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

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

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0 2 4 6 8 10 12 14 16

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ININC

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N-bit OutputN-bit Output

Oscillator Clk

StartConversion

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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 ErrorDifferential 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 DiagramWave

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 formLogic Flow

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.

The End

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