Data Acquisition. Data Acquisition System Analog Signal Signal Conditioner ADC Digital Processing...
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Transcript of Data Acquisition. Data Acquisition System Analog Signal Signal Conditioner ADC Digital Processing...
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.