analog to digital converter

Chapter 6: AD/DA CONVERTERS

EE301 : Electronic Circuits 1

Learning Outcomes

Upon completing this chapter, students should be able to:

• Understand the methods of converting digital signals to analogue signals (DAC).

• Understand the methods of converting analog signals to digital signals (ADC).


GO/SO1. Understand the methods of converting digital

signals to analogue signals (DAC). • Describe the methods of converting digital

signals to analogue signals • Explain the applications of D/A converter. • Construct the circuits to convert digital signals

to analogue signals– Resistive divider circuit. – R-2R ladder circuit.

3

Introduction-Data acquisition


Data converters : the devices that perform the interfacing

function between analog and digital worlds are analog-to-

digital (A/D) and digital-to-analog (D/A) converters

interface between the real world of physical

parameters, which are analog, and the artificial world

of digital computation and control.

Data acquisition

EE301 : Electronic Circuits

Data distribution

EE301 : Electronic Circuits6

Introduction –CONVERTERS


1. Digital-to Analog Converters (DACs)2. Analog-to-Digital Converters (ADCs)

OverviewDefinitions:Analog voltages – Voltage that may vary continuously

throughout some range.Digital Voltages – Has only 2 useful values; that is “high” and “low”.An analog-to-digital converter (ADC or A/D converter) converts an analog

value to digital equivalent in a process called digitizing.

An Digital-to-Analog converter (DAC or D/A converter) function is to convert this digital values (in binary) back to analog values.

Digital to Analog Converters (DAC)


What is a digital to analog converter (DAC)? Converts digital input signal to an analog

output signal

1001

0101

0011

0111

1001

1010

1011

DACDAC

10111001 10100111 10000110010101000011001000010000

Digital to Analog Converters (DAC)


Ana

log

Out

put

Sig

nal

Digital Input Signal

What a DAC Looks Like:


Digital to Analog Converters - Common Common ApplicationsApplications


Generic useCircuit ComponentsDigital Audio Function Generators/OscilloscopesMotor Controllers

DACs Common Applications -Generic-Generic


Used when a continuous analog signal is required. Signal from DAC can be smoothed by a Low pass

filter

0 bit

nth bit

n bit DAC011010010101010100101101010101011111100101000010101010111110011010101010101010101010111010101011110011000100101010101010001111

Digital Input

Filter

Piece-wise Continuous Output

Analog Continuous Output

DACs Common Applications -Circuit Components-Circuit Components


Voltage controlled Amplifier digital input, External Reference Voltage as control

Digitally operated attenuator External Reference Voltage as input, digital control

Programmable Filters Digitally controlled cutoff frequencies

DACs Common Applications -Digital Audio-Digital Audio


CD Players MP3 Players Digital Telephone/Answering Machines

1 2

3

1. http://www.electrorent.com/products/search/General_Purpose_Oscilloscopes.html

2. http://www.bkprecision.com/power_supplies_supply_generators.htm

3. http://www.toshiba.com/taistsd/pages/prd_dtc_digphones.html

http://configure.us.dell.com/dellstore/config.aspx?c=us&cs=19&l=en&oc=DDJ20M&s=dhs

DACs Common Applications- Function GeneratorsFunction Generators


Digital Oscilloscopes Digital Input Analog Ouput

Signal Generators Sine wave generation Square wave generation Triangle wave generation Random noise generation

1

1. http://www.electrorent.com/products/search/General_Purpose_Oscilloscopes.html

2

2. http://www.bkprecision.com/power_supplies_supply_generators.htm

http://www.electrorent.com/products/search/detail.aspx?Mfr=Lecroy+Corporation&MfrMod=LEC-LC584AL&Desc=1GHz+4CH+Digital+Scope

DACs Common Applications -Motor Controllers-Motor Controllers


Cruise Control Valve Control Motor Control

1

1. http://auto.howstuffworks.com/cruise-control.htm

2

2. http://www.emersonprocess.com/fisher/products/fieldvue/dvc/

3

3. http://www.thermionics.com/smc.htm

Types Of DACs


D/A conversion can be achieved using a number of different methods such as:

The Weighted-Resistor DAC

The Ladder Network (The R-2R Ladder DAC)

Comprised of switches, op-amps, and resistors

Binary Weighted Resistor DAC


Advantage:Easy principle (low bit DACs)Disadvantages:Requirement of several

different precise input resistor values: one unique value per binary input bit. (High bit DACs)

Larger resistors ~ more error.Precise large resistors –

expensive.High number of bits lead to

current changes in the magnitude of noise amplitudes.

Four-Bit Binary Weighted Resistor DAC


This approach is not satisfactory for a large number of bits because it requires too much precision in the summing resistors. This problem is

overcome in the R-2R network DAC.

Switches as Binary Representation


Rf = R

iI

Most Significant Bit, MSB

Least Significant Bit,

LSB

-VREF

Vo

R 2R 4R 8R

Switches as Binary Representation


-VREF

Least Significant Bit

Most Significant Bit

CLEAREDSET

( 1 1 1 1 )2 = ( 15 )10

Binary Weighted Resistor


Rf = R

8R4R2RR

Vo

-VREF

iI

LSB

MSB

“Weighted Resistors” based on bit

Reduces current by a factor of 2 for each bit

Binary Representation

• Choose a common ratio R

R1 = 20R = R

R2 = 21R = 2R

R3 = 22R = 4R

R4 = 23R = 8R

• Remember:- MSB line has smallest

resistor = R- LSB line has largest

resistor = 2n-1R



R

B

R

B

R

B

R

BVI REF 842

0123

842012

3

BBBBVRIV REFfOUT


Result:

Bi = Value of Bit i


Resolution/Value Digital2 1

REF

ini

REFOUT

V

BVV


More Generally:

Bi = Value of Bit i n = Number of Bits

Examples /Exercise:

1. Draw the diagram of an 8-bit weighted resistor

DAC. Given: Resistor in 25 bit line = 20kΩ

Reference voltage = -10 volt 8 bit parallel binary input Xp = 10101101Calculate :

1. Common ratio R of the network2. Value of each of the resistors in the network3. Feedback resistor of the OP AMP, Rf

4. Load Current, IL

5. Analog output Voltage, Vo

SOLUTION : DISCUSSION WITH LECTUREREE301 : Electronic Circuits 26

R-2R Ladder DAC


R-2R Ladder DAC


Only two resistor values- R and 2R Does not need the kind of precision as Binary

weighted DACs Easy to manufacture More popular Less errors

R-2R Ladder DAC


R-2R Ladder DAC


The most popular method incorporates a ladder network containing series-parallel combination of resistors, in R and 2R values.

R-2R Ladder DAC


Example circuit with 0110 input

Vref2

2D2D2D2DVo

4

33

22

11

00

R-2R Ladder DAC


VREF

MSB

LSB

R-2R Ladder DAC


Same input switch setup as Binary Weighted Resistor DAC

All bits pass through resistance of 2RVREF

MSB

LSB

R-2R Ladder DAC


The less significant the bit, the more resistors the signal must pass through before reaching the op-amp

The current is divided by a factor of 2 at each node

LSB MSB

R-2R Ladder DAC

248163210 DDDD

vR

Rv ref

fout


The summing amplifier with the R-2R ladder of resistances shown produces the output

where the D's take the value 0 or 1. The digital inputs could be TTL voltages which close

the switches on a logical 1 and leave it grounded for a logical 0.

This is illustrated for 4 bits, but can be extended to any number with just the resistance values R and 2R.

DAC -Performance Specifications --ResolutionResolution


Resolution: is the amount of variance in output voltage for every change of the LSB in the digital input.

Accuracy is a comparison of the actual output of a DAC with the expected output. It is expressed as a percentage of a full-scale, or maximum, output voltage. For Example: if a converter has a full scale output of 10V and the accuracy is ±0.1%, than the max error for any output voltage is (10V)(0.001) = 10mV.

NLSB

VV

2Resolution Ref

N = Number of bits


Voltage resolution: n2

Vref

Voltage step example : for 10 bit resolution. So n=10

if Vref = 10V

voltage step : 10V/1024 = 10mV

DAC -ResolutionDAC -Resolution

DACs Performance Specifications - ResolutionResolution


Better Resolution(3 bit)Poor Resolution(1 bit)

Vout

Desired Analog signal

Approximate output

2 V

olt.

Lev

els

Digital Input0 0

1

Digital Input

Vout

Desired Analog signal

Approximate output

8 V

olt.

Lev

els

000

001

010

011

100

101

110

111

110

101

100

011

010

001

000

Comparison Between Weighted Resistor and R-2R

Weighted resistor If all bits = 1, branch current

are weighted sum of branch current = IL

It uses n resistor Number of different resistor

values = n from R to 2n-1 R More difficult to match resistor

values Not convinient to make in IC

form Feedback resistor of OpAmp =

0.5R

R-2R

If all bit=1, branch currents are equal. Each contributes different fraction to IL

It uses 2n+1 resistors

No different resistor values = 2, i,e R & 2R

Easier to match Convenient to make in IC

form Feedback resistor of OpAmp

= 3REE301 : Electronic Circuits 39

Exercise & Example5.2 Given an 8-bit resistor 2-2R

ladder DAC with reference voltage VR = 10V. Shunt Arm Resistor = 10KΩ and Binary input Xp/Bin = 11001010.

Calculate : • a) Load Current, IL b) Analog output voltage, Vo

SOLUTION : DISCUSSION WITH LECTURER

5.3 Draw the circuit of R -2R DAC as mentioned in question 5.2



Analog to Digital ConvertersAnalog to Digital Converters ADC, A/D or A to D : an electronic integrated circuit

(i/c) that converts continuous signals to discrete

digital numbers.

ADC is an electronic device that converts an input

analog voltage ( or current ) to a digital number.

The digital output may be using different coding

schemes, such as binary and two's complement

binary

OVERVIEW Analog Signals


Analog signals – directly measurable quantities in terms of some other quantity

Examples: Thermometer – mercury height rises as temperature

rises Car Speedometer – Needle moves farther right as

you accelerate Stereo – Volume increases as you turn the knob.

OVERVIEW Digital Signals


Digital Signals – have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off.

Examples: Light switch can be either on or off Door to a room is either open or closed

Examples of A/D Applications


Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals

Strain Gages - determines the amount of strain (change in dimensions) when a stress is applied

Thermocouple – temperature measuring device converts thermal energy to electric energy

Voltmeters Digital Multimeters

Types of ADC• Digital Ramp ADC• Successive Approximation ADC

E2002 : Electronic System 2 45

Analog to Digital ConvertersAnalog to Digital Converters

Just what does an A/D converter DO?• Converts analog signals into binary

words


Analog to Digital ConvertersAnalog to Digital Converters

1. Digital Ramp ADC


Digital ramp ADC


Output : conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard.

Apply the analog voltage to one terminal of a comparator

and trigger a binary counter which drives a DAC. The

output of the DAC is applied to the other terminal of the

comparator. Since the output of the DAC is increasing with the

counter, it will trigger the comparator at some point when

its voltage exceeds the analog input. The transition of the comparator stops the binary counter,

which at that point holds the digital value corresponding

to the analog voltage.

Operation - Digital ramp ADC• Uses a DAC a binary counter to generate the digital value of an

analog input.• Initially, the counter is set to 0 (RESET). Vsx = 0volt. Since Vs >

Vsx, the comparator output is HIGH. The counter advances and the DAC output, Vsx increases one step at a time.

• This continues until Vs < Vsx. The comparator will go low and the counter stops counting and the contents of the counter are the digital representation of Vs.

• The control logic loads the binary count into the latches and resets the counter, thus beginning another count sequence to sample the input value. For an 8-bit conversion this means a maximum of 256 counter states. Notice that for each sample, the counter must count from zero up to the point at the stair-step reference voltage reaches the analog input voltage.


2. Successive Approximation ADC


Successive Approximation ADC


The successive approximation ADC

is much faster than the digital

ramp ADC because it uses digital logic to

converge on the value closest to

the input voltage. A

comparator and a DAC are used in

the process.

Practical considerations of ADC circuits

E2002 : Electronic System 2 52

Example /Exercise5.4 An eight-bit ADC has resolution of 20mV.

What will its digital output be for an analog input of 2.17 Volt?

Solution : DISCUSSION WITH LECTURER

END OF THIS TOPIC


analog to digital converter

Documents

data acquisition

converting

binary representation

analog output

2r ladder

analog voltage

resolution

analog input