Lecture 15: Digital to Analog Converters

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Lecture 15 Digital to Analog Converters

• Lecturers
• Professor John Devlin
• Mr Robert Ross

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Overview

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

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Interfacing with the analog world
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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)

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Digital to analog conversion

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

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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

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DAC Specifications

• Monotonicity Direction of change the same
  input increases output increases

Monotonic Function
Non-Monotonic Function
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Resolution

• Calculating the Resolution

Afs Analog full scale voltage n Number of bits
Resolution
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Calculations
Afs Analog full scale voltage n Number of bits
Resolution
Analog Output K X Digital Input
Digital Input Analog Output / K
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Types of DACs

• R-2R Ladder
• Pulse Width Modulation (PWM)
• Binary Weighted
• Thermometer coded

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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

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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)

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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

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Binary Weighted
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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

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DAC Applications

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

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Waveform Generation

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

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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