introduction of adc& dacshare.its.ac.id/pluginfile.php/808/mod_resource/content/1/adc_dan... ·...

Introduction of ADC& DAC

Presented by:M. Khoirul Effendi ST. MSc. (Eng.)

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Review of Control System (1/2)

controller

Control element

Process

Measurement

p

r

c

u

b

e = r-b

Summing points

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Review of Control System (2/2)

PROSES YANG INGIN DIKONTROL SENSOR

ANALOG TO DIGITAL CONVERTER (ADC)

OPERATIONAL AMPLIFIER

CONTROLLER SET POINTDIGITAL TO ANALOG CONVERTER (DAC)

ACTUATOR

FINAL ELEMENT

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Analog to Digital Converter (ADC)

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What is an ADC (1/2)Clic

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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. Able to control multivariable in the 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

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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 meansADC has a three-step process

• Sampling• Quantization• Coding

Sample and Hold (S/H)

Quantizer Coder

A/D Converter

Xa(t) X(n) Xq(n) 01011 …

Analog Signal Discrete-TimeSignal

QuantizedSignal

Digital Sinyal

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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.

Sample and Hold (S/H)Clic

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Commercially Available S/HClic

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Sampling DefinitionClic

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Sampling Theorem Clic

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Sampling TheoremClic

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An Ilustration of Aliasing

In physics, a moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes.

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http://en.wikipedia.org/wiki/Interference



Sampling TheoremClic

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8 MHz4 MHzVideo

40 kHz20 kHzAudio

8 kHz4 kHzSpeech

4 kHz2 kHzMechanical

2 kHz1 kHzBiomedical

1 kHz500 HzGeophysical

fsfmaxAplikasi

Sampling Rate of DSP Applications

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• X(n) = xa(nT); - < n < • t = nT = n/Fs

– Fs = Sampling rate– T = Sampling period

Sampling Formulation (1/3)Clic

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Relationship Among Frequency Variables

-Fs/2 F Fs/2- < F < -- < <

-½ f ½= /T, F = f Fs

-= T, f = F/Fs

(radians/sample) (cycles/sample)(radians/sec) Hz

= 2 f= 2 F

Discrete-Time SignalsContinuous-Time Signals


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Sampling TheoremIf the highest freq. contained in an analog signal xa(t) is Fmax = B and the signal is sampled at a rate Fs > 2 Fmax 2B, then xa(t) can be exactly recovered from its sample values using the interpolation function

g(t) = (sin 2 Bt)/(2 Bt)


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Sampling Example 1 (1/3)Clic

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Sampling Example 2Clic

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QuantizationClic

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Quantizing breaking down analog value into a set of finite states

Quantizing (1/2)Clic

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Quantizing (2/2)Clic

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Coding assigning a digital word or number to each state.

CodingClic

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DitheringClic

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LSB (Least Significant Bits)Clic

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AccuracyClic

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Accuracy ResolutionClic

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Accuracy Sampling rateClic

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Accuracy (Resolution & Sampling rate)

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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 ADC• Tracking ADC• Successive approximation• Dual slope• Voltage to frequency• Parallel (or flash)

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Counter Type ADC

OperationReset and Start CounterDAC convert Digital output of Counter to Analog signalA clock pulse is used to increment the counter value increase ViCompare Analog input and Output of DAC

• Vi < VDAC– Continue counting

• Vi = VDAC– Stop counting

Digital Output = Output of Counter

DisadvantageConversion time is varied

• 2n Clock Period for Full Scale input

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Tracking Type ADC

Tracking or Servo TypeUsing Up/Down Counter to track input signal continuously

• For slow varying input

Can be used as S/H circuitBy stopping desired instantDigital OutputLong Hold Time

Disabling UP (Down) control, Converter generate

Minimum (Maximum) value reached by input signal over a given period

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Successive Approximation ADC

Most Commonly used in medium to high speed ConvertersBased on approximating the input signal with binary code and then successively revising this approximation until best approximation is achievedSAR(Successive Approximation Register) holds the current binary value

Block Diagram

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Successive Approximation ADC

Circuit waveform

Logic Flow

Conversion Timen clock for n-bit ADCFixed conversion time

Serial Output is easily generated

Bit decision are made in serial order

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ADC FormulationClic

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An Example of SAR Calculation (1/5)Clic

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Dual Slope Integrating ADC

OperationIntegrateReset and integrateThus

ApplicationsDPM(Digital Panel Meter), DMM(Digital Multimeter), …

Excellent Noise RejectionHigh frequency noise cancelled out by integrationProper T1 eliminates line noiseEasy to obtain good resolution

Low SpeedIf T1 = 60Hz, converter throughput rate < 30 samples/s

1

0

T

iv dt2

0

t

rV dt

1 ( ) 2i AVG rT v t V2

( )1

i AVG rtv VT

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Voltage to Frequency ADC

VFC (Voltage to Frequency Converter)

Convert analog input voltage to train of pulses

CounterGenerates Digital output by counting pulses over a fixed interval of time

Low SpeedGood Noise ImmunityHigh resolution

For slow varying signalWith long conversion time

Applicable to remote data sensing in noisy environments

Digital transmission over a long distance

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Parallel or Flash ADC

Very High speed conversionUp to 100MHz for 8 bit resolutionVideo, Radar, Digital Oscilloscope

Single Step Conversion2n –1 comparatorPrecision Resistive NetworkEncoder

Resolution is limitedLarge number of comparator in IC

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Type of ADC Speed Price Noise Immunity

Conversion Time

Voltage to frequency

Constant

Dual slope Vary Staircase ramp

Vary

fT

n2max

Successive approximation

Constant

fnT

Parallel (or flash)

Not feasible for high resolution

Constant

Advantages AND DisadvantagesClic

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In practice, an ADC is usually in form of an integrated circuit (IC). ADC0808 and ADC0809 are two typical examples of 8-bit ADC with 8-channel multiplexer using successive approximation method for its conversion.

ADC0809National Semiconductor

For more information,

http://www.national.com/ads-cgi/viewer.pl/ds/AD/ADC0808.pdf

An Example of ADC Clic

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-Vref

Start Clock

End of Conversion

8-bit OutputOutputLatchBuffer

OutputEnable

256R Resistor Ladder

S.A.R.

8-bit ADC

Control & Timing

Switch Tree

Comparator

8 ChannelsMultiplexing

Switches

AddressLatch andDecoder

8 Analog Inputs

3-bit Address

Address LatchEnable

VCC GND +Vref

Block Diagram

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ADC on EWB

(SOC = Start of Conversion)

(EOC = End of Conversion)

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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.

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How to select and use an ADC

Range of commercially available ADCs

Guidelines for using ADCs

Use the full input range of the ADCUse a good source of reference signalLook out for fast input signal changesKeep analog and digital grounds separateMinimize interference and loading problem

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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 ± ½LSB.

• Resolution: V to cause 1 bit change in output• Output Voltage Range Input Voltage Range• Output Settling Time Conversion Time• Output Coding (usually binary)

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Commercially available monolithic ADCs

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Commercially available hybrid ADCs

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DAS (Data Acquisition System)

DAS performs the complete function of converting the raw outputs from one or more sensors into equivalent digital signals usable for further processing, control, or displaying applications

ApplicationsSimple monitoring of a single analog variableControl and Monitoring of hundreds of parameters in a nuclear plant

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Single Channel System

TransducerGenerate signal of low amplitude, mixed with undesirable noise

Amplifier, FiltersAmplifyRemove noiseLinearize

S/H (Sample and Hold)Reduce uncertainty error in the converted output when input changes are fast compared to the conversion timeIn Multi-channel system

• To hold a sample from one channel while multiplexer proceed to sample next one

• Simultaneous sampling of two signal

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Multi-channel System

Analog multiplexer and a ADC

Low cost

Local ADCs and digital multiplexer

Higher sampling rate

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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

ssf

nT cc 12810128

102256 6

6

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(b) For a 8-bit successive approximation ADC, the conversion time is constant and equal to

ssfnTc 4104

1028 6

6

It can be noted that the conversion speed of successive approximation ADC is much faster than the staircase ramp type.

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Question 2.

Find out the percentage resolution of a DAC of n bits, and hence determine the value for n = 12.

Solution:Percentage resolution =

For n = 12,

%10021

n

Percentage resolution =

million)per (part ppm244%0244.0%1002112

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Suatu alat pengukur temperatur menggunakan suatu sensor dengan persamaanoutput 6,5 mV/C. Sebuah ADC 6 bit dengan tegangan referensi 10 V digunakandalam peralatan ini. a. Rancang sebuah sirkuit yang menghubungkan sensor dengan ADCb. Cari resolusi dari sensor temperatur tsb!

An example of ADC (1/3)Clic

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Digital to Analog Converter (DAC)

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In an electronic circuit, a combination of high voltage (+5V) and low voltage (0V) is usually used to represent a binary number. For example, a binary number 1010 is represented by

Weighting 23 22 21 20

Binary Digit 1 0 1 0

State +5V 0V +5V 0V

DACs are electronic circuits that convert digital, (usually binary) signals (for example, 1000100) to analog electrical quantities (usually voltage) directly related to the digitally encoded input number.

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DACs are used in many other applications, such as voice synthesizers, automatic test system, and process control actuator. In addition, they allow computers to communicate with the real (analog) world.

Regi

ster

VoltageSwitch

ResistiveSummingNetwork

Amplifier

Input BinaryNumber

Analog VoltageOutput

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Register: Use to store the digital input (let it remain a constant value) during the conversion period.

Voltage: Similar to an ON/OFF switch. It is ‘closed’ when the input is ‘1’. It is ‘opened’ when the input is ‘0’.

Resistive Summing Network: Summation of the voltages according to different weighting.

Amplifier: Amplification of the analog according to a pre-determined output voltage range. For example, an operation amplifier

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The three most popular types of resistive summing networks are:

Weighted binary resistance type, and

Ladder resistance (R-2R) type

Multiplying ADC

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Weighted Binary Resistance Type (1/3)

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R2R Ladder Network (1/5)

2R

S1

2R

Rf

R1

R

2RR

2RR

2RR

S2

S3

S4

Vout

VrefMSB

LSB

The basic principle of this circuit is formed to overcome great The basic principle of this circuit is formed to overcome great obstacles obstacles that occur when the resistor circuit increases the number of bitthat occur when the resistor circuit increases the number of bitss

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R2R Ladder Network (2/5)Clic

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Multiplying ADC (1/3)Clic

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Selection of DACClic

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Selection of DAC

For the selection of an IC DAC, there are several parameters that can determine the suitability of a particular device.

ResolutionThe number of bits making up the input data word that will ultimately determine the output step voltage as a percentage of full-scale output voltage. Example: Calculate the resolution of an 8-bit DAC.

Solution: Resolution = 8 bits

Percentage resolution =

%391.0%1002561%100

21

8

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Output Voltage Range

This is the difference between the maximum and minimum output voltages expressed in volts.

Example:

Calculate the output voltage range of a 4-bit DAC if the output voltage is +4.5V for an input of 0000 and +7.5V for an input of 1111.

Solution:

Output voltage range = 7.5 – 4.5 = 3.0V


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Accuracy

The accuracy is usually expressed by the error in output voltage compared with the expected output voltage. The higher the accuracy, the lower will be the error. Due to the incremental nature of the digital input word, an error can be tolerated but it should not exceed ±½LSB or ½resolution.

Example. The error at full-scale for an 8-bit DAC with 10V maximum output is 50mV. Calculate the error and compare it with the resolution.

Solution: Error =

Resolution = ; ½ Resolution = 0.195%

%5.0%1001005.0

%391.0%1002561


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The accuracy is not as good as the error = ½ resolution, but for many applications, it is quite satisfactory. Some commercially available DACs have their accuracy specified as worse than ½resolution.

Sources of errors may be broadly classified under four categories:Non-monotonicityFull Scale-factor errorOffset errorGain errorDiff non linearity errorSettling time and overshoot errorResolution error


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Selection of DAC

LSB = Least Significant Bit The LSB is the smallest level that an ADC can convert, or is the smallest increment a DAC outputs

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Input coding

The digital input can be in binary format or it can be in binary coded decimal format depending on the application. Binary format is more commonly used.


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introduction of adc& dacshare.its.ac.id/pluginfile.php/808/mod_resource/content/1/adc_dan... ·...

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