltages, one positive, the other negative one. This characteristic is common to many analogical circuits.
R3 resistance, connected to the noninverseuse entry (pin 2) LM 747 has as a function to minimize the tension of offset at exit. This R3 resistance must be equivalent to the resistance which “sees” the inverseuse entry.
The latter is consisted the parallelization of R1 and R2 without omitting the trimmer in series with R1.
The equivalent resistance of this trimmer for the inverseuse entry varies from 0 W with 250 W. This resistance is null when the cursor of the trimmer is located at the one of the two ends and it is maximum (250 W) when the cursor is located in the middle of the trimmer. In this case, there are 500 W in parallel with 500 W, which gives well a value of 250 W.
Finally, one has (R1 + 250 W) in parallel with R2, that is to say 10 250 W // 10 kW what gives 5.061 W.
In this experiment, R3 resistance is worth 5,6 kW.
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