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Data Acquisition

Data Acquisition. Data Acquisition System. Signal Conditioner. Analog Signal. ADC. Digital Processing. Communication . Analog vs. Digital Signal. Analog signals: Continuous, expressed in decimal system No limitation on the maximum/minimum value Can not be processed by computer

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Data Acquisition

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  1. Data Acquisition

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

  3. 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

  4. 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

  5. Analog-to-Digital Conversion (ADC) Function: convert analog signals into digital signals • Sample & hold • Quantization • Coding y(t)=f(t) yk=f(tk)

  6. 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: N=2n, n is # of bits • Quantization resolution: Q=(Vmax-Vmin)/N • Quantization Error:

  7. 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

  8. Example of Data Acquisition Card

  9. Timing • Aperture time: the duration of the time window that the analog is sampled • Jitter:

  10. Sampling • Sampling: Numerical evaluate the signal at discrete distance in time, yk=y(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 > Nyquist Frequency

  11. 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).

  12. Aliasing • Matlab example of aliasing

  13. 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.

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