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Data Acquisition
Data Acquisition SystemData Acquisition System
Analog Signal
Signal Conditioner
ADC
Digital Processing
Communication
Analog vs. Digital SignalAnalog 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
Signal ConditioningSignal 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
Analog-to-Digital Conversion (ADC)Analog-to-Digital Conversion (ADC)
Function: convert analog signals into digital signals
– Sample & hold– Quantization– Coding
y(t)=f(t) yk=f(tk)
QuantizationQuantization
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:
Nkk ftf )(
Select a Data Acquisition CardSelect 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
Example of Data Acquisition CardExample of Data Acquisition Card
TimingTiming
• Aperture time: the duration of the time window that the analog is sampled
• Jitter:
SamplingSampling
• Sampling: Numerical evaluate the signal at discrete distance in time, yk=y(kt)
• Digitized Signal: a sequence of numbers that is an approximation to an analog signal
• Sampling time/Period: time duration between two consecutive samples, t
• Sampling rate (Hz): 1/t• Nyquist Frequency: 2fmax
• Sampling theory: fs > Nyquist Frequency
Sampling TheorySampling Theory
• Shannon-Nyquist sampling theorem– The maximum frequency component a
sampled data system can accurately handle is its Nyquist limit (i.e., Nyquist frequency).
AliasingAliasing
• Matlab example of aliasing
Anti-aliasing FilterAnti-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.