1. Introduction
Among the valid signals for digital and analog signals that are normally found in nature is necessary conversion allow digital systems to communicate with the outside.
This communication is passed in two directions:
1. from the digital system to the outside, making a conversion digital / analog (D / A).
2. from the outside to the digital system, making a conversion to an analog / digital (A / D).
There are a lot of converters implemented through a combination
of analog and digital electronics. The choice between one or another design will depend on the performance required for each application.
2. Converting digital / analog (D / A) binary weighted
mount a simple system of digital to analog conversion, which will convert the signal generated by a digital system to obtain an analog signal the output of the circuit. Our digital system will be a counter 74LS93A integrated circuit, very versatile, which allows changing the external connections, configure it to make a note that we truncated at any point in the sequence.
will use to build digital signals that we want to convert, 74LS93A configurations shown in Figure 1 (a reasoned analysis by the integrated block diagram of which is attached, as is generated each sequence):
Figure 1: Set a timer module 16, 10 and 5 respectively
obtained signals Q0, Q1, Q2 and Q3 are digital signals whose value (4 bits) we will convert into an analog signal. The value is a simple digital signal that goes from zero hasta el modulo dado por la configuración respectiva, y por tanto su conversión analógica será una rampa lineal hasta dicho valor. Uno de los conversores D/A más simple es el de ponderación binaria, que se construye mediante una red de resistencias en el que los valores de las resistencias son inversamente proporcionales a los pesos binarios de cada bit digital de la señal de entrada al conversor. Mediante un amplificador operacional, convertimos la intensidad de corriente que circula por cada rama del circuito (y que será proporcional al peso del bit correspondiente), en una señal de voltaje analógica cuyo valor corresponderá al valor binario dado por los cuatro bits de la señal digital. Montar el siguiente circuito for D / A converter will use the given resistance (nominal value then gives slightly from ideal standard shown in the diagram), and integrated operational amplifier LM741. To power the operational amplifier is necessary to connect the integrated power 12 V / +12 V, for which we use the two voltage sources interconnected as shown in the figure, creating an intermediate node to ground, which is the basis of land for all the circuit. Feeding the other integrated circuits is between 0 and 5 V, using the TTL power supply voltage. Before connecting the integrated check the polarity of the feeds to the millimeter.
input signals Q0, Q1, Q2 and Q3 of the converter, are the ones that get to the counter output that we connect 74LS93A and respecting the weight of each bit.
After mounting the circuit, perform the following steps:
1. Check, displaying on the oscilloscope signals Q0, Q1, Q2 and Q3, if the sequence generated by the counter in three different configurations are correct.
2. Overlay to display on the oscilloscope clock signal introduced at the counter, and the output signal Vout to get the converter. We must get the correct conversion for the inverted output signal (press and hold the button for the channel 2 on the oscilloscope to see the inverse of Vout), ie, a stepped ramp that increases linearly and becomes
Figure 2: Circuit binary weighting
start after reaching the modulo value for which we set the converter. Make this observation for the three configurations given the counter and graph the signal obtained.
Choose one of the settings of the counter to perform the following steps:
1. Change the value of RVAR and observe its effect on the ramp conversion. Build
a graph that is rendered (for five values) the value of variable resistance and the maximum of the ramp. > It saturates the ramp? Get the maximum value of RVAR for the amplifier is not saturated. Modify slightly (to + -13.5 V) supply of 741 and observe the effect on the conversion. RVAR get the maximum value of -13.5 + V supply.
2. Modify one of the strengths of the resistive network (Eg changing a value slightly higher 2R, using the potentiometer, which we will _ja replaced by a resistance of 100 k) and observe its effect on the ramp conversion. Several different branches and note that steps are modified in each case.
3. Measure time delays for each step of the ramp (with respect to the reference clock signal) and the initialization delay time of conversion (zero ramp). Increasing the clock frequency and see how the signal is distorted. Get the maximum frequency at which the converter operates without errors.
reasonably explain the results of the measures obtained in the last three points.
3. Analog / digital converter (A / D) parallel
in this case is to digitize an analog input signal to obtain the value of the magnitude of the signal expressed by a binary sequence. There are a lot of designs, a wide variety of A / D, whose accuracy depends on the number of output bits by which digitize the signal.
We will set up a simple scan converter in parallel, giving an output of two bits (a limited accuracy), an analog input signal generating a signal that we will get synod with the function generator. We will use four 100 k resistors, three of the four operational amplifiers having the LM324 and a 74LS148 priority encoder. (NOTE: In this case the integrated operational fed with (0, +5 V) and the other, which simplifies the mounting board).
Before mounting the circuit, we will make a trial of digital conversion using a single operational which will control the signal input to the inverting input of the amplifier via the power supply (Vcont), we can modify and convert the analog sine signal introduced by the non-inverting input to a square wave (digital) we obtain at the output. Observe the effect of the value of continuous control signal on output Vcont get (make a sketch of the display of the conversion on the oscilloscope, which will observe the original analog signal and digital signal simultaneously). Observe the delay incurred and obtain the maximum frequency at which the amplifier ceases to scan properly.
After verifying the operational digitizing function, fit the full two-converter bits and display on the oscilloscope the sequence 0 {3 generated by the converter as the analog signal is digitized (using a sine signal ranging from 0 to 4 V).
Figure 3: Setting 74LS148
Note the following issues in the assembly of the figure:
1. The operational with the inverting input connected to the signal we want to digitize and noninverting each of the voltage levels that we have divided the route 0 {VCC. It is the opposite we did in the test with a single operational because in this way we get the scan and we can introduce inverted output signals of the operational input directly to the decoder (inputs denied, active low) for proper operation of A / D converter
2. EI entry serves to introduce a signal that marks the sampling rate. In our case, when connected to LOW (active low) we will carry out continuous sampling, so that the pace of digitization will be marked by operational delays and decoder.
3. The decoder outputs are also active low, so to display them properly on the oscilloscope without using additional investors, we will use the investor's own scope, which is activated by holding down the button a few seconds next to the entrance of the canal.
Given the foregoing, and having verified the correct operation of A / D,
1. Compare the analog signal and each output significant bits (A0 and A1). Make a chart that represents the analog signal and the two bits of output as a function of time. Reason converter operation in two stages: the operational and decoder. Indicate that the decoder should be a priority.
2. On the chart above, study the effect of operational delays by measuring the delay of the intermediate signal on line 3 of entry to the decoder. Calculate the delay by the asymmetry of conversion. Measure the total delay of the decoder, as challenged finding corresponds to the operational and how the decoder.
3. Estimate the precision of digitization.
4. D / A converters and A / D integrated
In electronic systems converters are usually not set built from discrete components of lesser rank, but are used directly integrated circuits containing the full converter, in which all components have been lithographed monolithically in silicon. There is a great deal on the market. Burr-Brown house, which was originally dominated market converters. It has now been absorbed by Texas Instruments, and all products have been included in the catalog of this multinational.
mention two of the most widely used converters, of which I provided some of its spec sheet appended to this practice, although we will not use them.
ADC0804C is an analog-digital converter 8-bit successive approximation monolithic CMOS technology. The conversion is performed by a modified potentiometric network of 256 resistors. Includes flip-flops to control the data bus used, making it very versatile and suitable for inclusion in systems containing microprocessors.
DAC80 is a digital-analog converter 12 bit. Its analog output is very robust and stable, capable of supplying up to 2; 5mA to an external load without degrading the D / A. It also works well in a wide range of temperatures and supply voltages, consuming enough low. Since it is also low cost, this converter is one of the most used.
Mario Dominguez Zambrano
EES Section: 02
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