CHAPTER 1 Transducers, Signals, and Signal Conditioning

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CHAPTER 1 Transducers, Signals, and Signal
Conditioning
Lesson 8Data Acquisition and Waveforms

• Topics
• Data Acquisition Overview
• Transducers
• Signals
• Signal Conditioning

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System Overview
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Transducer Overview

• Topics
• What is a Transducer?
• Types of Transducers

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What is a Transducer?
A transducer converts a physical phenomena into
a measurable signal
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Signal Overview

• Topics
• Types of Signals
• Information in a Signal
• State, Rate, Level, Shape, and Frequency

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Signal Classification

• Your Signal

Analog
Digital
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Digital Signals
Your Signal
Digital

• Two possible levels
• High/On (2 - 5 Volts)
• Low/Off (0 - 0.8 Volts)
• Two types of information
• State
• Rate

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Digital Signal Information
Your Signal
Digital
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Analog Signals

• Your Signal

Analog

• Continuous signal
• Can be at any value with respect to time
• Three types of information
• Level
• Shape
• Frequency (Analysis required)

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Analog Signal Information

• Your Signal

Analog
Analysis Required
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Signal Conditioning Overview

• Topics
• Purpose of Signal Conditioning
• Types of Signal Conditioning

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Why Use Signal Conditioning?

• Signal Conditioning takes a signal that is
  difficult for your DAQ device to measure and
  makes it easier to measure
• Signal Conditioning is not always required
• Depends on the signal being measured

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Amplification

• Used on low-level signals (i.e. thermocouples)
• Maximizes use of Analog-to-Digital Converter
  (ADC) range and increases accuracy
• Increases Signal to Noise Ratio (SNR)

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DAQ Hardware Overview

• Topics
• Types of DAQ Hardware
• Components of a DAQ device
• Configuration Considerations

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Data Acquisition Hardware

• DAQ Hardware turns your PC into a
• measurement and automation system

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Terminal Block and Cable
50 pin connector

• Terminal Block and Cable route your signal to
  specific pins on your DAQ device
• Terminal Block and Cable can be a combination of
  68 pin or 50 pin

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DAQ Device

• Most DAQ devices have
• Analog Input
• Analog Output
• Digital I/O
• Counters
• Specialty devices exist for specific applications
• High speed digital I/O
• High speed waveform generation
• Dynamic Signal Acquisition (vibration, sonar)
• Connect to the bus of your computer
• Compatible with a variety of bus protocols
• PCI, PXI/CompactPCI, ISA/AT, PCMCIA, USB,
  1394/Firewire

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Configuration Considerations

• Analog Input
• Resolution
• Range
• Gain
• Code Width
• Mode (Differential, RSE, or NRSE)
• Analog Output
• Internal vs. External Reference Voltage
• Bipolar vs. Unipolar

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Resolution

• Number of bits the ADC uses to represent a signal
• Resolution determines how many different voltage
  changes can be measured
• Example 12-bit resolution
• Larger resolution more precise representation
  of your signal

of levels 2resolution 212 4,096 levels
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Resolution Example

• 3-bit resolution can represent 8 voltage levels
• 16-bit resolution can represent 65,536 voltage
  levels

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Range

• Minimum and maximum voltages the ADC can digitize
• DAQ devices often have different available ranges
• 0 to 10 volts
• -10 to 10 volts
• Pick a range that your signal fits in
• Smaller range more precise representation of
  your signal
• Allows you to use all of your available resolution

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Range

• Proper Range
• Using all 8 levels to represent your signal

• Improper Range
• Only using 4 levels to represent your signal

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Gain

• Gain setting amplifies the signal for best fit in
  ADC range
• Gain settings are 0.5, 1, 2, 5, 10, 20, 50, or
  100 for most devices
• You dont choose the gain directly
• Choose the input limits of your signal in LabVIEW
• Maximum gain possible is selected
• Maximum gain possible depends on the limits of
  your signal and the chosen range of your ADC
• Proper gain more precise representation of your
  signal
• Allows you to use all of your available resolution

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Gain Example

• Input limits of the signal 0 to 5 Volts
• Range Setting for the ADC 0 to 10 Volts
• Gain Setting applied by Instrumentation Amplifier
  2

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Code Width

• Code Width is the smallest change in the signal
  your system can detect (determined by resolution,
  range, and gain)
• Smaller Code Width more precise representation
  of your signal
• Example 12-bit device, range 0 to 10V, gain
  1

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Grounding Issues

• To get correct measurements you must properly
  ground your system
• How the signal is grounded will affect how we
  ground the instrumentation amplifier on the DAQ
  device
• Steps to proper grounding of your system
• Determine how your signal is grounded
• Choose a grounding mode for your Measurement
  System

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Signal Source Categories
Signal Source
Grounded
Floating
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Grounded Signal Source
Signal Source

• Signal is referenced to a system ground
• earth ground
• building ground
• Examples
• Power supplies
• Signal Generators
• Anything that plugs into an outlet ground

Grounded
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Floating Signal Source
Signal Source

• Signal is NOT referenced to a system ground
• earth ground
• building ground
• Examples
• Batteries
• Thermocouples
• Transformers
• Isolation Amplifiers

Floating
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Measurement System

• Three modes of grounding for your Measurement
  System
• Differential
• Referenced Single-Ended (RSE)
• Non-Referenced Single-Ended (NRSE)
• Mode you choose will depend on how your signal is
  grounded

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Differential Mode

• Differential Mode
• Two channels used for each signal
• ACH 0 is paired with ACH 8, ACH 1 is paired with
  ACH 9, etc.
• Rejects common-mode voltage and common-mode noise

Measurement System
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RSE Mode

• Referenced Single-Ended (RSE)
• Measurement made with respect to system ground
• One channel used for each signal
• Doesnt reject common mode voltage

Measurement System
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NRSE Mode

• Non-Referenced Single-Ended (NRSE)
• Variation on RSE
• One channel used for each signal
• Measurement made with respect to AISENSE not
  system ground
• AISENSE is floating
• Doesnt reject common mode voltage

Measurement System
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Choosing Your Measurement System
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Options for Grounded Signal Sources
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Options for Floating Signal Sources
BEST Rejects Common-Mode
Voltage - Cuts Channel Count in Half - Need bias
resistors
Differential
BETTER Allows use of entire
channel count Dont need bias resistors -
Doesnt reject Common-Mode Voltage
RSE
GOOD Allows use of entire
channel count - Need bias resistors - Doesnt
reject Common-Mode Voltage
NRSE
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DAQ Software Overview

• Topics
• Levels of DAQ Software
• NI-DAQ Overview
• Measurement Automation Explorer (MAX) Overview

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Levels of Software
User
Application Level
LabVIEW
Diagnostic Level
MAX
Driver Level
NI-DAQ
DAQ Device
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What is NI-DAQ?

• Driver level software
• DLL that makes direct calls to your DAQ device
• Supports the following National Instruments
  software
• LabVIEW
• Measurement Studio
• Also supports the following 3rd party languages
• Microsoft C/C
• Visual Basic
• Borland C
• Borland Delphi

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What is MAX?

• MAX stands for Measurement Automation Explorer
• MAX provides access to all your National
  Instruments DAQ, GPIB, IMAQ, IVI, Motion, VISA,
  and VXI devices
• Used for configuring and testing devices
• Functionality broken into
• Data Neighborhood
• Devices and Interfaces
• Scales
• Software

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Data Neighborhood

• Provides access to the DAQ Channel Wizard
• Shows configured Virtual Channels
• Includes utilities for testing and reconfiguring
  Virtual Channels

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DAQ Channel Wizard

• Interface to create Virtual Channels for
• Analog Input
• Analog Output
• Digital I/O
• Each channel has
• Name and Description
• Transducer type
• Range (determines Gain)
• Mode (Differential, RSE, NRSE)
• Scaling

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Devices and Interfaces

• Shows currently installed and detected National
  Instruments hardware
• Includes utilities for configuring and testing
  your DAQ devices
• Properties
• Test Panels

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Properties

• Basic Resource Test
• Base I/O Address
• Interrupts (IRQ)
• Direct Memory Access (DMA)
• Link to Test Panels
• Configuration for
• Device Number
• Range and Mode (AI)
• Polarity (AO)
• Accessories
• OPC

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Test Panels

• Utility for testing
• Analog Input
• Analog Output
• Digital I/O
• Counters
• Great tool for troubleshooting

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Scales

• Provides access to DAQ Custom Scales Wizard
• Shows configured scales
• Includes utility for viewing and reconfiguring
  your custom scales

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DAQ Custom Scales Wizard

• Interface to create custom scales that can be
  used with Virtual Channels
• Each scale has its own
• Name and Description
• Choice of Scale Type (Linear, Polynomial, or
  Table)

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Sampling Considerations

• Analog signal is continuous
• Sampled signal is series of discrete samples
  acquired at a specified sampling rate
• Faster we sample the more our sampled signal will
  look like our actual signal
• If not sampled fast enough a problem known as
  aliasing will occur

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Aliasing
Adequately Sampled Signal
Aliased Signal
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Nyquist Theorem

• Nyquist Theorem
• You must sample at greater than 2 times the
  maximum frequency component of your signal to
  accurately represent the FREQUENCY of your signal
• NOTE You must sample between 5 - 10 times
  greater than the maximum frequency component of
  your signal to accurately represent the SHAPE of
  your signal

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Nyquist Example
Aliased Signal
Adequately Sampled for Frequency Only (Same of
cycles)
Adequately Sampled for Frequency and Shape
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Data Acquisition Palette
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DAQ Channel Name Data Type
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Analog Input Palette
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Single-Point AI VIs

• Perform a software-timed, non-buffered
  acquisition
• Good for battery testing, control systems
• - Not good for rapidly changing signals due
  to software timing

• AI Sample Channel
• Acquires one point on one channel
• AI Sample Channels
• Acquires one point on multiple channels

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Multiple-Point (Buffered) AI VIs

• Perform a hardware-timed, buffered acquisition
• Highly recommended for most applications
• Allows triggering, continuous acquisition,
  different input limits for different channels,
  streaming to disk, and error handling

• AI Config
• Configures your device, channels, buffer
• AI Start
• Starts your acquisition, configure triggers
• AI Read
• Returns data from the buffer
• AI Clear
• Clears resources assigned to the acquisition

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AI Config

• Interchannel Delay
• Determines the time (in seconds) between samples
  in a scan
• Input Limits
• Max and Min values for your signal
• Used by NI-DAQ to set gain
• Device
• Number of the device (from MAX) you are
  addressing
• Channels
• Chooses what channel(s) you are addressing

• Buffer Size
• Number of scans the buffer can hold
• A scan acquires one sample for every channel you
  specify
• 1000 scans x 2 channels 2000 total samples
• Task ID
• Passes configuration information to other VIs
• Error In/Out
• Receives/Passes any errors from/to other VIs

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Different Gains for Different Channels

• AI Config allows different gains for different
  channels
• The first element of the input limits array
  corresponds to the first element of the channel
  array

Gain 2
Gain 20
Range 0 to 10V
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AI Start

• Task ID In/Out
• Receives/Passes configuration information to/from
  other VIs
• Number of Scans to Acquire
• Total number of scans acquired before the
  acquisition completes
• Default value (-1) sets of Scans to Acquire
  Buffer Size (AI Config)
• A value of 0 acquires continuously
• Scan Rate
• Chooses the number of scans per second
• Error In/Out
• Receives/Passes any errors from/to other VIs

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AI Read AI Clear

• Number of Scans to Read
• Specifies how many scans to retrieve from the
  buffer
• Default value (-1) sets of Scans to Read of
  Scans to Acquire (AI Start)
• If of Scans to Acquire (AI Start) 0, default
  for of Scans to Read is 100
• Scan Backlog
• Number of unread scans in the buffer
• Waveform Data
• Returns t0, dt (inverse of scan rate), and Y
  array for your data

• Clears resources assigned to the device

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Error Cluster
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Buffered Acquisition Flowchart
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Buffered Acquisition

• AI Start begins the acquisition
• Acquisition stops when the buffer is full
• AI Read will wait until the buffer is full to
  return data
• If error input is true then Config, Start, and
  Read pass the error on but dont execute Clear
  passes AND executes

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Continuous Acquisition Flowchart
NO
YES
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Continuous Buffered Acquisition

• Differences from a buffered acquisition
• of scans to acquire 0
• While loop around AI Read
• Number of Scans to read does not buffer size
• Scan backlog tells how well you are keeping up

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Analog Output Architecture
Channel 0
Channel 1

• Most E-Series DAQ devices have a
  Digital-to-Analog Converter (DAC) for each analog
  output channel
• DACs are updated at the same time
• Similar to Simultaneous Sampling for Analog Input

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Analog Output Palette
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Single-Point AO VIs

• Perform a software-timed, non-buffered generation
• Good for generating DC voltages, or control
  systems
• - Not good for waveform generation because
  software timing is slow

• AO Update Channel
• Generates one point on one channel
• AO Update Channels
• Generates one point on multiple channels

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AO Update Channels

• Device
• Number of the device (from MAX) you are
  addressing
• Ignored if using virtual channel
• Channels
• Chooses what channel(s) you are addressing
• Can either be a number or a virtual channel name
• Uses the DAQ Channel Name control

• Values
• 1-D array of data
• The first element of the array corresponds to the
  first channel in your channels input

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Multiple-Point (Buffered) AO VIs

• Perform a hardware-timed, buffered generation
• Highly recommended for most applications
• Allows continuous generation, triggering, and
  error handling

• AO Config
• Configures your device, channels, buffer
• AO Write
• Writes data to the buffer
• AO Start
• Starts your generation
• AO Wait
• Waits until the generation is complete
• AO Clear
• Clears resources assigned to the generation

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Buffered Generation Flowchart
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Buffered Generation

• AO Write fills the buffer with waveform data
• AO Start begins the generation
• Without AO Wait the generation would start (AO
  Start) and then end immediately after (AO Clear)
• If error input is true then Config, Write, Start,
  and Wait pass the error on but dont execute
  Clear passes AND executes

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AO Write One Update

• Your analog output channel will continue to
  output the last value written to it until either
• The device is reset (power off, reset VI)
• A new value is written
• Use AO Write One Update at the end of your
  generation to set the channel back to 0

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Continuous Generation Flowchart
NO
YES
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Continuous Generation

• Differences from a buffered generation
• number of buffer iterations 0
• No AO Wait
• AO Wait would hang because the generation never
  completes
• While loop with AO Write
• The second AO Write is used for error checking
  ONLY