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ELET 3132L – Experiment #7
Conversion Between Analog and Digital Signals
Revised 21-August-2010
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 binary numbers 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 converter used 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 the conversion.
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. The conversion 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.
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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:
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Based upon the voltage resolution per bit that you obtained, you should be able to predict the expected
output 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
4.00 Volts
4.50 Volts
5.00 Volts
Expected
Output
Output
Obtained
Vo
Step 2: Adjust the potentiometer to the indicated analog voltages in Table 6.2 and record in the table the
binary output that was actually obtained. Ignore the column labeled “Vo”
Describe any variations in the expected results: ___________________________________________________
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The resolution obtained for a single bit is also expressed as a percentage of the full-scale voltage that is
determined by a single bit. That is,
%Res =
1
x100
2N
where N is the number of binary bits in the conversion.
Based upon this equation, what is the percentage of full-scale resolution obtained with this 8-bit analog to
digital converter?
________________________ans.
Instructor’s Signature: _______________________________
Date: ____________
Operative: ____
Inoperative: ____
PART TWO. Digital to Analog Conversion
In order to convert an 8-bit digital value to its respective analog value, we will use a DAC0808 chip. This
device utilizes an R-2R ladder network to drive a current summing junction resulting in an appropriate analog
value. We will use the circuit of Part One to provide the digital signals for our D/A conversion.
Step 3: Construct the circuit of Figure 6.3.
FIGURE 6.3. Addition of Digital to Analog Conversion Circuit
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Note that the DAC0808 D/A converter generates a varying analog current at its output, Io, on Pin-4. The fullscale value of this current is determined by the current input at Vref+, on Pin 14, (2.00 mA.) Consequently, this
current must be converted back to a voltage to obtain Vo. To do this, an LM741 operational amplifier is
configured as a current to voltage converter using the same size current loop (5.1K resistor.)
Step 4: For each entry in Table 6.2, measure and record the output voltage (Vo) from the Op-Amp (Pin
6). Explain any variation between the input analog value and the output analog value.
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Instructor’s Signature: _______________________________
Date: ____________
Operative: ___
Inoperative: ___
PART THREE. Ambient Temperature Measurement.
Figure 6.4 illustrates the replacement of the 100K potentiometer with an LM34 temperature sensor and an
operational amplifier as the voltage source for the A/D converter. Note that nothing else has changed from
Figure 6.3. The LM34 temperature sensor generates a voltage output equal to 10mV per degree Fahrenheit with
an error of less than .1 degrees. Consequently, at room temperature of 72°F, the sensor will generate 200.0mV.
In order to measure a voltage that is within the full-scale input of the A/D converter, an LM741 operational
amplifier is configured as a non-inverting amplifier with a gain of 2, resulting in a representation of 2.00V for a
temperature of 72°F. Note that the pin-out shown for the LM34 is the bottom view. Note also that the inputs to
the LM741 are configured for non-inverting amplification.
FIGURE 6.4. Temperature Sensing Circuit.
Step 5: Construct the circuit of Figure 6.4, removing the potentiometer of Figure 6.3, and replacing it
with the LM34 temperature sensor and the LM741 operational amplifier circuit. Record in the first line
of Table 6.3 the (1) voltage at Pin 2 of the LM34, (2) the corresponding ambient temperature it
represents, (3) the voltage at Pin 6 of the ADC0804 verifying the gain of the input amplifier, (4) the
digital value at the inputs of the DAC0808, and (5) the voltage at Pin 6 of the LM741 at the output of the
circuit. You may have to wait a little while for the temperature to stabilize.
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TABLE 6.3 Temperature Acquisition Data
(1)
(2)
(3)
LM33 Output
Ambient
A/D Input
Voltage
Voltage
Temperature, °F
(4)
Digital
Equivalent
(5)
Output
Voltage, Vo
Step 6: Heat the temperature sensor by grasping it with your fingers. As the temperature indicates
higher values, add respective lines of data to Table 6.3.
Record here your comments concerning the circuit operation and reasons for unexpected results. (Why is the
temperature seemingly higher than it should be?, etc.)
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Instructor’s Signature: _______________________________
Date: ____________
Operative: ___
Inoperative: ___
NOTES:
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Parts List:
1. Logic Trainer
2. DC Power Supply
3. ADC0804 A-to-D
Converter
4. DAC0808 D-to-A
Converter
5. LM741 Op-Amp
6. 100kΩ Potentiometer
7. 20KΩ Resistor
8. 10KΩ Resistor
9. 5.1KΩ Resistor
10. 2.2KΩ Resistor
11. 150pF Capacitor
12. 0.1µF Capacitor
13. LM34
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