Download Operation of the successive-approximation A/D

Data Acquisition Systems
Data acquisition is the process of sampling
signals that measure real world physical
conditions and converting the resulting
samples into digital numeric values that can
be manipulated by a computer.
Data acquisition systems (abbreviated with
the acronym DAS or DAQ) typically convert
analog waveforms into digital values for
processing.
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Contd. Data Acquisition System
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A data acquisition system consists of many
components that are integrated to:
Sense physical variables (use of
transducers)
Condition the electrical signal to make it
readable by an A/D board.
Convert the signal into a digital format
acceptable by a computer
Process, analyze, store, and display the
acquired data with the help of software
Data Acquisition System
Analog
Signal
Signal
Conditioner
ADC
Communication
Digital
Processing
Data Acquisition System
Block Diagram
The components of data acquisition
systems include:
Sensor(Transd
ucers)
Sensors that convert physical parameters to electrical signals
Sense physical phenomena and translate it
into electric signal.
Temperature
 Pressure
 Light
 Force
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Displacement
 Level
 Electric
signals
 ON/OFF
switch
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Signal Conditioning
Signal conditioning circuitry to convert sensor
signals into a form that can be converted to digital
values.
Functions: modify the analog signal to match
the performance of the ADC
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Pre-filtering: remove undesirable high
frequency components
Amplification: amplify the signal to match the
dynamic range of the ADC
Analog-to-Digital Conversion
•
Most signals are naturally analog (e.g. a voltage)
•
Digital techniques are often more useful: data
storage, processing, computing, error free signal
transmission etc.
•
We need ways to convert analog signals to digital
•
Want A/D converters to be fast, accurate and
cheap
Before a computer can process analog information,
we must first use an analog-to-digital converter
(ADC) to transform the analog values into digital
binary values.
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Data Acquisition Software
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It can be the most critical factor in
obtaining reliable, high performance
operation.
Transforms the PC and DAQ hardware into
a complete DAQ, analysis, and display
system.
Different alternatives:
 Programmable software.
 Data acquisition software packages.
Successive-approximation A/D converter
The successive approximation ADC has
been the mainstay of data acquisition systems
for many years.
A successive approximation ADC is a type of
analog-to-digital converter that converts a
continuous analog waveform into a discrete
digital representation via a binary search
through all possible quantization levels before
finally converging upon a digital output for
each conversion.
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Contd. Successive-approximation A/D
converter
The successive-approximation A/D converter
consists of:
• D/A converter
• Comparator
• Success-approximation register .
• Conversion process starts at Maximum range
(MSB) and steps down through a defined
sequence to the (LSB) until correct reading
is found using a DAC and an analog
comparator circuit.
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Contd. Successive-approximation A/D
converter
Successive Approximation ADC Block Diagram.
Key
DAC = Digital-to-Analog converter
EOC = end of conversion
SAR = successive approximation register
S/H = sample and hold circuit
Vin = input voltage
Vref = reference voltage
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Contd. Successive-approximation A/D
converter
The successive approximation Analog to
digital converter circuit typically consists of
four chief sub circuits:
A sample and hold circuit to acquire the
input voltage (Vin).
An analog voltage comparator that
compares Vin to the output of the
internal DAC and outputs the result of the
comparison to the successive
approximation register (SAR).
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Contd. Successive-approximation A/D
converter
A successive approximation register sub
circuit designed to supply an approximate
digital code of Vin to the internal DAC.
An internal reference DAC that, for
comparison with VREF, supplies
the comparator with an analog voltage equal
to the digital code output of the SARin.
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Operation of the successiveapproximation A/D converter
The successive approximation register is
initialized so that the most significant
bit (MSB) is equal to a digital 1.
This code is fed into the DAC, which then
supplies the analog equivalent of this digital
code (Vref/2) into the comparator circuit for
comparison with the sampled input voltage.
If this analog voltage exceeds Vin the
comparator causes the SAR to reset this bit;
otherwise, the bit is left a 1.
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Operation of the successiveapproximation A/D converter
Then the next bit is set to 1 and the same test
is done, continuing this binary search until
every bit in the SAR has been tested.
The resulting code is the digital
approximation of the sampled input voltage
and is finally output by the SAR at the end of
the conversion (EOC).
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Operation of the successiveapproximation A/D converter
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Example:Operation of the successiveapproximation A/D converter
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Successive Approximation ADC
Example
Goal: Find digital value
Vin
• 8-bit ADC
• Vin = 7.65
• Vfull scale = 10
Successive Approximation ADC
Example
• MSB  LSB
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 7
• (Vfull scale +0)/2 = 5
• 7.65 > 5  Bit 7 = 1
1
Vfull scale = 10, Vin = 7.65
Successive Approximation ADC
Example
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 6
• (Vfull scale +5)/2 = 7.5
• 7.65 > 7.5  Bit 6 = 1
1
1
Vfull scale = 10, Vin = 7.65
Successive Approximation ADC
Example
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 5
• (Vfull scale +7.5)/2 = 8.75
• 7.65 < 8.75  Bit 5 = 0
1
1
0
Vfull scale = 10, Vin = 7.65
Successive Approximation ADC
Example
Vin = 7.65
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 4
• (8.75+7.5)/2 8.125
• 7.65 < 8.125  Bit 4 = 0
1
1
0
0
Successive Approximation ADC
Example
Vin = 7.65
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 3
• (8.125+7.5)/2 = 7.8125
• 7.65 < 7.8125  Bit 3 = 0
1
1
0
0
0
Successive Approximation ADC
Example
Vin = 7.65
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 2
• (7.8125+7.5)/2 = 7.65625
• 7.65 < 7.65625  Bit 2 = 0
1
1
0
0
0
0
Successive Approximation ADC
Example
Vin = 7.65
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 1
• (7.65625+7.5)/2 = 7.578125
• 7.65 > 7.578125  Bit 1 = 1
1
1
0
0
0
0
1
Successive Approximation ADC
Example
Vin = 7.65
• MSB  LSB
• Average high/low limits
• Compare to Vin
• Vin > Average  MSB = 1
• Vin < Average  MSB = 0
• Bit 0
• (7.65625+7.578125)/2 =
7.6171875
• 7.65 > 7.6171875  Bit 0 = 1
1
1
0
0
0
0
1
1
Successive Approximation ADC
Example
• 110000112 = 19510
• 8-bits, 28 = 256
• Digital Output
• 195/256 = 0.76171875
• Analog Input
• 7.65/10 = 0.765
Exercise
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Given an analog input signal whose voltage should
range from 0 to 15 volts, and an 8-bit digital
encoding, calculate the approximate encoding for
5 volts. Trace the successive-approximation
approach to find the correct encoding.