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Lecture 15: Digital to Analog Converters

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Title: Lecture 15: Digital to Analog Converters


1
Lecture 15 Digital to Analog Converters
  • Lecturers
  • Professor John Devlin
  • Mr Robert Ross

2
Overview
  • Introduction to DACs
  • Types of DACs
  • Applications of DACs
  • Further Reading
  • R.J. Tocci, Digital Systems, Principles and
    Applications, Prentice Hall (Chapter 10)

3
Interfacing with the analog world
4
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 continuous-like
    analog signals allowing digital electronics to
    output real world analog signals
  • DACs are Mixed Signal Devices as they combine
    analog circuits with DSP
  • Reverse of the operation of the ADC (Analog to
    Digital Converter)

5
Digital to analog conversion
  • Convert a digital value to a proportional current
    or voltage.
  • Vref is used to set the full scale output.

6
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

7
DAC Specifications
  • Monotonicity Direction of change the same
    input increases output increases

Monotonic Function
Non-Monotonic Function
8
Resolution
  • Calculating the Resolution

Afs Analog full scale voltage n Number of bits
Resolution
9
Calculations
Afs Analog full scale voltage n Number of bits
Resolution
Analog Output K X Digital Input
Digital Input Analog Output / K
10
Types of DACs
  • R-2R Ladder
  • Pulse Width Modulation (PWM)
  • Binary Weighted
  • Thermometer coded

11
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

12
R-2R Ladder
  • Resistor Ladders (or R-2R 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-gt10000)

13
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

14
Binary Weighted
15
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

16
DAC Applications
  • Audio
  • CD Players
  • MP3 Players
  • Video
  • DVD Analog TV
  • Signal Generators
  • Ramp Function
  • Sine Wave

17
Waveform Generation
  • Waveforms can be generated by supplying digital
    representations of the desired analog output

18
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
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