Digital to Analog Converters DAC

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Digital to Analog Converters (DAC)

• Ryan Johnson
• Reinhard Powell
• Erick Rios

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What is a DAC?

• A digital-to-analog converter (DAC) is a
  circuit that produces an analog current or
  voltage that is proportional to an analog
  reference (voltage or current) and an N-bit
  binary word.
• Vout k x Vref x (Binary Word)

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DAC Configuration
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DAC Types

• Multiplying DAC
• - reference source is external to the DAC
  package
• Nonmultiplying DAC
• - reference source is inside the DAC package

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DAC Circuit Types

• Two types of DAC Circuits
• 1. Binary weighted
• 2. R-2R ladder

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4-Bit Binary Weighted DAC
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Binary Weighted Principles
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Principles Contd

• V0 -RfI0
• V0 voltage output from amplifier
• Rf feedback resistance
• Note For a gain of 1, R 2Rf

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Example

• Find output voltage and current for a binary
  weighted resistor DAC of 4 bits where
• R 10 k Ohms, Rf 5 k Ohms and Vref 10
  Volts. Applied binary word is 1001.

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Solution Contd
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Solution Contd
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Solution Contd

• Binary input 1001 9
• From example, V0 5.625V
• V0/Vref 5.625V/10V 9/16

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Limitations of the Binary Weighted DAC

• Has problems if bit length is longer than 8 bits
• For example, if R 10 k Ohms
• R8 28-1(10 k Ohms) 1280 k Ohms
• If Vref 10 Volts,
• I8 10V/1280 k Ohms 7.8 ?A
• Op-amps to handle those currents are rare
• and expensive.

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Limitations Contd

• If R 10 Ohms and Vref 10 V
• I Vref/R 10V/10 Ohms 1 A
• This current is more than a typical op-amp
• can handle.