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.