Introduction of ADC

Review of Control System

Analog to Digital Converter (ADC)
What is an ADC (1/2)
Why it should be digitalized?

Computer/controller uses digital data to be processed for controlling a process.

The advantages using computer in process control are
1. Too control multivariable in a control system
2. Linearization
3. The difficult equation able to solve as quickly as possible, and it can be
modified.
4. Computer network able to solve control issues in large scales
Analog to Digital Converter (ADC)

 Most signals are analog

• e.g. speech, biological signals, seismic signals, radar signals,
sonar signals, etc.
 ADC is applied to process analog signals by digital means
 ADC has a three-step process
• Sampling
• Quantization
• Coding
A/D
Converter

Xa(t) X(n) Xq(n) 01011

Sample and Quantizer Coder …
Hold (S/H)

Analog Signal Discrete-Time Quantized Digital Sinyal

Signal Signal
Sample and Hold (S/H)

1. Electronic switch is opened 

isolating capacitor from input.
2. Capacitor will hold (stay
charged)  switch is opened.
3. The voltage of Capacitor will be
used as voltage input of ADC 
but does not discharge because
the high resistance of voltage
follower.
ADC Specifications

 Acquisition Time is the time that it takes for the ADC to

acquire and convert an analog signal to a digital value.
The conversion time is a sum of: (Sample Time) +
(Calibration Time) + (Charge Distribution Time) +
(Synchronization Time)
 Aperture time is the period during which the ADC is
reading the input signal. The larger the aperture time, the
better the resolution. Select short aperture times for faster
measurement speed.
 Settling time is the time elapsed which it used by ADC to
build convergence output from an input signal.
Commercially Available S/H
Turbine Supervisory Instrument (TSI)
Resistive Vibration Sensor

Alat ini terdiri dari 3 komponen penting

1. Flexure
2. Strain gauge
3. Bridge circuit
Prinsip Kerja

Strain gauge
1
SG1 Vibration

V
Flexure
Strain
gauge2
SG2

Ketika Flexure terdeflexi akibat gerak vibrasi benda kerja, maka nilai hambatan strain gauge 1
dan 2 akan berubah sebanding dengan defleksi yang terjadi karena perubahan percepatan
pada benda kerja. Perubahan hambatan ini kemudian disambungkan dengan bridge sirkuit
yang menghasilkan tegangan mengikuti persamaan sbb
V  R3 V  SG2
V  
R1  R3 SG1  SG2
Sampling Definition
Sampling Theorem

Nquist theorem :
𝑓𝑠 ≥ 2𝑓𝑚
Dimana
fs = number of sampling
(sample/s)
fm = maximum frekuensi (cycle/s)

𝑓𝑚
Sampling rate = 𝑓𝑠

Number of sampling of a cycle

(NC)
𝑓
𝑁𝐶 = 𝑓𝑠
𝑚
Contoh 1 : Proper sampling

Jika diketahui
f = 90 cycle/second
fs = 1000 sample/second

Sampling rate = 0.09

1000
𝑁𝐶 =
90
= 11.1 𝑠𝑎𝑚𝑝𝑙𝑒/𝑐𝑦𝑐𝑙𝑒
Contoh 2 : Proper sampling

Jika diketahui
f = 90 cycle/second
fs = 280 sample/second

Sampling rate = 0.32

280
𝑁𝐶 =
90
= 3.2 𝑠𝑎𝑚𝑝𝑙𝑒/𝑐𝑦𝑐𝑙𝑒
Improper sampling number

 Terjadinya aliasing. Aliasing adalah sebuah efek yang menyebabkan hasil

sampling sinyal sangat jauh berbeda dengan aslinya

Jika diketahui
f = 90 cycle/second
fs = 95 sample/second

Sampling rate = 0.95

95
𝑁𝐶 =
90
= 1.05 𝑠𝑎𝑚𝑝𝑙𝑒/𝑐𝑦𝑐𝑙𝑒
Sampling Rate of DSP
Applications

Aplikasi fmax fs

Geophysical 500 Hz 1 kHz

Biomedical 1 kHz 2 kHz

Mechanical 2 kHz 4 kHz

Speech 4 kHz 8 kHz

Audio 20 kHz 40 kHz

Video 4 MHz 8 MHz

Quantization
Quantizing (1/2)

Quantizing  breaking down analog value into a set of finite states

Quantizing (2/2)
Coding

Coding  assigning a digital word or number to each state.

Accuracy
Accuracy  Resolution
Accuracy  Sampling rate
Accuracy  (Resolution &
Sampling rate)
Commonly Used Methods of ADC

Numerous methods are used for converting analog signals to

digital form. Five most commonly used methods are listed
below:

• Counter / staircase ramp ADC

• Tracking ADC
• Successive approximation
• Dual slope
• Voltage to frequency
• Parallel (or flash)
Counter / Staircase Ramp ADC

 Operation
 Reset and Start Counter
 DAC convert Digital output of Counter
to Analog signal
 A clock generated pulse
 Binary counter generates an n bit
digital output which is applied as an
input to the DAC
 Compare Analog input and Output of
DAC
• VA> VDAC
– Continue counting
• VA < VDAC
– Stop counting
 Digital Output = Output of Counter

 Disadvantage
 Conversion time is varied
• 2n Clock Period for Full Scale input
Comparator Principles
Contoh soal ADC

- Ketika direset counter = 0,

maka tegangan yang keluar
DAC dirumuskan sbb
0
- 𝑉𝐷𝐴𝐶 = 23 × 5 = 0 volt
- Di komparator karena 𝑉𝐷𝐴𝐶 >
𝑉𝐴 maka output komparator = 1
(asumsi +𝑉𝐶𝐶 ) = 1
- Di logika and ketika output
Sebuah ADC digunakan untuk komparator bernilai 1, maka
menerka tegangan input (VA) sebesar output ADC bernilai 1 dan 0.
4 Volt. Spec ADC (3 bit dan output 0 - Counter mengupgrade nilainya
s/d 5 Volt). Uraikan proses menjadi 1, ketika mendeteksi
penerkaan tegangan yang keluar dari nilainya 1 dan 0.
1
ADC - 𝑉𝐷𝐴𝐶 = 23 × 5 = 0.625 volt
- Proses berjalan lagi dari awal
ADC Process
Tracking Type ADC

 Tracking or Servo Type  Can be used as S/H circuit

 Using Up/Down Counter to  By stopping desired instant
track input signal  Digital Output
continuously  Long Hold Time
• For slow varying input  Disabling UP (Down) control,
Converter generate
 Minimum (Maximum) value
reached by input signal over a
given period
Tracking ADC
Successive Approximation
ADC
 Most Commonly used in  Block Diagram
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
Successive Approximation
ADC
 Circuit waveform  Conversion Time
 n clock for n-bit ADC
 Fixed conversion time
 Serial Output is easily
generated
 Bit decision are made in
serial order
 Logic Flow
ADC Formulation
An Example of SAR Calculation (1/5)
An Example of SAR Calculation (2/5)
An Example of SAR Calculation (3/5)
An Example of SAR Calculation (4/5)
An Example of SAR Calculation (5/5)
Advantages and Disadvantages

Advantages Disadvantages

• Capable of high speed  Higher resolution

• Medium accuracy compared
to other ADC types
successive
• Good tradeoff between approximation ADCs
speed and cost will be slower
 Speed limited
~5Msps
Dual Slope Integrating ADC

 Operation T1
 
Integrate 0 vi dt
t2


Reset and integrate
Thus T v

0
Vr dt
1 i ( AVG )  t2Vr
  t2
vi ( AVG )  Vr
T1
 Applications
 DPM(Digital Panel Meter),
DMM(Digital Multimeter), …

VS=-VA/RC×t1
VS=Vref/RC×t2
Vref/RC×t2=-VA/RC×t1
VA=-Vref×t1/t2
 Kelebihan dan kekurangan
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
Contoh Dual Slope

 For example, consider the clock frequency is 1 MHz, the reference voltage
(Vref) is -1 V, the fixed time period t1 is 1 ms and the RC time constant is also
1 ms.
 Assuming the unknown analog input voltage amplitude as VA = 5 V, during
the fixed time period t1 , the integrator output Vs is
VS = -VA/RC × t1= (-5)/1 ms × 1 ms=-5 V
 During the time period t2, ramp generator will integrate all the way back to 0V.
t2 = VS/Vref × RC = (-5)/(-1) × 1 ms = 5ms = 5000μs
 Hence the 4-bit counter value is 5000, and by activating the decimal point of
(most Significant Digit) MSD seven segment displays, the display can directly
read as 5 V.
VS=-VA/RC×t1
VS=Vref/RC×t2
Vref/RC×t2=-VA/RC×t1
VA=-Vref×t1/t2
Advantages and Disadvantages

Advantages Disadvantages

 Input signal is  Slow

averaged  High precision
 Greater noise external components
immunity than other required to achieve
ADC types accuracy
 High accuracy
Voltage to Frequency ADC

 VFC (Voltage to Frequency  Low Speed

Converter)  Good Noise Immunity
 Convert analog input voltage  High resolution
to train of pulses
 For slow varying signal
 Counter
 With long conversion time
 Generates Digital output by
counting pulses over a fixed  Applicable to remote data
interval of time 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
 Single Step Conversion
 2n –1 comparator
 Precision Resistive Network
 Encoder
 Resolution is limited
 Large number of comparator in
IC
Simulasi EWB

Flash_ADC.ewb
Advantages and Disadvantages

Advantages Disadvantages
 Very fast  Needs many parts
(255 comparators
for 8-bit ADC)
 Lower resolution
 Expensive
 Large power
consumption
Advantages AND Disadvantages

Type of ADC Speed Price Noise Conversion

Immunity Time
Voltage to    Constant
frequency
Dual slope    Vary
Staircase    Vary
ramp 2n
Tmax 
f
Successive    Constant
approximation n
T 
f
Parallel (or    Constant
flash) Not feasible
for high
resolution
Transformasi antar base
(GENAP)
Transformasi antar base
(GANJIL)
Convert basis 16 ke basis 2

HEKSA NUMBER DESIMAL 23= 8 22= 4 21= 2 20= 1

0H 0 0 0 0 0
1H 1 0 0 0 1
2H 2 0 0 1 0
3H 3 0 0 1 1
4H 4 0 1 0 0 Misal
5H 5 0 1 0 1 DH16 = 11012
6H 6 0 1 1 0
FH16 = 11112
7H 7 0 1 1 1
8H 8 1 0 0 0 9H16 = 10012 dst
9H 9 1 0 0 1
AH 10 1 0 1 0
BH 11 1 0 1 1
CH 12 1 1 0 0
DH 13 1 1 0 1
EH 14 1 1 0 0
FH 15 1 1 1 1
Convert basis 16 ke basis
10

Misal

Check
Convert basis 8 ke basis 10

Misal
OCTA NUMBER 22 = 4 21 = 2 20 = 1 68 = 1102
0 0 0 0 48 = 1002
1 0 0 1 78 = 1112

2 0 1 0 dst

3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1
ADC on EWB

(EOC = End of Conversion)

(SOC = Start of Conversion)

ADC simulation on EWB

MSB = D7 adc_basic.ewb
LSB = D0
Kenapa keluar 7F H  lihat bab DSC (digital signal conditioning)
Explanation

0111 1111

= 1 × 23 + 1 × 22 + 1 × 21 + 1 × 20
= 8 + 4 + 2 + 1 = 15  FH

= 0 × 23 + 1 × 22 + 1 × 21 + 1 × 20
= 0 + 4 + 2 + 1 = 7  7H
How It is works?

When this ADC is connected to a computer, the sequence of operation is

listed below:

1. The computer reads the EOC to check the ADC is busy or not.
2. If the ADC is not busy when the computer selects the input channel
and send out the “Start” signal. Otherwise, step (1) is repeated.
3. The computer monitors the EOC.
4. When the EOC is activated, the computer reads the digital output.

When there is more than one ADCs being linked to the computer, they
can be connected in parallel. Using the ‘output enable’ can do the
selection of ADC output.
How to select and use an
ADC
 Range of commercially  Guidelines for using
available ADCs ADCs
 Use the full input range of
the ADC
 Use a good source of
reference signal
 Look out for fast input
signal changes
 Keep analog and digital
grounds separate
 Minimize interference and
loading problem
Selection of ADC

The parameters used in selecting an ADC are very similar to those

considered for a DAC selection.

• Error/Accuracy: Quantising error represents the difference

between an actual analog value and its digital representation.
Ideally, the quantising error should not be greater than ± ½
LSB.
• Resolution: V to cause 1 bit change in output
• Output Voltage Range  Input Voltage Range
• Output Settling Time  Conversion Time
• Output Coding (usually binary)
Commercially available
monolithic ADCs
Commercially available
hybrid ADCs
DAS (Data Acquisition System)

 DAS performs the  Applications

complete function of  Simple monitoring of a
converting the raw single analog variable
outputs from one or more  Control and Monitoring of
sensors into equivalent hundreds of parameters in
a nuclear plant
digital signals usable for
further processing, control,
or displaying applications
Single Channel System
 S/H (Sample and Hold)
 Transducer  Reduce uncertainty error in
the converted output when
 Generate signal of low input changes are fast
amplitude, mixed with compared to the conversion
undesirable noise time
 Amplifier, Filters  In Multi-channel system
• To hold a sample from one
 Amplify channel while multiplexer
 Remove noise proceed to sample next one
 Linearize • Simultaneous sampling of
two signal
Multi-channel System

 Analog multiplexer and a  Local ADCs and digital

ADC multiplexer
 Low cost  Higher sampling rate
Worked Examples

Question 1. Calculate the maximum conversion time of (a) a 8-bit

staircase ramp ADC and (b) a successive approximation ADC, if
the clock rate is 2MHz.
Solution:
(a) For a 8-bit staircase ramp ADC, the maximum number of
count is
nc = 28 = 256
Therefore, the maximum conversion time is
nc 256
Tc    128  10 6
s  128s
f 2  10 6
(b) For a 8-bit successive approximation ADC, the conversion
time is constant and equal to

n 8
Tc    4  10 6
s  4 s
f 2  10 6

It can be noted that the conversion speed of successive

approximation ADC is much faster than the staircase ramp
type.
Question 2.

Find out the percentage resolution of a DAC of n bits, and hence

determine the value for n = 12.

Solution:
1
Percentage resolution = n
 100%
2
For n = 12,

Percentage resolution =

1
12
 100%  0.0244%  244ppm (part per million)
2
An example of ADC (1/3)

Suatu alat pengukur temperatur menggunakan suatu sensor dengan persamaan

output 6,5 mV/C. Sebuah ADC 6 bit dengan tegangan referensi 10 V digunakan
dalam peralatan ini.
a. Rancang sebuah sirkuit yang menghubungkan sensor dengan ADC
b. Cari resolusi dari sensor temperatur tsb!
An example of ADC (2/3)

9.84375
𝑔𝑎𝑖𝑛 = = 15.14
0.65
An example of ADC (3/3)