ECE 2799 ECE 2799 Electrical and Computer Engineering DesignElectrical and Computer Engineering Design

ANALOG to DIGITAL ANALOG to DIGITAL

CONVERSION CONVERSION

Prof. BitarProf. Bitar

Last Update: 04-08-11

ADC SymbolADC Symbol(Parallel Output)(Parallel Output)

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ADC N Bits

D0

DN

AIN

Important ADC ParametersImportant ADC Parameters

ResolutionResolution AccuracyAccuracy Conversion TimeConversion Time

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ADC ResolutionADC Resolution Number of BitsNumber of Bits

Example: N=8 BitsExample: N=8 Bits

Number of Discrete LevelsNumber of Discrete Levels 22NN = 2 = 288 = 256 = 256

Number of Steps (Intervals)Number of Steps (Intervals) (2(2NN-1) = 255-1) = 255

Voltage per Step Voltage per Step ΔV = VΔV = VFullScaleRange FullScaleRange / (2/ (2NN-1) OR V-1) OR VFSR FSR / 2/ 2NN

Method 1: ΔV = 5V / 255 = 19.608 mVMethod 1: ΔV = 5V / 255 = 19.608 mV Method 2: ΔV = 5V / 256 = 19.531 mVMethod 2: ΔV = 5V / 256 = 19.531 mV

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Conversion TableConversion Table(for ΔV = 5V / 256 = 19.531 mV )(for ΔV = 5V / 256 = 19.531 mV )

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Dec. Hex Binary V (mV)

0 00 0000 0000

0.000

1 01 0000 0001

19.531

2 02 0000 0010

39.063

.

.128

.

.

.

.80..

.

.1000 0000

.

.

.

.2.500 V

.

.

255 FF 1111 1111

4.980 V Q: How would you digitize 20mV?Q: How would you digitize 20mV?

Voltage to Binary Voltage to Binary Transfer CharacteristicTransfer Characteristic

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Courtesy: Analog Devices AD7819 Datasheet

Accuracy: Quantization Accuracy: Quantization ErrorError

Quantization Error is often equal to ½ Quantization Error is often equal to ½ the Least-Significant-Bit voltage.the Least-Significant-Bit voltage.

In our example, that would be, In our example, that would be, 19.531mV / 2 = 9.766mV19.531mV / 2 = 9.766mV

As a percentage of VAs a percentage of VFSRFSR, that would , that would be,be, 9.766mV / 5.000 x 100 = 0.195%9.766mV / 5.000 x 100 = 0.195%

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Conversion TimeConversion Time The time required for the ADC to convert a The time required for the ADC to convert a stablestable analog input voltage to a binary analog input voltage to a binary number. (Implies the use of a S/H circuit.)number. (Implies the use of a S/H circuit.)

Depends greatly on the architecture of the Depends greatly on the architecture of the ADC. There are different types.ADC. There are different types.

SAR (Successive Approximation Register)SAR (Successive Approximation Register) Sigma-DeltaSigma-Delta FlashFlash

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How do You Choose ?How do You Choose ?

Well, how often do you need to Well, how often do you need to sample your analog waveform, if you sample your analog waveform, if you want to reproduce it accurately?want to reproduce it accurately?

Nyquist Rate (minimum) Nyquist Rate (minimum) For audio, typically 44.1 kSPS is used.For audio, typically 44.1 kSPS is used. That’s 22.67µsec per sample, so the That’s 22.67µsec per sample, so the

conversion time has to be faster than conversion time has to be faster than that!that!

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A Look at Two ADC’sA Look at Two ADC’s

Analog Devices AD7819 Analog Devices AD7819

Texas Instruments MSP430xx Texas Instruments MSP430xx series microcontrollers with built-series microcontrollers with built-in ADC’sin ADC’s

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AD7819 8-Bit Parallel DACAD7819 8-Bit Parallel DAC

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AD7819 Block DiagramAD7819 Block Diagram

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AD7819 Pin DescriptionsAD7819 Pin Descriptions

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Package Pin AssignmentsPackage Pin Assignments

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AD7819 Converter AD7819 Converter OperationOperation

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AD7819 Typical CircuitAD7819 Typical Circuit

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AD7819AD7819Equivalent Analog Input Equivalent Analog Input

ModelModel

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AD7819 DC Acquisition AD7819 DC Acquisition TimeTime

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AD7819AD7819Transfer CharacteristicTransfer Characteristic

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AD7819 Microcontroller AD7819 Microcontroller InterfacingInterfacing

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AD7819 Timing and ControlAD7819 Timing and Control

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MSP430xx ADC FeaturesMSP430xx ADC Features Maximum conversion rate 200 kspsMaximum conversion rate 200 ksps Monotonic 10-bit converter Monotonic 10-bit converter Internal sample-and-holdInternal sample-and-hold Timer control optionTimer control option Optional on-chip reference (1.5 V or 2.5 V)Optional on-chip reference (1.5 V or 2.5 V) Up to 12 inputs (depends on chip)Up to 12 inputs (depends on chip) Internal temp referenceInternal temp reference Selectable clock sourceSelectable clock source Multiple conversion modesMultiple conversion modes Auto-conversion storage / data transfer Auto-conversion storage / data transfer

modesmodes S. J. Bitar - 2010S. J. Bitar - 2010

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Conversion FormulaConversion Formula

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Analog MultiplexerAnalog Multiplexer

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Sample TimingSample Timing

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Analog Input ModelAnalog Input Model

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Conversion Mode SummaryConversion Mode Summary

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Conversion State DiagramConversion State Diagram

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Transferring Data to Transferring Data to MemoryMemory

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Interrupt Driven ConversionInterrupt Driven Conversion

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ADC RegistersADC Registers

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Example: Control Register 1Example: Control Register 1

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Example: Input Channel Example: Input Channel SelectSelect

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