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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