Presentation on theme: "Digital to Analog Converters"— Presentation transcript:
1 Digital to Analog Converters Tyler SmithBrent NelsonJerry Jackson10/14/04
2 Topics Discussed What is a DAC? Choosing a DAC Resistor String DAC Weighted Resistor DACR-2R DACPWMDAC associated errorsApplicationsConclusion
3 What is a DACA digital to analog converter (DAC) is a device that converts digital numbers (binary) into an analog voltage or current output.
4 Choosing a DACThere are six main parameters that should be considered when choosing a DAC for a particular project.Reference VoltageResolutionLinearitySpeedSettling timeError
5 Choosing a DAC Reference Voltage To a large extent the output properties of a DAC are determined by the reference voltage.Multiplier DAC – The reference voltage is constant and is set by the manufacturer.Non-Multiplier DAC – The reference voltage can be changed during operation.
6 Choosing a DAC Resolution The resolution is the amount of voltage rise created by increasing the LSB of the input by 1. This voltage value is a function of the number of input bits and the reference voltage value.- Increasing the number of bits results in a finer resolution- Most DACs in the bit range
7 Choosing a DAC Linearity The linearity is the relationship between the output voltage and the digital signal input.
8 Choosing a DACSpeedUsually specified as the conversion rate or sampling rate. It is the rate at which the input register is cycled through in the DAC.High speed DACs are defined as operating at greater than 1 millisecond per sample (1MHz).Some state of the art bit DAC can reach speeds of 1GHzThe conversion of the digital input signal is limited by the clock speed of the input signal and the settling time of the DAC.
9 Choosing a DAC Settling Time Ideally a DAC would instantaneously change its output value when the digital input would change. However, in a real DAC it takes time for the DAC to reach the actual expected output value.
10 Choosing a DACErrorThere are multiple sources of error in computing the analog output.
11 Example of a DAC - AD7224An example of a DAC would be the Analog Devices AD 7224 D/A Converter. The AD7224 is a precision 8-bit, voltage-output, digital-to-analog converter with an output amplifier.Specifications:DAC Type – R-2R Voltage OutInput – Dual 8 BitReference voltage – Non-Multiplier2v – 12.5vSettling Time - 7μsCost - Under $4.00
14 Resistor String DAC 3 Bit Resistor String DAC Components of a String DACResistor StringSelection SwitchesOpamp
15 Resistor String DACHow many internal components would be needed to create an 8 bit resistor string DAC?Number of Resistors =Number of Switches =Impractical for a DAC with more than a couple bits input.
16 Weighted Resistor DAC Basic Idea: Use a summing op-amp circuit Use transistors to switch between high and groundUse resistors scaled by two to divide voltage on each branch by a power of two-+R2R4R2nRR/2Vout
18 Weighted Resistor Summary AdvantagesSimpleFastDisadvantagesNeed large range of resistor values (2000:1 for 12-bit) with high precision in low resistor valuesNeed very small switch resistancesSummaryUse in fast, low-precision converter
19 R-2R DAC Basic Idea: Use only 2 resistor values Use equal resistances in parallel to halve the resistanceCreates a series of voltage dividers cutting voltages in halfAnother summing op-amp
20 R-2R Example Digital word = 001 V0 has two 2R resistances in parallel connected to groundEquivalent of R between V0 and groundV1 now has a resistance R to V0 and R to groundV0 = V1/2V1 has two 2R resistances to groundEquivalent of R between V1 and groundV2 now has a resistance R to V1 and R to groundV1 = V2/2V2 = VrefV0 = V2/4V0 = Vref/4Vout = -V0/2Vout = -Vref/8
21 R-2R Summary Advantages Summary Only 2 resistor values Better than weighted resistor DAC
22 Pulse Width Modulation Approximate analog signal by switching on/off at high frequencyIntegral of output voltage from PWM ideally is the same as integral of desired output voltageN-bit digital words updated at rate fDAC clock must run at rate 2n*fExample:Desired output = 7V, supply voltage = 10VOperate 10V at 70% duty cycle to approximate 7VIn practice: use counter, comparator, clock, integrator
23 PWM Summary Advantages Disadvantages Summary All digital Cheap High sampling rate requiredSensitive to clock variationsSummaryBest when load is a (relatively) slowly responding system
36 Programmable gain OpAmps Voltage controlled Amplifier (digital input, Vref as control)Digitally operated attenuators (Vref as input, digital control)
37 Programmable Filters Integrate DACs in filters Variable cutoff frequency commanded by a digital signal
38 DAC ApplicationsUsed at the end of a digital processing chain when analog signals are requiredDigital AudioCD Players, digital telephones, etc.Industrial Control SystemsMotor speed, valves, etc.Waveform Function GeneratorsCruise Control
39 ReferencesAlciatore, “Introduction to Mechatronics and Measurement Systems,” McGraw-Hill, 2003Horowitz and Hill, “The Art of Electronics,” Cambridge University Press, 2nd EdAnalog Devices AD 7224 DAC General Overview and SpecificationsD/A Converter Fundamentals and Definition Of TermsData Converter Fundamentals