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Lecture 15: Digital to Analog Converters

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).

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Lecture 15: Digital to Analog Converters

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  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 • DAC’s 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. 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->10000) R-2R Ladder

  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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