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ECE 2799 Electrical and Computer Engineering Design. ANALOG to DIGITAL CONVERSION Prof. Bitar Last Update: 04-08-11. ADC Symbol (Parallel Output). D N. ADC. A IN. N Bits. D 0. Important ADC Parameters. Resolution Accuracy Conversion Time. ADC Resolution. Number of Bits
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ECE 2799 Electrical and Computer Engineering Design ANALOG to DIGITAL CONVERSION Prof. Bitar Last Update: 04-08-11
ADC Symbol(Parallel Output) DN ADC AIN N Bits D0 S. J. Bitar - 2010
Important ADC Parameters • Resolution • Accuracy • Conversion Time S. J. Bitar - 2010
ADC Resolution • Number of Bits • Example: N=8 Bits • Number of Discrete Levels • 2N = 28 = 256 • Number of Steps (Intervals) • (2N-1) = 255 • Voltage per Step • ΔV = VFullScaleRange/ (2N-1) OR VFSR/ 2N • Method 1: ΔV = 5V / 255 = 19.608 mV • Method 2: ΔV = 5V / 256 = 19.531 mV S. J. Bitar - 2010
Conversion Table(for ΔV = 5V / 256 = 19.531 mV ) Q: How would you digitize 20mV? S. J. Bitar - 2010
Voltage to Binary Transfer Characteristic Courtesy: Analog Devices AD7819 Datasheet S. J. Bitar - 2010
Accuracy: Quantization Error • Quantization Error is often equal to ½ the Least-Significant-Bit voltage. • In our example, that would be, • 19.531mV / 2 = 9.766mV • As a percentage of VFSR, that would be, • 9.766mV / 5.000 x 100 = 0.195% S. J. Bitar - 2010
Conversion Time • The time required for the ADC to convert a stable analog input voltage to a binary number. (Implies the use of a S/H circuit.) • Depends greatly on the architecture of the ADC. There are different types. • SAR (Successive Approximation Register) • Sigma-Delta • Flash S. J. Bitar - 2010
How do You Choose ? • Well, how often do you need to sample your analog waveform, if you want to reproduce it accurately? • Nyquist Rate (minimum) • For audio, typically 44.1 kSPS is used. • That’s 22.67µsec per sample, so the conversion time has to be faster than that! S. J. Bitar - 2010
A Look at Two ADC’s • Analog Devices AD7819 • Texas Instruments MSP430xx series microcontrollers with built-in ADC’s S. J. Bitar - 2010
AD7819 8-Bit Parallel DAC S. J. Bitar - 2010
AD7819 Block Diagram S. J. Bitar - 2010
AD7819 Pin Descriptions S. J. Bitar - 2010
Package Pin Assignments S. J. Bitar - 2010
AD7819 Converter Operation S. J. Bitar - 2010
AD7819 Typical Circuit S. J. Bitar - 2010
AD7819Equivalent Analog Input Model S. J. Bitar - 2010
AD7819 DC Acquisition Time S. J. Bitar - 2010
AD7819Transfer Characteristic S. J. Bitar - 2010
AD7819 Microcontroller Interfacing S. J. Bitar - 2010
AD7819 Timing and Control S. J. Bitar - 2010
MSP430xx ADC Features • Maximum conversion rate 200 ksps • Monotonic 10-bit converter • Internal sample-and-hold • Timer control option • Optional on-chip reference (1.5 V or 2.5 V) • Up to 12 inputs (depends on chip) • Internal temp reference • Selectable clock source • Multiple conversion modes • Auto-conversion storage / data transfer modes S. J. Bitar - 2010
Conversion Formula S. J. Bitar - 2010
Analog Multiplexer S. J. Bitar - 2010
Sample Timing S. J. Bitar - 2010
Analog Input Model S. J. Bitar - 2010
Conversion Mode Summary S. J. Bitar - 2010
Conversion State Diagram S. J. Bitar - 2010
Transferring Data to Memory S. J. Bitar - 2010
Interrupt Driven Conversion S. J. Bitar - 2010
ADC Registers S. J. Bitar - 2010
Example: Control Register 1 S. J. Bitar - 2010
Example: Input Channel Select S. J. Bitar - 2010
