Computerized Data Acquisition Systems

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Computerized Data Acquisition Systems
Transducer
Signal Conditioning
A/D Conversion
D/A Conversion
Computer
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Typical PC-based DAQS

• Data Acquisition
• Data saving
• Data manipulation and display
• Process control by making use of the results

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SCX Signal Conditioning

• Amplification
• Attenuation
• Multiplexing
• Filtering
• Excitation
• Linearization

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Signal Conditioning

• Amplification
• Amplify low level signal
• Max of amplified signal equals to max input range
  of the analog to digital converter (ADC)
• Attenuation
• Reduce high-level signals
• Multiplexing
• Measure multiple signals
• Filtering
• Remove undesirable frequency information
• Noise filter
• Antialiasing filter
• Excitation
• Generate excitation for some transducers strain
  gauge
• Linearization
• Account for nonlinear response to change in
  phenomena being measured, thermocouples

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Signal conditioning board
SC-2345
SC-2075
CA-1000
SCB-68
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Plug-In Data Acquisition Boards

• Key Benefits
• Lowest cost
• Large selection to choose from
• Established platform

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Example of Computer DAQ System
DAQ Board
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Inside the Case of a Computer

• It contains a system board (mother board). This
  board hosts most of the circuits connecting
• CPU,
• memory,
• I/O ports.
• buses,
• storage
• Power supplier

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Sampling Concepts

• Sampling Rate
• How often the AD conversion take place
• Multiplexing
• A common technique to measure several signals
  with a single ADC

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Aliasing
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Representing Numbers in Computers
Everyday life Decimal
Computers Binary
One-to-one correspondence
A bit has bistable states on (1) and off (0) 8
bits 1 byte
01011100 0?20 0?21 1?22 1?23 1?24
0?25 1?26 0?2792
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Representing Negative Numbers
MSB is 0 for positive numbers MSB is 1 for
negative numbers
2s complement

• Convert the integer to binary as if it were
  positive
• Invert all of the bits
• Add 1 least significant bit to produce the final
  result

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Major DAQ Components

• Multiplexers (MUX) multi-channel handling
• Analog-to-digital converters
• Digital-to-analog converters
• Simultaneous sample-and-hold subsystems

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Basics of A/D Converters

• The output has 2N possible values quantization
• N number of bits.
• Characteristics
• N determines the resolution of the output
• Input range unipolar and bipolar
• Conversion speed

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Data Acquisition Terminology

• Resolution - Determines how much different
  voltage changes can be measured
• Higher resolution ? more precise representation
  of signal
• Range - minimum and maximum voltages
• Smaller range ? more precise representation of
  signal
• Gain - Amplifies or attenuates signal for best
  fit in range
• Code width (LSB) smallest detectable change in
  voltage depend on range, resolution, and gain

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Resolution Error
Input resolution error Example A 12-bit A/D
converter has an analog input range of -10 to 10
V. Find the resolution error of the converter for
the analog input.
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Single-slope integrating A/D Converter
Analog input voltage
voltage
Integrator voltage
Input range
Time (counter)
Counter
Integrator
Comparator
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Successive-approximation converters

• Employ an interval-halving rule
• Example
• An 8-bit AD unipolar successive-approximation
  converter has an input range of 0 to 5V. An
  analog voltage of 3.15 V is applied to the input.
  Find the analog output

Reference voltage increment (?V) input span/2N
0.01953 V
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Demo AD Conversion
saturation error
Quantization error
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D/A Conversion
Convert n-digital binary word into an analog
voltage