Download Analog to Digital conversion

Analog to Digital conversion
Introduction
* The process of converting an analog signal into an equivalent
digital signal is known as Analog to Digital (AD) conversion.
* The conversion time depends upon the frequency of input
clock signal.
Analog to digital conversion
03 07 10 14 09 02 00 04
Conversion Methods
* Ladder Comparison
* Successive Approximation
* Slope Integration
* Flash Comparison
Ladder Comparison
Successive Approximation
*
*
*
*
Charge a capacitor at constant
current
Count clock ticks
Stop when the capacitor voltage
matches the input
Cannot achieve high resolution
* Capacitor and/or comparator
Start
Conversion
Voltage accross the capacitor
Slope integration
20
18
16
14
12
10
8
6
4
2
0
Vin
0
Start
Conversion
2
4
6
8
10
Counting time
S
Q
C
IN
Oscillator
14
16
Time
Enable
Counter
R
12
Clk
N-bit Output
Flash comparison
* If N is the number of bits in the
output word….
* Then 2N comparators will be required.
* With modern microelectronics this is
quite possible, but will be expensive.
ADC Essentials
Basic I/O Relationship
– ADC is Rationing System
• x = Analog input /
Reference
– Fraction: 0 ~ 1
ADC Essentials
n bits ADC
– Number of discrete output
level : 2n
– Quantum
• LSB size
• Q = LSB = FS / 2n
Quantization Error
– 1/2 LSB
– Reduced by increasing n
Converter Errors
Offset Error
Gain Error
Converter Errors
Integral Linearity Error
Differential Linearity Error
A/D conversion Techniques
 Counter or Tracking ADC
 Successive Approximation ADC
 Most Commonly Used
 Dual Slop Integrating ADC
 Voltage to Frequency ADC
 Parallel or Flash ADC
 Fast Conversion
 Software Implementation
 Shaft Encoder
Counter type ADC
Block Diagram
Wave form
Counter type ADC
Operation
– Reset and Start Counter
– DAC convert Digital output of Counter to Analog signal
– Compare Analog input and Output of DAC
• Vi < VDAC
– Continue counting
• Vi = VDAC
– Stop counting
– Digital Output = Output of Counter
Tracking type ADC
Tracking or servo type
– Using Up/Down Counter
to
track
input
signal
continuously
• For slow varying input
Successive Approximation ADC
 Most Commonly used in medium to
high speed Converters
 Based on approximating the input signal
with binary code and then successively
revising this approximation until best
approximation is achieved
 SAR(Successive Approximation Register)
holds the current binary value
Block Diagram
Successive Approximation ADC
Circuit wave form
Logic Flow
Dual slope integrating ADC
Operation
 Integrate

T1
0
vi dt
 Reset and integrate
 Thus vi ( AVG )  Vr

t2
T1
T1vi ( AVG )  t2Vr

t2
0
Vr dt
Dual slope integrating ADC
Excellent Noise Rejection
 High frequency noise
cancelled out by integration
 Proper T1 eliminates line
noise
 Easy to obtain good
resolution
Low speed
 If T1 = 60Hz, converter
throughput rate < 30
samples/s
Voltage to Frequency ADC
VFC (Voltage to Frequency Converter)
 Convert analog input voltage to
train of pulses
Counter
 Generates Digital output by
counting pulses over a fixed
interval of time
Voltage to Frequency ADC
 Low speed
 Good noise immunity
 High resolution
• For slow varying signal
• With long conversion time
 Applicable to remote data sensing in noisy environments
• Digital transmission over a long distance
Parallel or Flash ADC
Single step Conversion
Very high speed conversion
 Up to 100MHz for 8 bit
resolution
 Video, Radar, Digital
Oscilloscope
Resolution is limited
 Large number of comparator
in IC
 2n –1 comparator
 Precision Resistive Network
 Encoder
Software Implementation
Implementing
software
Microprocessor
 Counting
using
Limited practical use
 Availability if Good
 Shifting
performance with very
 Inverting
responsible cost
 Code conversion
 …..
Shaft Encoder
Electromechanical ADC
 Convert shaft angle to digital
output
Encoding
 Optical or magnetic sensor
Applications
 Machine tools, industrial
robotics, Numerical control
Shaft Encoder
Binary Encoder
 Misalignment of mechanism
causes large error.
Ex: 011 -> 111 (180 deg)
Gray Encoder
 Misalignment causes of 1 LSB
error.
The End
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