Convertidor Analogico Digital

ELET 3156 DL - Laboratory #6

Conversion Between Analog and Digital Signals
There is no pre-lab work required for this experiment. However, be sure to read through the assignment completely prior to starting the practicum. Task: To become familiar with the conversion between analog and digital electronic signals and the properties and definitions associated with those conversions.

PART ONE.Analog to Digital Conversion
There are many applications of the process related to the conversion of varying analog voltages to correlated binary values. This process is often referred to as digitization and is accomplished by sampling an analog voltage at repeated intervals and then storing, in binary form, a number that identifies the analog value of each sample. The resulting table of binarynumbers can be stored, transmitted without electronic interference, and converted back to analog form if need be. There are many different methods to convert analog signals to digital values. Some electronic circuits that deserve attention include the dual-slope integrator used in digital meters, flash converters used for high-speed applications, and the successive approximation type converterused for many digital instrumentation applications. We will be using an ADC0804 8-bit Analog to Digital Converter IC in this experiment. This circuit is a state machine that utilizes a clock to drive an internal counting circuit (successive approximation) and an internal digital-to-analog converter to perform the conversion process. Figure 6.1 illustrates the circuit we will use to perform theconversion. The analog input to this circuit is generated at Pin 6 (V+) by using a voltage divider created by the 100K potentiometer placed between Vref (5V) and ground. The RC pair made up of the 10K resistor and the 150pF capacitor create a digital oscillator that is used to provide clock pulses to the converter chip. The digital output can be connected directly to the trainer LED displays. Theconversion is started with a transition on Pin 3, WR .

Figure 6.1: Analog to Digital Conversion Circuitry.

Step 1: Construct the circuit of Figure 6.1. Apply power, and adjust the potentiometer until the voltage at Pin 6 is equal to 0V. Then, by SLOWLY increasing the voltage at Pin 6, indicate on Table 6.1 the lowest voltage at which the digital output first switches to the indicated binary value.Page - 1

TABLE 6.1 Analog to Digital Conversion Data Analog Voltage Binary Value Analog Voltage 0.00 Volts 00000000 00000001 00000010 00000011 00000100 00000101 00000110 00000111

Binary Value 00001000 00001001 00001010 00001011 00001100 00001101 00001110 00001110

On the graph below (Figure 6.2) plot the analog voltage versus the binary value.

Digital Value

Analog Voltage(Millivolts)

FIGURE 6.2 Transfer Function Segment of 8-Bit Analog to Digital Conversion. Since this circuit uses an 8-bit conversion, it will divide the voltage between V+ and V- into 256 discrete steps. This full-scale voltage of 5V when divided by 256, produces steps of about 20mv each. Based upon your data, how much of an input voltage swing actually produces a one-bit change on the output?________________. If this value (voltage resolution per bit) varies from the expected value of 20mV, explain: ____________________________________________________________

______________________________ ____________________________________________________________

______________________________ Based upon the voltage resolution per bit that you obtained, you should be able to predict the expectedoutput binary value for any analog input by dividing the analog input by the resolution per bit. Based upon this calculation, fill in the columns in Table 6.2 that are labeled “Expected Output.”

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TABLE 6.2 Analog to Digital Conversion Data Analog Expected Output Vo Voltage Output Obtained 0.50 Volts 1.00 Volts 1.50 Volts 2.00 Volts 2.50 Volts

Analog Voltage 3.00 Volts 3.50 Volts...