week 9 -data acquisition systems (daq)

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Universiti Malaysia Perlis

DNT245

SENSORS & MEASUREMENT

Week 9: Data AcquisitionSystem (DAQ)

1

Date: 01/02/2012

Prepared by: Maizatul Nurul Bariah AhmadEmail: [email protected]/0194052335



Data acquisition systems are products or

processes used to collect information to

document or analyze some phenomenon

Designed to capture basic electro-mechanical

phenomena by measuring the electrical output

from a variety of transducers

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3

Transducer1

Transducer3

Transducer4

Transducer2

SignalConditioner

1

SignalConditioner

4

SignalConditioner

3

Signal

Conditioner2

M

ULTIPLEX

ER

A/DConverter

Analog

Data

- Recorders

- Desplay

- Meter

Printer

Digital Display

Magnetic Tape

Transmission

Computer

Processing



1. Sensors and Transducers

2. Signals

3. Signal Conditioning

4. DAQ Hardware5. Driver and Application Software

(a) Driver Software

(b) Application Software

4



1. Sensors and Transducers

A device that converts a physical phenomenon

into a measurable electrical signal

the ability of a data acquisition system tomeasure different phenomena depends on the

transducers to convert the physical phenomena

into signals measurable by the data acquisition

hardware.

Transducers are synonymous with sensors in data

acquisition system.

There are specific transducers for many different

applications.

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2. Signals

The appropriate transducers convert physicalphenomena into measurable signals. However,

different signals need to be measured indifferent ways.

For this reason, it is important to understand thedifferent types of signals and their correspondingattributes.

Signals can be categorized into two groups:·Analog·Digital

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Analog Signal

Exist at any value with respect to time

Eg: voltage, temperature, pressure, sound and load

Primary characteristic: level, shape, and frequency

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Digital Signal

Cannot take any value with respect to time

Has two possible levels: high and low

Useful information measured from a digital signal –state and rate.

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3) Signal Conditioning

Sometimes transducers generate signals too difficult or toodangerous to measure directly with a DAQ device.

Signal conditioning is essential for an effective data acquisition

system – for instance, when dealing with high voltages, noisyenvironments, extreme high and low signals, or simultaneous

signal measurement – too difficult to measure directly with a

data acquisition device.

Maximize accuracy of a system, allow sensors to operate

properly and guarantees safety

Types of signal conditioning – amplification, attenuation,

isolation, bridge completion, simultaneous sampling, sensor

excitation, multiplexing.

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4) DAQ Hardware

DAQ hardware acts as the interface between the

computer and the outside world

It primarily functions as a device that digitizesincoming analog signals so that the computer can

interpret them.

Other data acquisition functionality includes:

Analog input/output Digital input/output

Counter/timers

Multifunction – a combination of analog, digital, and counter

in a single device

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5) Driver and Application Software

Driver Sofware

Software transforms the PC and the DAQ hardware into acomplete data acquisition, analysis, and presentation tool.

Without software to control or drive the hardware, theDAQ device will not work properly.

Driver software is the layer of software for easilycommunicating with the hardware.

It forms the middle layer between the application softwareand the hardware.

Driver software also prevents a programmer from having todo register-level programming or complicated commands

in order to access the hardware functions.11



Application Software

The application layer can be either a developmentenvironment in which you build a custom application thatmeets specific criteria, or it can be a configuration-basedprogram with preset functionality.

Application software adds analysis and presentationcapabilities to driver software.

To choose the right application software, evaluate thecomplexity of the application, the availability of

configuration-based software that fits the application, andthe amount of time available to develop the application. If the application is complex or there is no existing program,use a development environment.

Eg: LabView, Matlab, Visual Basic, etc..

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Introduction to ADC

An electronic device that converts analog signals

to an equivalent digital form

Digital Computer: Binary (discrete) values

Physical World: Analog (continuous) values

Example: Temperature, Humidity, Pressure

Output: Voltage or Current

Microcontroller? -----> Digital Therefore, ADC is needed to translate (convert)

the analog signals to digital numbers

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Introduction to ADC (Cont’d)

ADC Resolution

Assuming VREF = 5V

* Step Size (Resolution): is the smallest change that can be discerned by an ADC

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ADC CPU DISPLAYSENSOR

Microcontroller Connection to Sensor via ADC

n-bit No. of steps Step size (mV)

8 28

=256 5/256 = 19.5310 210 =1024 5/1024 = 4.88

12 212 =4096 5/096 = 1.2



ADC Reference Voltage (Vref)

Vref: Input voltage used for the referencevoltage

The voltage connected to this pin , with the

resolution of the ADC chip, dictate the step

size Example: If we need the analog input to be

0 to 4 volts, Vref is connected to 4 volts

Digital data output: 8-bit (D0-D7), 10-bit

(D0-D9)

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ADC Reference Voltage (Vref)(Cont’d)

Vref Relation to Vin Range for an 10-bit ADC

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V ref Vin(V) Step Size (mV)

5.00 0 to 5 5/1024 = 4.88

4.096 0 to 4.096 4.096/1024 = 4

3.0 0 to 3 3/1024 = 2.93

2.56 0 to 2.56 2.56/1024 = 2.5

2.048 0 to 2.048 2.048/1024 = 2



ADC Digital Data Output

Dout = Vin / Step Size

Example:Vref = 2.56, Vin = 1.7V.Calculate the Do-D9 output?

Solution:Step Size = 2.56/1024 = 2.5mVDout = 1.7/2.5mV = 680(Decimal)

D0-D9 = 1010101000

Digital data output(in decimal):

8-bit (D0-D7)= 25610-bit (D0-D9) = 1024

Analog Input Voltage

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An electronic component found in many dataacquisition devices that produce an analogoutput signal

To convert digital values to analog voltages

Performs inverse operation of the Analog-to-Digital Converter (ADC)

DACDigital Value Analog Voltage

Reference Voltage

ValueDigital∝OUT V

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Types

Binary Weighted Resistor R-2R Ladder Multiplier DAC

The reference voltage is constant and is set by the manufacturer.

Non-Multiplier DAC The reference voltage can be changed during operation.

Characteristics Comprised of switches, op-amps, and resistors

Provides resistance inversely proportion tosignificance of bit

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20

R f = R

8R 4R 2R R Vo

-VREF

∑ i I

LSB

MSB



21

R f

= R

8R 4R 2R R Vo

-

VREF

∑ i I

LeastSignificant Bit

Most

Significant Bit



22

-VREF

LeastSignificant Bit

MostSignificant Bit

CLEAREDSET

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



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R f = R

8R 4R 2R R

Vo

-VREF

∑ i I

LSB

MSB

“Weighted

Resistors”

based on bit

Reducescurrent by a

factor of 2

for each bit



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Result:

Bi = Value of Bit i

+++=∑

R

B

R

B

R

B

R

BV I REF

842

0123

+++=⋅=

842

0123

B B B BV R I V REF f OUT



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More Generally:

Bi = Value of Bit i

n = Number of Bits

ResolutionValueDigital2 1

⋅⋅=

= ∑ −−

REF

in

i REF OUT

V

BV V



26

VREFMSB

LSB



Same input switch setup as Binary WeightedResistor DAC

All bits pass through resistance of 2R

VREFMSB

LSB

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

LSB MSB

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

Analysis for current from (001)2 shown below

0 I

VREF

RR R R 2R2R2R2R

Op-Amp input

“Ground”

B0

2

0 I

4

0 I

8

0 I

R

V

R R R

V I REF REF

32220 =

+

−=

B1 B2

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30

Result:

Bi = Value of Bit i

ref ampop R I V OUT .−=

R f

++=−

8423

012B B B

R

V I REF ampop



0 I

VREF

RR R R 2R

2R2R2R

Op-Amp input“Ground”

B0B2

0 I

VREF

Example:

Input = (101)2

VREF = 10 V

R = 2 Ω

Rf = 2R

mA67.132220−==

+

−=

R

V

R R R

V

I

REF REF

mA04.128

00 −=+=−

I I I ampop

V17.4=−= − f ampopOUT R I V

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32

Binary Weighted R-2R

Pros Easily understood

Only 2 resistor values

Easier implementation

Easier to manufacture

Faster response time

Cons

Limited to ~ 8 bits

Large #of resistors

Susceptible to noise

ExpensiveGreater Error

More confusing analysis



Most computers perform instructions

sequentially. Thus, computer take data from sensors one at

one time using device called multiplexer (MUX)

A concept of electronic switch, selecting

particular channels, read and the processthem.

Figure 4.4 shows a mechanical analog to the

DAS, switches are semiconductor devices (eg:

transistors) Subject to errors:

Crosstalk

Transfer accuracy

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