Transcript Digital Processing - Electrical & Computer Engineering

ECE 2799
Electrical and Computer Engineering Design
ANALOG to DIGITAL
CONVERSION
Prof. Bitar
Last Update: 11-12-12
ADC Symbol
(Parallel Output)
DN
AIN
N Bits
ADC
D0
S. J. Bitar - 2010
Important ADC Parameters
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Resolution
Accuracy
Conversion Time
S. J. Bitar - 2010
ADC Resolution
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Number of Bits
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Number of Discrete Levels
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Example: N=8 Bits
2N = 28 = 256
Voltage per Step
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ΔV = VFullScaleRange / 2N
= 5V / 256
= 19.53125 mV
S. J. Bitar - 2010
Conversion Table
(for ΔV = 1LSB = 5V / 256 = 19.53125 mV )
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?
S. J. Bitar - 2010
Voltage to Binary
Transfer Characteristic
Courtesy: Analog Devices AD7819 Datasheet
S. J. Bitar - 2010
Accuracy: Quantization Error
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Quantization Error is often equal to ½ the
Least-Significant-Bit voltage.
In our example, that would be,
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19.531mV / 2 = 9.766mV
As a percentage of VFSR, that would be,
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9.766mV / 5.000 x 100 = 0.195%
S. J. Bitar - 2010
Conversion Time
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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.
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SAR (Successive Approximation Register)
Sigma-Delta
Flash
S. J. Bitar - 2010
How do You Choose ?
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Well, how often do you need to sample
your analog waveform, if you want to
reproduce it accurately?
Nyquist Rate (minimum)
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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
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Analog Devices AD7819
Texas Instruments MSP430xx series
microcontrollers with built-in ADC’s
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AD7819 8-Bit Parallel DAC
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AD7819 Block Diagram
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AD7819 Pin Descriptions
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Package Pin Assignments
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AD7819 Converter Operation
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AD7819 Typical Circuit
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AD7819
Equivalent Analog Input Model
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AD7819 DC Acquisition Time
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AD7819
Transfer Characteristic
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AD7819 Microcontroller
Interfacing
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AD7819 Timing and Control
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MSP430xx ADC Features
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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
S. J. Bitar - 2010
Conversion Formula
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Analog Multiplexer
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Sample Timing
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Analog Input Model
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Conversion Mode Summary
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Conversion State Diagram
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Transferring Data to Memory
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Interrupt Driven Conversion
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ADC Registers
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Example: Control Register 1
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Example: Input Channel Select
S. J. Bitar - 2010