Computer Based Data Acquisition Basics

Outline

• Basics of data acquisition

• Analog to Digital Conversion– Quantization– Aliasing

Acquisition Basics

Physical

Phenomenon Transducer

Signal

Conditioning

Interface Card

Computer

Analog to Digital Conversion

• In this process, we convert an analog voltage into a number

• Computers store numbers in “bits”• Typically use a 12 bit converter - converts

each input voltage into some number between 0 and 4095 (212-1)

• Maximum sample rate - if 100 kHz – takes 10 sec to perform the conversion

Sources of Error

• The fact that we are using a converter which has a finite number of bits introduces an error called “quantization error”

• The fact that it takes a finite (non-zero) amount of time to perform the conversion introduces the possibility of an error called “aliasing”

Quantization Error

V

n+3

n+2

n+1

n

n-1

n-2

n-3

V

Analog to Digital Conversion

Quantization Error

• For example, if we have a – 10 volt input range– n ranges from 0-4095

• Uncertainty of 1.22 mV

mV44.24096

V 10V

Aliasing

• Aliasing can lead to incorrect frequency information

• Sampling Theorem - Maximum frequency component that can be correctly identified is that frequency which is 1/2 the sample frequency. This maximum frequency is called the Nyquist frequency.

Voltage vs Time

Time (sec)

Vo

lta

ge

Voltage vs Time

Time (sec)

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Voltage vs Time

Time (sec)

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Voltage vs Time

Time (sec)

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Voltage vs Time

Time (sec)

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Aliasing• Once the observed frequency exceeds 1/2

half the sampling frequency, trouble starts

Relation between Measured Frequency, Sampling Frequency and Signal

Frequency

0

0.2

0.4

0.6

0 0.5 1 1.5 2 2.5 3fsignal/fsample

f mea

sure

d/f

sig

nal