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Lecture 15: Digital to Analog
Converters
Lecturers:
Professor John Devlin
Mr Robert Ross
Overview
• Introduction to DACs
• Types of DACs
• Applications of DACs
• Further Reading:
– R.J. Tocci, Digital Systems, Principles and
Applications, Prentice Hall (Chapter 10)
Interfacing with the analog world
Introduction DACs
• The real world is full of analog, continuous
signals
• Microprocessors use digital electronics (discrete
binary values) for processing
• Digital to Analog Converters (DAC or D/A)
convert discrete digital numbers into continuouslike analog signals – allowing digital electronics
to output real world analog signals
• DAC’s are ‘Mixed Signal Devices’ as they
combine analog circuits with DSP
• Reverse of the operation of the ADC (Analog to
Digital Converter)
Digital to analog conversion
• Convert a digital value to a proportional current or
voltage.
• Vref is used to set the full scale output.
DAC Specifications
• Resolution: The smallest possible change
that can occur in the analog output due to
a change in the digital input (Step Size)
• Offset Error: Signal may be artificially
offset, yet remain linear
• Linearity: How much two adjacent analog
values deviate from the ideal 1LSB step
DAC Specifications
• Monotonicity: Direction of change the
same – input increases = output increases
Monotonic Function Non-Monotonic Function
Resolution
Calculating the Resolution
Resolution =
K
A fs
2 1
n
Afs: Analog full scale
voltage
n: Number of bits
Calculations
Resolution =
K
A fs
2n  1
Afs: Analog full scale
voltage
n: Number of bits
Analog Output = K X Digital Input
Digital Input = Analog Output / K
Types of DACs
•
•
•
•
R-2R Ladder
Pulse Width Modulation (PWM)
Binary Weighted
Thermometer coded
Pulse Width Modulation DAC
• Pulse Width Modulation (PWM)
• The duty cycle of a square wave is
modulated, thereby altering the average
voltage of the waveform
• Period remains constant
R-2R Ladder
• Resistor Ladders (or R2R Ladders) is the
simplest DAC
• Operates as an array of
voltage dividers
• Requires one output pin
for each bit of resolution
• Requires well matched
resistors
• Voltage spikes at major
crossings (eg. 01111>10000)
Binary Weighted
•
•
•
•
Contains a resistor for each bit of the DAC
Resistors are arranged in binary decades
All resistors fed into a summing point
Difficult to produce – with accurate
resistors for each binary bit
Binary Weighted
Thermometer coded
• A resistor or current source for each
possible value of the DAC Output
• 8 bit DAC would have 256 resistors
• Fast and high precision, but very
expensive
DAC Applications
• Audio
– CD Players
– MP3 Players
• Video
– DVD – Analog TV
• Signal Generators
– Ramp Function
– Sine Wave
Waveform Generation
• Waveforms can be generated by supplying
digital representations of the desired
analog output
Summary
• Digital to Analog converters allow digital
electronics to output signals which are
similar to real world continuous signals
• Pulse Width Modulation is a simple and
widely used high resolution technique for
implementing DACs