Lab 4

Lab # 04 Introduction and implementation of ADC and DAC with microcontroller
Lab # 04
Introduction and implementation of ADC and DAC

with Arduino
Objectives
• Understand Analog to Digital and Digital to Analog Converters.

• Implementing Analog to Digital Converter and Digital to Analog Converters.
Tools
• Arduino
• Proteus ISIS
Pre Lab
Please read the theoretical background of the using DAC in our lab.
Analog to Digital Converter

In almost all digital systems there is a frequent need to convert analog signals generated by analog
devices such as microphone, sensors and potentiometers into digital values that can be stored and
processed. To serve this purpose analog to digital converters are used.
There are various analog to digital convertors: high speed ones, precise ones and economical ones
and so on. Most of them can be directly connected to microcontroller as shown below.
From the processor-ADC interface as shown in figure above, we conclude that an activation signal
is required to be generated by the processor to prompt the ADC to start conversion. When the ADC
finishes conversion, it should generate a ‘conversion complete’ signal to tell the processor that the
conversion is done. This signal can interrupt microcontroller or be polled by the processor.
You want to read the voltage on an analog pin. Perhaps you want a reading from a potentiometer
or a device or sensor that provides a voltage between 0 and 5 volts
This sketch reads the voltage on an analog pin and flashes an LED in a proportional
rate to the value returned from the analogRead function.

Connecting a potentiometer to Arduino
In-Lab Task 1:
Interface variable resistor at analog pin of Arduino, on virtual terminal please show it’s equivalent
digital value, percentage out of max value and equivalent analog value received.
In-Lab Task 2:
Interface 24Volts DC source with Arduino analog pin, use voltage divider circuit to calculate the
identified voltage when varied from 0V – 24V.
Digital to Analog Convertors

In digital systems data can be processed only if it is represented in digital format (0,1) while the
majority of real world applications require analog signals. This leads to the conversion between
analog and digital formats. For example most audio signals are stored in digital form and in order
to be heard through signal they must be converted to analog signal.
Digital to analog converters are an interface between the abstract digital world and analog real life.
Usually DACs are used to convert 8 bit digital data into an analog signal. If greater precision is
needed, chips with 12-bit, 14-bit or 16-bit data convertibility are available.
Lets assume that we have a 3-bit DAC that has three digital lines (D2,D1,D0) and has one output
analog line. Assume that we assign references of analog output to Vref- = 0V and Vref+= 1V, then
the input/output relation will be as shown below:
From the table we draw the following points:

• The three bit DAC has 2^3 = 8 possible combinations. If a converter has n input lines
then it has 2^n input combinations.
• If the high and low references for analog output are V1 and V2 respectively then the
change in the output corresponding to each increment of the n-bit digital input is (V2-
V1)/(2^n).This value is known as resolution. In 3-bit case and V1,V2: 0 and 1 volts
resolution becomes 1/8 volts.
• When MSB is 1 and other bits are zeros, the output analog is half of the full scale. In
3-bit DAC example it leads to 0.5 volts.
• For the maximum input, the output is equal to the value of full scale minus the value
of resolution. In the 3-bit example it gives 1-1/8=7/8 V.
DAC0808 8-bit Digital to Analog Conversion IC

The DAC0808 is an 8-bit digital-to-analog converter (DAC) featuring a full scale output current
settling time of 150 ns while dissipating only 33 mW with ±5V supplies.
R2R Digital to Analog Convertor

A basic R-2R resistor ladder network is shown in Figure below. Bit D7 MSB (most significant bit)
to Bit D0 LSB (least significant bit) are driven from digital logic gates. Ideally, the bits are
switched between 0 volts (logic 0) and Vref (logic 1). The R-2R network causes the digital bits to
be weighted in their contribution to the output voltage Vout.
For a digital value VAL, of a R-2R DAC of N bits of 0 V/Vref, the output voltage Vout is:
Vout = Vref × VAL / 2N

In the example shown, N = 8 and hence 2N = 256. With Vref = 5 V (typical TTL logic 1 voltage),
VAL will vary between 00000000, VAL = 0 and 11111111, VAL = 255.
Minimum (single step) VAL = 1, we have
Vout = 5 × 1 / 256 = 0.0195 volts

Maximum output (11111111 VAL = 255, we have
Vout = 5 × 255 / 256 = 4.980 volts
In-Lab Task 3:
Use the same principle as before to identify 0V-5V from output of variable resistor, but the output
of Arduino is digital 10-bit digital value, use R2R ladder as shown above to calculate the digital
to analog output, which will eventually be the same as given at input. Please code and show circuit.
Post-Lab Task 1:
Let’s suppose the input signal you want to monitor using circuit that uses Arduino is -5V to 5V
peak to peak sinusoidal wave with 100Hz frequency, please identify the circuit and Arduino code
which takes this input sinusoid and regenerates this signal again at the out which can be verified
on oscilloscope.
Critical Analysis / Conclusion

(By Student about Learning from the Lab)
Lab Assessment
Pre Lab /1
In Lab /5
Data Analysis /4 /10
Data
Post Lab /4 /4
Presentation
Writing Style /4
Instructor Signature and Comments

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Lab # 04 Introduction and implementation of ADC and DAC with microcontroller

Introduction and implementation of ADC and DAC

• Understand Analog to Digital and Digital to Analog Converters.

Analog to Digital Converter

rate to the value returned from the analogRead function.

Connecting a potentiometer to Arduino

Digital to Analog Convertors

From the table we draw the following points:

DAC0808 8-bit Digital to Analog Conversion IC

R2R Digital to Analog Convertor

Vout = Vref × VAL / 2N

Vout = 5 × 1 / 256 = 0.0195 volts

Vout = 5 × 255 / 256 = 4.980 volts

Critical Analysis / Conclusion

Data Analysis /4 /10

Instructor Signature and Comments

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