3 bit Flash type ADC (ES – 106)

Lord Bharath Bhushan Lohray I am the author of this document 2006.10.08 14:06:04 +05'30' Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I Pramukhswami Institute of Electronic Sciences Sardar Patel University Vallabh Vidyanagar 2006

CONTENTS No.

Topic

Page No.

1

Acknowledgements

1

2

Introduction

2

3

About the Project

4

4

Circuit Diagrams

6

5

Component List

9

6

Component Description

10

7

PCB Fabrication

14

8

Testing

16

9

Troubleshooting

17

10

Applications

18

11

Further Scope

19

12

Flash Type ADC Experiment

21

13

Reference

22

14

Appendix Datasheets

23

3 Bit Flash type ADC

Acknowledgements We are thankful to Dr. Lakshminarayana, HOD, Department of

Electronic

Sciences

and

all

staff

members

for

permitting us to use the facilities at the Department. We are also grateful to Dr. S. S. Patel and Dr. Vibha Vaishnav for their guidance.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

1

3 Bit Flash type ADC

Introduction Generally

speaking,

the

world

around

us

is

analog

in

nature i.e. all changes and physical phenomenon occurring are continuous. However due to the various advantages of having data in digital format like storage, processing, transmission etc., we prefer to convert these continuous varying

quantities

into

discreet

digital

values.

This

process in known as quantization and the device employed for

quantization

is

known

as

an

Analog

to

Digital

Converter – ADC. Various circuits commonly used as ADCs [1] are •

Successive Approximation Type ADC

•

Counter Type ADC

•

Single Slope type ADC

•

Dual Slope type ADC

•

Ramp type ADC

•

Sigma Delta ADC

•

Flash type ADC

The

most

commonly

used

ADC

is

the

successive

approximation type of ADC due to its low cost. However, the

successive

approximation

type

of

ADC

have

maximum

conversion time of all types of ADCs. The Flash type of ADC has the least conversion time and is used in time critical applications such as a sample and hold circuit of a digital oscilloscope. The chief drawback hardware

of

the

flash

requirements.

type The

of

ADC

is

hardware

it’s

cost

required

and for

realizing the ADC increases as 2resolution. i.e. a 3 bit ADC Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

2

3 Bit Flash type ADC

requires 8 op-amps while a 4 bit ADC needs 16 op-amps while a 5 bit ADC would need 32 op-amps. Apart from conversion time and economy, the various other parameters of ADCs [2][3] areResolution – The minimum change at the input that can register a change in state of the output. Higher the resolution, more accurate is the digital output. Quantization Noise – The difference between the actual analog value and approximated digital value due to the "rounding"

that

occurs

while

converting

is

called

quantization error or quantization noise. Generally the quantization error is in between 0 to ½ the LSB value. The

RMS

value

of

the

quantization

error

is

given

by

the

range

of

1 LSB 12

Full

scale

Measurement

Range

–

This

is

voltages that the ADC can convert from analog form to digital form. Dither - Dither is a form of noise, or 'erroneous' signal or data which is added to sample data for the purpose of minimizing quantization error. Aperture error - This is due to a clock jitter during the sample and hold time of the conversion.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

3

3 Bit Flash type ADC

About the Project The Flash Type of ADC is the fastest of all types of ADC ADCs

but

[3]

has

a

large

die

size,

a

high

input

capacitance, and are prone to produce glitches on the output (by outputting an out-of-sequence code). They are often used for video or other fast signals. The

Flash

type

ADC

can

be

realised

on

a

PCB

for

demonstration purpose using readily available components at a low cost. The Flash type ADC however requires a large number of comparators that increases as 2resolution. So a 4 bit ADC would

need

8

comparators.

This

is

not

difficult

to

incorporate of a PCB without using advanced fabrication techniques. The Flash type ADC, though is hardware intensive, has a simple operation and is easy to understand. The flash type ADC consists of an array of parallel comparators that is fed a threshold voltage derived from a reference voltage

and

potential

divider

to

the

inverting

input

terminal of the op-amp comparator and the input analog voltage to the non-inverting input terminal of the opamp.

On

crossing

the

threshold

(the

voltage

at

the

inverting input), the output of the comparator goes high. Since the threshold is derived from a potential divider, each comparator is set to go high at different threshold levels. However, comparator

as

comparator

below

n

go

for high

level as

n

well.

goes So,

high, a

all

priority

encoder is used to convert these input lines form the comparator into binary coded output. The 74148 priority encoder accepts an 8 line input and gives out a 3 bit Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

4

3 Bit Flash type ADC

binary output. Each of the input line has a progressively increasing

priority.

The

output

is

a

function

of

the

highest priority input irrespective of the state of any lower

priority

inputs

unlike

simple

encoders

which

outputs a bit-wise OR of all outputs corresponding to the various inputs. Since a priority encoder is a logic circuit and exhibits a propagation delay. Any output that is received during this period is invalid. A strobe signal should be used to latch the valid output. The conversion speed of a flash type ADC depends on the slew rate of the op-amps used and the propagation delay of the priority encoder. The output of the priority encoder is encoded to the 7segment code by 7447, which is a binary to 7-segment encoder and driver. This data is displayed on a 7-segment display as a digit. The

output

of

the

priority

encoder

and

the

7-segment

driver is so wired so as to blank the display (character F) on overflow of the ADC. The output of the comparator array is also used to glow a row of LEDs to indicate the comparators that are at the given point of time giving a high output.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

5

3 Bit Flash type ADC

Component List Passive Components Resistors 47 KΩ

8

560

9

0

Ω

Ω

3

Semiconductors LM 324

2

7447

1

74148

1

LTS 542

1

LED

8

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

9

3 Bit Flash type ADC

Component Description LM324 These devices consist of four independent high-gain frequencycompensated

operational

amplifiers

that

are

designed

specifically to operate from a single supply over a wide range of voltages. Operation from split supplies also is possible if the difference between the two supplies is 3 V to 32 V, and VCC is at least 1.5 V more positive than the input common-mode voltage. The low supply-current drain is independent of the magnitude

of

the

supply

voltage.

Applications

include

transducer amplifiers, dc amplification blocks, and all the conventional operational-amplifier circuits that now can be more easily implemented in single-supply-voltage systems. For example, the LM124 can be operated directly from the standard 5-V supply that is used in digital systems and easily provides the

required

interface

electronics

without

requiring

additional ±15-V supplies.

LM 324 Pin Diagram

74148 Pin Diagram

74148 The 74148 TTL encoders feature priority decoding of the inputs to ensure that only the highest-order data line is encoded. The ’148 and ’LS148 devices encode eight data lines

to

three-line

(4-2-1)

binary

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

(octal).

Cascading

10

3 Bit Flash type ADC

circuitry (enable input EI and enable output EO) has been provided to allow octal expansion without the need for external

circuitry.

For

all

types,

data

inputs

and

outputs are active at the low logic level. All inputs are buffered

to

represent

one

normalized

Series

54/74

or

54/74LS load, respectively.

74148 Logic Diagram

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

11

3 Bit Flash type ADC

7447 The 7447 feature active-LOW outputs designed for driving common-anode All

of

LEDs

the

input/output

above

incandescent

circuits

controls

identification following

or

and

page.

have

and

a

resultant

Display

full

lamp

patterns

are

unique

symbols

conditions.

All

of

circuits

for to

directly.

ripple-blanking

test

displays

nine

the

indicators

input. are

BCD

Segment

shown input

a

counts

authenticate

incorporate

on

input

automatic

leading and/or trailing-edge, zero-blanking control (RBI and

RBO).

Lamp

test

(LT)

of

these

devices

may

be

performed at any time when the BI/RBO node is at a HIGH logic

level.

input

(BI)

All

types

which

can

contain be

used

an to

overriding control

blanking the

lamp

intensity (by pulsing) or to inhibit the outputs.

7447 Connection Diagram

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

Segment Identification

12

3 Bit Flash type ADC

LTS 542 The LTS 542 is a common anode 7 segment display. The display has bright red LEDs made of GaP on GaP substrate and has a grey face and white segments. The segments are designated as a, b, c, d, e, f and g. There is also a decimal point located to the right of the digit. The anode of the LEDs is common and two pins are provided for connection with the anode.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

13

3 Bit Flash type ADC

PCB Fabrication A

PCB

is

needed

to

host

the

circuit

components

and

provide mechanical rigidity against shock and jerks that a device may experience in real world environment. The circuit

board

conductive

also

path

has

for

a

vital

the

flow

function of

of

providing

signals

between

components mounted over it. PCBs

used

may

be

either

a

general

purpose

or

may

be

specially etched as per the requirements. The specially etched PCBs may made by first marking the required parts of a copper clad board and developing the masked board in an etchant, viz. FeCl3 . The masking of the copper board may be done in several ways, the popular way of masking is by the use of oil paints, permanent markers or by toners. The toner method that was used to develop the PCB used. The circuit layout diagram of the PCB was first drawn using and CAD software (Dip Trace). The design “artwork” was printed on to a photographic quality

paper

renderable

using

resolution.

a

laser The

printer

image

taken

at

the

is

the

highest mirror

image of the image that is expected to be on the PCB looking from the signal side. The image is then transferred on to the copper board by the process of thermal lithography. The image is placed facing the copper side of the board and ironed for a few minutes under pressure. This causes the toner from the paper to melt and stick on to the copper board. The toner on the paper is a type of fine plastic powder.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

14

3 Bit Flash type ADC

The paper is then pealed off the board patiently under water in the presence of a dish washing detergent. The soap reduced the viscosity of water and facilitates the paper to soak more water. After the removal of the paper, the board is ready to be etched. In a strong FeCl3 solution, the etching should be complete in 20 minutes given appropriate agitation, to prevent the deposition of etchant by products viz. Fe The

etched

board

is

then

drilled

at

the

appropriate

places and is ready to be mounted.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

15

3 Bit Flash type ADC

Testing Testing of the board after mounting the components may be carried out by 1. tying the Aref to Vcc 2. The

analog

input

is

obtained

from

a

preset

potentiometer which is wired to vary between Vcc and GND. 3. The

potentiometer

is

varied

using

a

screw

driver

from zero position to maximum position gradually. 4. On varying the potentiometer, the LEDs should glow one

by

one

until

all

eight

LEDs

glow

and

the

corresponding decimal number is displayed on the 7segment display.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

16

3 Bit Flash type ADC

Troubleshooting The likely problems and their causes are as given

Symptom

Possible Causes

Flickering numbers

No

common

between the

Troubleshooting

ground Tie

ADC

analog

all

GND

and connections input together.

source.

Blank 7-segment

Over voltage.

Reduce the analog input

display

voltage.

Connect to GND and get digit 0 on the display.

Unstable output

Unstable Aref.

Provide stable

a

proper analog

reference source.

LEDs glow but the

Faulty

7447

or Try replacing the

Digit is not

74148

ICs.

Faulty LM324

Try replacing the

displayed.

LEDs do not glow.

ICs.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

17

3 Bit Flash type ADC

Application The ADCs have extensive applications in our daily lives. The

telephone

exchanges

use

PCM

to

transmit

voice

channels between exchanges. The ADC is used to convert the analog signals on the local loop into digital signals for

PCM.

The

sensor

data

is

stored

and

processed

in

digital format. This is done with the help of ADCs. The

Flash

type

of

ADC

is

used

in

time

critical

applications which requires very high sampling rates. A few applications where flash type ADCs are used [4] are 1. Data acquisition 2. Satellite communication 3. Radar processing 4. Sampling oscilloscopes 5. High-density disk drives.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

18

3 Bit Flash type ADC

Further Scope [5] Sub-Ranging ADC - When higher resolution converters or smaller die size and power for a given resolution are needed [4], multi-stage conversion is employed. This architecture is known as a sub-ranging converter. This is also sometimes referred to as a multi-step or half-flash converter. This approach combines ideas from successive approximation and flash architectures.

Sub-ranging ADCs reduce the number of bits to be converted into smaller groups, which are then run through a lower resolution flash converter. This approach reduces the number of comparators and reduces the logic complexity, compared to a flash converter. The trade off results in slower conversion speed compared to flash. The Pipelined ADC – A pipelined ADC employs a parallel structure in which each stage works on one to a few bits of successive samples concurrently. This improves speed at the expense of power and latency. However, each pipelined stage is much slower than a flash section. The pipelined ADC requires accurate amplification in the DACs and interstage amplifiers, and these stages have to settle to the desired linearity level. For a given resolution, pipelined ADCs are around 10 times slower compared to flash converters of similar resolution. Pipelined converters are possibly the optimal architecture for ADCs that need to sample at rates up to Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

19

3 Bit Flash type ADC

around 100Msps with resolution at 10-bits and above. At resolutions of up to 10-bits, and conversion rates above a few hundred MS/s, flash ADCs dominate. Interestingly, there are some situations where flash ADCs are hidden inside a converter employing another architecture to increase its speed.

Here, the analog input VIN is first sampled and held steady by a sample-and-hold (S&H), while the flash ADC in stage one quantizes it to 3 bits. The 3-bit output is then fed to a 3-bit DAC (accurate to about 12 bits), and the analog output is subtracted from the input. This "residue" is then gained up by a factor of 4 and fed to the next stage (stage two). This gained-up residue continues through the pipeline, providing 3 bits per stage until it reaches the 4-bit flash ADC, which resolves the last 4LSB bits. Because the bits from each stage are determined at different points in time, all the bits corresponding to the same sample are time-aligned with shift registers before being fed to the digitalerror-correction logic. Note that as soon as a certain stage finishes processing a sample, determining the bits and passing the residue to the next stage, it can start processing the next sample due to the sample-and-hold embedded within each stage. This pipelining action accounts for the high throughput.

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

20

3 Bit Flash type ADC

Flash Type ADC Experiment Reference Voltage (Aref): ______________________v Observation Table No.

Analog Input (Volts)

Comparators C0

C1

C2

C3

C4

C5

Digital C6

C7

Output

01 02 03 04 05 06 07 08 09 Graph A graph of analog input voltage v/s digital binary output is plotted. ADC characteristic

Analog Input (Volts)

6 5 4 3 2 1 0 0000

0001

010

0011

0100

0101

0110

0111

1000

Digital Output

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

21

3 Bit Flash type ADC

Reference [1]

A.

Anand

Kumar,

“Fundamentals

of

Digital

Circuits”, PHI New Delhi, 2004, pp 563-571. [2]

Floyd

Thomas,

“Digital

Fundamentals”,

Pearson

Education Inc., 8th ed., pp 759-775. [3]

Wiki Users, “Wikipedia – The free encyclopedia”, http://en.wikipedia.org/w/index.php?title=Analog-to-digital_converter&oldid=76779215

,

22nd

September 2006 [4]

MAXIM

Website,

http://www.maxim-ic.com/appnotes.cfm/appnote_number/810,

Understanding Flash ADCs. [5]

MAXIM

Website,

http://www.maxim-ic.com/appnotes.cfm/appnote_number/2094,

Simple ADC Comparison Matrix

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

22

3 Bit Flash type ADC

Appendix Datasheets

Bharath Bhushan Lohray & Bhavĭsha S Patel M.Sc. Semester – I, 2006 Department of Electronics, Sardar Patel University, Vallabh Vidyanagar, Gujarat, India.

23