Data Acquisition

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Data Acquisition
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Data Acquisition System
Signal Conditioner
Analog Signal
ADC
Digital Processing
Communication
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Analog vs. Digital Signal

• Analog signals
• Continuous, expressed in decimal system
• No limitation on the maximum/minimum value
• Can not be processed by computer
• Digital signals binary number system
• All numbers are expressed by a combination of 1
  0
• The maximum value is limited by of bits
  available

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Signal Conditioning

• Functions modify the analog signal to match the
  performance of the ADC
• Pre-filtering remove undesirable high frequency
  components
• Amplification amplify the signal to match the
  dynamic range of the ADC

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Analog-to-Digital Conversion (ADC)

• Function convert analog signals into digital
  signals
• Sample hold
• Quantization
• Coding
• y(t)f(t) ?ykf(tk)

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Quantization

• Definition transformation of a continuous analog
  input into a set of discrete output state
• Coding the assignment of a digital code word or
  number to each output states
• of possible state N2n, n is of bits
• Quantization resolution Q(Vmax-Vmin)/N
• Quantization Error

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Select a Data Acquisition Card

• Functions A/D, D/A, Digital I/O, signal
  conditioning (amplification, prefiltering),
  timer, trigger, buffer
• Features
• A/D resolution ( of bits used)
• Maximum sampling rate
• of channels
• Total throughput
• Aperture time

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Example of Data Acquisition Card
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Timing

• Aperture time the duration of the time window
  that the analog is sampled
• Jitter

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Sampling

• Sampling Numerical evaluate the signal at
  discrete distance in time, yky(kDt)
• Digitized Signal a sequence of numbers that is
  an approximation to an analog signal
• Sampling time/Period time duration between two
  consecutive samples, Dt
• Sampling rate (Hz) 1/Dt
• Nyquist Frequency 2fmax
• Sampling theory fs gt Nyquist Frequency

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Sampling Theory

• Shannon-Nyquist sampling theorem
• The maximum frequency component a sampled data
  system can accurately handle is its Nyquist limit
  (i.e., Nyquist frequency).

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Aliasing

• Matlab example of aliasing

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Anti-aliasing Filter

• One way of avoiding the problem of aliasing is to
  apply an anti-aliasing filter to the signal,
  prior to the sampling stage, to remove any
  frequency components above the "folding" or
  Nyquist frequency (half the sampling frequency).
• An anti-aliasing filter is a low-pass filter.