CHAPTER 7:

DATA ACQUISITION
SYSTEM
Nor Farhani Zakaria

EMT 478 INSTRUMENTATION 2020/21

 Chapter Outline
• Data Acquisition System (DAS) Overview
• Multichannel DAS
• Computer Based DAS
• Digital to Analog and Analog to Digital
Converters
• Data Loggers
 DAS Overview
• Data Acquisition = process of sampling signals that measure real
world conditions and converting them to a data readable by a
system
• gathering of information in system or process.
• Parameter information (i.e temperature,
pressure or flow) gathered by sensors →
convert the information into electrical signals.
• The sensors signals transferred to instrument
using medium (i.e wire, optical fiber or
wireless link)
 Objective of DAS
• Acquire necessary data, at correct speed and
time
• Monitor complete plant operation to maintain on
line and safe operations
• Must be able to collect, summarize and store
data for diagnosis of operation and record
purpose
• Must be able to compute unit performance
indices on-line, real time data
• Must be reliable, easy to operate and user
friendly
Data Acquisition System (DAS) Elements:
➢ Signals
➢ Transducers :Sense physical variables
➢ Signal-conditioning hardware: to make it readable by an A/D
board
➢ DAQ device or module :Convert the signal into a digital format
acceptable by a computer
➢ Application software :Process, analyze, store, and display the
acquired data
DAS Block Diagram
Microprocessor/
RF measurement
Microcontroller
and display
 Types of DAS

DAS

Serial
Communication USB Data
Data Acquisition Acquisition Systems
Systems

Wireless Data Data Acquisition

Acquisition Systems Plug-in Boards
 Considering the configuration and
sub-systems of DAS

❑ Numbers of channel to be monitored – single/multi channel

❑ Analog or digital signal :
→ Digital signals : information regarding the voltage state (hi/low)
and/or the rate of change of these states
→ Analogue signal: provides voltage level, shape or frequency
content information.
❑ Sampling rate per channel - the rate at which the DAQ device samples
an incoming analogue signal
❑ accuracy & resolution : the number of bits used to represent the
analogue signal
❑ Signal conditioning requirements of each channel (i. e. : linearization,
isolation, filtering, amplification )
❑ Cost
 Single channel DAS

-Common DAS
-Consist of signal conditioner followed by analog to digital ADC
converter
 Multi-Channel DAS
• multiplexed system = timely
shared of various sub systems of
the data acquisition from two or
more sources
 Multiplexing
Why multiplex?

Several types of multiplexing system used in multiple channels data

acquisition;
• Multi-Channel Analog Multiplexed System
• Multiplexing the Outputs of Sample-Holds
• Multiplexing after ADC
• Multiplexing Low-level Data
 i) Multi-Channel Analog Multiplexed System
• Individual signal applied directly or after
signal conditioning if necessary
• Multiplexer controlled by logic circuit
seeks the next channel to be converted.
• Previous data stored in Sample Hold(S/H)
before convert to digital forms.
• After conversion is complete status line
from converter causes S/H to return to
sample mode and acquire signal
• On completion of acquisition, the S/H
switched to hold mode & conversion
begins again.
• Slow but low cost.
• Sufficient accuracy achieved
 ii) Multiplexing the Outputs of S/H
• Used to monitor large number of channels at the same time
but moderate speeds.
• Individual S/H assigned to each channel is updated
synchronously by logic.
• Application: radar and fire alarm system, wind-tunnel
measurement
 iii) Multiplexing After ADC
• ADC is employed for each analog input and multiplexed digital outputs
• As ADC is assigned to an individual channel, the conversion rate must be
as fast as is needed for that channel, it would need a higher conversion
rate if it were used in a multichannel multiplexed system
• Provides additional advantages to large plant areas where many sensors
:LVDT, gauges, thermocouples are used
• Since analog signal digitized at the source, digital transmission of data to data
centre provides enhanced immunity against line frequency and interferences
 Computer Based DAS
• In DAS, it utilizes a computer driven visual as an operator aid for
displaying and measuring purpose.
• This DAS aids operates in the following manner.
1. Display understandable information
2. Display visual of plant subsystems
3. Display vital parameters, eliminating the need of others
instrumentation
4. Display alarms indicating abnormal plant operating condition
5. Provide recording of sequence events, in case of emergency
• Can used programmable software i.e. C++/ Visual C++, Fortran or DAS
software packages i.e. LabView, MATLAB etc
Digital to Analog Converter (DAC)
Purpose:
• To convert digital information into equivalent analog
information
• Performs inverse operation of the Analog-to-Digital
Converter (ADC)
• VOUT  Digital Value

Reference Voltage

Digital Value DAC Analog Voltage

100101…
 Types of DACs
– Binary Weighted Resistor
– R-2R Ladder Network
– DAC using Op-Amp
– Weighted Converter using Transistor Switches
 i) Binary Weighted Resistor
• Utilizes a summing op-amp circuit
• Weighted resistors are used to distinguish each bit from the
most significant to the least significant
• Transistors are used to switch between Vref and ground (bit
high or low)

I i
Rf = R

R 2R 4R 8R Vo
MSB

LSB
-VREF
 Binary Representation
Rf = R

I i

R 2R 4R 8R Vo
Most
Significant Bit

Least
Significant Bit
-VREF
Consider the truth table for 3 bit binary signal:
- there are 8 possible states of digital
signals (23)
- Say 000 = 0 V and 111=+7V
- The least significant bit (LSB) weight
(increment change): 1/(2n-1)= 1/7
- Output = 1/7 x 7 V = 1 V

→ Resistive divider is designed to be 1 in 20

position
EXAMPLE 1 (textbook p. 640)
 Binary Representation

SET CLEARED
Most
Significant Bit

Least
-VREF Significant Bit

( 1 1 1 1 )2 = ( 15 )10
 Milman’s Theorem

• “the voltage appearing at any node in a

resistive network is equal to the summation
of the current entering the node (assuming
the node voltage is zero) divided by the
summation of the conductance connected to
the mode“
• Mathematically,
 Rf = R
I i

• “Weighted
Vo
Resistors”
based on bit R 2R 4R 8R

• Reduces MSB
current by a
factor of 2 for -VREF LSB
each bit

 B3 B2 B1 B0 
 I = VREF  R + 2R + 4R + 8R 
Bi = Value of bit i

 B2 B1 B0 
VOUT = I  R f = VREF  B3 + + + 
 2 4 8 
• More Generally:

Bi
VOUT = VREF  n −i −1
2

Bi = Value of Bit i
n = Number of Bits
Drawbacks of Binary Weighted Resistor

• Resistors used in the network have a wide range of values, so

it is very difficult to ensure the absolute accuracy and stability
of all the resistors.
• It is very difficult to match the temperature coefficients of all
the resistors. This factor is specially important in D/A
converters operation over a wide temperature range.
• When n is so large, the resistance corresponding to LBS can
assume a large value, which may be comparable with the
input resistance of the amplifier. This leads to erroneous
results.
 ii) R-2R Ladder Network
VREF
MSB Each bit corresponds
to a switch:

If the bit is high,

the corresponding
LSB
switch is connected to
the inverting input of
the op-amp.

If the bit is low, the

corresponding switch
is connected to ground.
• Same input switch setup as Binary Weighted
Resistor DAC
• All bits pass through resistance of 2R
VREF
MSB

LSB
• The less significant the bit, the more resistors must pass
through before reaching the op-amp
• The current is divided by a factor of 2 at each node

LSB MSB
 V
• Analysis for current from (001)2 :
𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹 2R 2R
2 4 8

R R R 2R
R 2R 2R 2R
∑I Req =
(2 R )(2 R ) = R
(2 R + 2 R )
IREF
Op-Amp input
VREF B1 B2
“Ground”

𝑉𝑅𝐸𝐹 𝑉𝑅𝐸𝐹
B0 𝐼𝑅𝐸𝐹 = =
2𝑅 + (2𝑅ԡ2𝑅) 3𝑅

VREF  B2 B1 B0  VREF Bi
I=  + + =
3R  2 4 8  3R
 2 n −i
• Output voltage:
VREF Bi Bi = Value of Bit i
I=
3R
 2 n −i
Rf Bi
VOUT =R f I = VREF  n −i
3R 2
Rf

R R R 2R
R 2R 2R 2R
∑I
𝐼𝑅𝐸𝐹
Op-Amp input
VREF B1 B2
“Ground”

B0
• If Rf = 6R, VOUT :

Rf  B2 B1 B0 
VOUT = VREF  + + 
3R  2 4 8
Bi = Value of Bit i
Bi
VOUT = VREF  n −i −1
2

Equal to Binary weighted resistor type DAC VOUT:

Bi
VOUT = VREF  n −i −1
2
 EXAMPLE 2
Figure below shows a R-2R ladder network. Given
Input = (101)2 ,VREF = 10 V, R = 2 Ω, Rf = 2R. Evaluate:

i) Refence current B0 B1 B2
ii) Output current through op-amp (1 0 1 )2 = 5
iii) Output voltage

R R R 2R
R 2R 2R 2R
𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹
Op-Amp input
VREF VREF
“Ground”
B1
B0 B2

38
 V

Solution:
𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹
2R 2R
2 4 8

R R R 2R
R 2R 2R 2R
𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹
Req =
(2 R )(2 R ) = R
Op-Amp input (2 R + 2 R )
VREF VREF
“Ground”
B0 B2

−𝑉𝑅𝐸𝐹 −𝑉𝑅𝐸𝐹
𝐼𝑅𝐸𝐹 = = = −1.67 mA
2𝑅 + 2𝑅 ԡ2𝑅 3𝑅
𝐼𝑅𝐸𝐹 𝐼𝑅𝐸𝐹
𝐼= + = −1.04 mA
8 2
VOUT = − IR f = 4.17 V
EXAMPLE 3
 Pros & Cons

Binary Weighted R-2R

Only 2 resistor values
Easier implementation
Pros Easily understood
Easier to manufacture
Faster response time

Limited to ~ 8 bits
Large # of resistors
Cons Susceptible to noise More confusing analysis
Expensive
Greater Error
 Chapter Outline
• Data Acquisition System (DAS) Overview
• Multichannel DAS
• Computer Based DAS
• Digital to Analog Converters
• Analog to Digital Converters
• Data Loggers
ANALOG DIGITAL CONVERTER (ADC)
• Converts analog signals into binary words
 Examples of A/D Applications
• Microphones - take your voice varying pressure waves in the air
and convert them into varying electrical signals

• Strain Gauges - 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 Multimeter
 Why ADC is important?

• All microcontrollers store information using digital logic

• Compress information to digital form for efficient storage
• Medium for storing digital data is more robust
• Digital data transfer is more efficient
• Digital data is easily reproducible
• Provides a link between real-world signals and data storage
 ADC Basic Principle

• The basic principle of operation is to use the

comparator principle to determine whether
or not to turn on a particular bit of the
binary number output.
• It is typical for an ADC to use a digital-to-
analog converter (DAC) to determine one of
the inputs to the comparator.
 Elements of a Flash A/D Converter

Encoder

Comparator
 ADC Types
3 Basic Types:

• Digital-Ramp ADC (Tracking ADC)

• Successive Approximation ADC
• Flash ADC
Others:
• Dual-slope ADC
• Delta-Sigma ADC
 Digital-Ramp ADC
• Using comparator : value of the analog
voltage at some point in time is compared
with some standard.
• Apply analog voltage to one terminal of a
comparator and trigger a binary counter
which drives a DAC.
Digital-Ramp ADC
• 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.
• Since the counter's output continuously
tracks the input (rather than counting to
meet the input and then resetting back to
zero), the binary output is legitimately
updated with every clock pulse.
 Successive approximation ADC
• The only change in this design is a very special counter circuit
known as a successive-approximation register.
• Instead of counting up in binary sequence, this register counts
by trying all values of bits starting with the most-significant bit
and finishing at the least-significant bit
• Throughout the count process, the register monitors the
comparator's output to see if the binary count is less than or
greater than the analog signal input.
 Successive approximation ADC
– VIN goes into comparator
– the successive approximation end of conversion
register (SAR) counter
increments each clock
as long as it is enabled by
the comparator
– the output of the SAR is fed
to a DAC that generates a
voltage for comparison with
VIN
– when the output of the DAC =
VIN the value of SAR is the
digital representation of VIN
 Successive approximation ADC

Illustration of 4-bit SAC with 1 volt step size

 Bit Weight
Digital Bit Bit Weight (V)
Each bit is weighted with an
analog value, such that a 1 in 7 Vref/21=10/2 =
that bit position adds its analog 5
value to the total analog value 6 10/4 = 2.5
represented by the digital
encoding. 5 10/8 = 1.25

4 10/16 = 0.625
Example: 10 V analog range, n=8
3 10/32 = 0.313

2 10/64 = 0.157

1 10/128 = 0.078

0 10/256 = 0.039
 Successive Approximation
Example
• 10 bit resolution or
0.0009765625V of Vref
• Vin= 0.6 volts
• Vref=1volts
• Find the digital value of
Vin
Successive Approximation

• MSB (bit 9)
– Divided Vref by 2 (1V/2)
– Compare Vref /2 (V=0.5 V) with Vin (0.6 V)
– If Vin is greater than Vref /2 , turn MSB on (1)
– If Vin is less than Vref /2 , turn MSB off (0)
– Vin =0.6V and V=0.5
– Since Vin>V, MSB = 1 (on)
Successive Approximation

• Next Calculate MSB-1 (bit 8)

– Compare Vin=0.6 V to V=Vref/2 + Vref/4= 0.5+0.25 =0.75V
– Since 0.6<0.75, MSB is turned off
• Calculate MSB-2 (bit 7)
– Go back to the last voltage that caused it to be turned on (Bit 9)
and add it to Vref/8, and compare with Vin
– Compare Vin with (0.5+Vref/8)=0.625
– Since 0.6<0.625, MSB is turned off
Successive Approximation

• Calculate the state of MSB-3 (bit 6)

– Go to the last bit that caused it to be turned on (In
this case MSB-1) and add it to Vref/16, and
compare it to Vin
– Compare Vin to V= 0.5 + Vref/16= 0.5625
– Since 0.6>0.5625, MSB-3=1 (turned on)
Successive Approximation
• This process continues for all the remaining
bits.
 Flash ADC
• the fastest type of ADC available, but
requires a comparator for each value of
output.
(63 for 6-bit, 255 for 8-bit, etc.)
• available in IC form up to 8-bit ADC
• The encoder logic executes a truth
table to convert the ladder of inputs to
the binary number output.
• The resistor net and comparators
provide an input to the combinational
logic circuit→conversion time is just the
propagation delay through the network
- not limited by the clock rate

23-1 = 7 Comparators
3-bit flash ADC with resolution 1 volt
 Flash ADC

Advantages Disadvantages

• Simplest in terms of •Each additional bit of

operational theory resolution requires twice
the number of
•Simple operational comparators
theory •Expensive
• Speed is limited only by • Prone to produce
gate and comparator glitches in the output
propagation delay
 Data Loggers

• Introduction
• Instumentation Protocols
• Types
• Choosing a Data Logger
• Applications
• Data logging versus data
acquisition
 Introduction
• A data logger (also data recorder) = electronic device that
records data over time or in relation to location either with a
built in or via external instruments and sensors
• available in various shapes, sizes, range from simple
economical single channel fixed function loggers to more
powerful programmable devices capable of handling
hundreds of inputs.
Types of Data Loggers
 Choosing a Data Logger

• Input Signal
• Number of Inputs
• Size
• Speed/Memory
• Real Time Operation
 Examples of data logger

• A flight data recorder (FDR),

• An event data recorder (EDR),
• A voyage data recorder (VDR),
• Ultra Wideband Data Recorder,
• A Depth Recorder
 Applications

• Unattended weather station recording (such as

wind speed / direction, temperature, relative
humidity, solar radiation).
• Unattended hydrographic recording (such as
water level, water depth, water flow, water pH,
water conductivity).
• Unattended soil moisture level recording.
• Unattended gas pressure recording.
• Offshore buoys for recording a variety of
environmental conditions.
• Road traffic counting.
• Environmental monitoring.
• Vehicle Testing
• Monitoring of relay status in railway signalling.
 Data logging VS data acquisition
Data logging Data acquisition
• Data logger is a data • Data acquisition system is not
acquisition system necessarily a data logger.
• Typically have slower • Typically have fast sample
sample rates. rates.
• Data loggers are implicitly • Data acquisition system must
stand-alone devices remain tethered to a
computer to acquire data.
• Data loggers used magnetic • Data acquisition used Static
tape , punched paper tape RAM, flash memory, EEPROM.
,directly viewable recorders
Such as strip chart recorders