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.
Conversion Methods
* Ladder Comparison
* Successive Approximation
* Slope Integration
* Flash Comparison
Slope integration
* 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
02468
101214161820
0 2 4 6 8 10 12 14 16
Time
Vo
lta
ge
acc
ross
th
e c
ap
aci
tor
Vin
Counting time
ININC
R
S Enable
N-bit OutputN-bit Output
Oscillator Clk
StartConversion
StartConversion
StartConversion
StartConversion
Q
Co
un
ter
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
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
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
Dual slope integrating ADC
Operation
Integrate
Reset and
integrate
Thus
1
0
T
iv dt2
0
t
rV dt
1 ( ) 2i AVG rT v t V
2( )
1i AVG r
tv V
T
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
Very high speed
conversion
Up to 100MHz for 8 bit
resolution
Video, Radar, Digital
Oscilloscope
Resolution is limited
Large number of
comparator in IC
Single step Conversion
2n –1 comparator
Precision Resistive
Network
Encoder
Software Implementation
Implementing software
using Microprocessor
Counting
Shifting
Inverting
Code conversion
…..
Limited practical use
Availability if Good
performance with very
responsible cost
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.
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