Chapter 5 Signal Conditioning

Chapter 5 Signal Conditioning

Measurement and Instrumentation / EEE 2223 By : Elya Mohd Nor and Azrena Abu Bakar

1

Signal Conditioning A major concern in transducer desin is sim!ly identi"yin some material that chanes in a !hysical !arameter bein sensed#  $he am!litude and linearity o" the transducer out!ut sinal are are usually not "irst% order concerns in transducer desin& but they t hey are o" considerable concern in data ac'uisition system#  $here"ore&  $here"ore& sinal conditionin to increase amplitude and improve linearity is usually necessary in data ac'uisition systems#

2

Signal Conditioning A major concern in transducer desin is sim!ly identi"yin some material that chanes in a !hysical !arameter bein sensed#  $he am!litude and linearity o" the transducer out!ut sinal are are usually not "irst% order concerns in transducer desin& but they t hey are o" considerable concern in data ac'uisition system#  $here"ore&  $here"ore& sinal conditionin to increase amplitude and improve linearity is usually necessary in data ac'uisition systems#

2

Temperature Sensor Curve  $hermistor  $hermistor

$hermocou!le $hermocou!le

($)

3

Principles o !nalog Signal Conditioning 1. Signal-Level and Bias Change Example 1An I( !yrometer !ro+ides an out!ut +oltae +aries "rom ,#2 to ,#- . as a tem!erature in the annealin o" lasses chanes "rom ,o to -,,o# 0oe+er& the transmitter to hich this transducer out!ut must be connected re'uire a +oltae that +aries "rom , to .& "or the same +ariation o" the !rocess +ariable# "hat to do# $o% to do it## *

Solution to Example 1 e !er"orm the re'uired sinal conditionin by "irst chanin the ,. to occur hen the transducer out!ut is ,#2.#  $his is done by &ias ad'ustment& or (ero shit# No e ha+e a +oltae that +aries "rom ,. to ,#*.& so e need to make the +oltae larer# I" e multi!ly the +oltae by 12#& the ne out!ut ill +ary "rom , to . 412# 5 ,#*.6#  $his is called ampliication& and 12# is called the gain# 

Principles o !nalog Signal Conditioning ). *mprove Linearity  $hermistor resistance +ersus tem!erature is hihly nonlinear and usually has a neati+e slo!e#

.a!or tem!erature measurement

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Principles o !nalog Signal Conditioning ). *mprove Linearity Example )7u!!ose a thermistor +aries nonlinearly ith a !rocess +ariable# A linearization circuit is re'uired to conditioned the transducer/sensor out!ut so that a +oltae as !roduced hich as linear ith the !rocess +ariable#

8

Principles o !nalog Signal Conditioning +. Conversions Example + , 7u!!ose a metal resistance +aries nonlinearly ith the tem!erature inside the o+en# In order to measure the o+en tem!erature& e need a circuit to con+ert the resistance chane to a +oltae or current sinal#  $his is done by brides hen the "ractional resistance chane is small and/or am!li"iers hose ain +aries ith resistance#

9

Principles o !nalog Signal Conditioning . Signal Transmission An im!ortant ty!e o" con+ersion is associated ith the standard o" transmittin sinals as *%to%2,mA current le+els in ire#

"hy current transmission than voltage# 

Principles o !nalog Signal Conditioning 5. igital *nterace ;  $he





use o" com!uters in measurement re'uires con+ersion o" analog data into a digital ormat by interated circuit de+ices called !C# Analo sinal con+ersion is needed to adjust the analo measurement sinal to match the in!ut re'uirements o" the A)# Example : A) need a +oltae beteen , to .& but sensor !ro+ides sinal that +aries "rom 3, to 9, m.# 7inal con+ersion circuits must be de+elo!ed to inter"ace the out!ut re'uirement to the re'uired A)#

Principles o !nalog Signal Conditioning /. 0iltering Industrial en+ironments in hich data ac'uisition systems are o"ten !laced tend to introduce s!urious inter"erence sinals into the ac'uisition system#  $hese unanted sinals are noise& and o"ten due to -,%0z or *,,%0z !oer line inter"erence caused by inducti+e load chanes such as startin motors# A "ilter is desined to reduce such inter"erence#

11

In summary& the re'uirements "or sinal conditionin in instrumentation are "or:%

1# 2# 3# *# #

7inal%le+el and bias chane ne o" the most important concerns in analog signal conditioning is the loadin o" one circuit by another#  $his introduces uncertainty in the am!litude o" a +oltae as it is !assed throuh the measurement !rocess# I" this +oltae re!resents some !rocess +ariable& then e ha+e many uncertainty in the +alue o" the +ariable# 13

The Thvenin e2uivalent circuit o a sensor allo%s easy visuali(ation o ho% loading occurs.

 $he +oltae that a!!ear across the load is reduced by the +oltae dro!!ed across the internal resistance# 0o to reduce the e""ect o" loadin?? @ by makin (< much larer than (5 4that is (<  (56#

1*

Example  An am!li"ier out!uts a +oltae that is 1, times the +oltae on its in!ut terminals# It has an in!ut resistance o" 1, k# A sensor out!uts a +oltae !ro!ortional to tem!erature ith a trans"er "unction o" 2, m./o#$he sensor has an out!ut resistance o" #, k# I" the tem!erature is ,o& "ind the am!li"ier out!ut#

1

Solution I" loadin is inored& serious errors can occur in e5!ected out!uts o" circuits and ains o" am!li"iers#

1-

ommon !assi+e circuits that can !ro+ide some o" the re'uired sinal conditionin o!erations are: 1# 2# 3#

)i+ider circuit Bride circuit ( "ilters

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ivider Circuit  $o  $o !ro+ide !ro+ide con+ersion o" resistance resistance into a +oltae +ariation#

.) C su! su!!l !ly y +ol +olta tae e (1& 1&( (2 C di di+i +id der resis esisto torrs Either (1 or (2 can be the sensor hose resistance +aries ith some measured +ariable# 19

ivider Circuit in Signal-level change  $he most "re'uently "re'uently !er"ormed ty!e o" sinal conditionin is a le+el or am!litude chane# A chane in sinal le+el may re'uire either attenuation or am!li"ication o" the in!ut sinal#

!ttenuation is accom!lished ith a resistive voltage divider netork#

!mpliication re'uires an acti+e de+ice such as a transistor or an op-amp# It is common !ractice to use both attenuation and am!li"ication "or sinal%conditionin !ur!oses in many electronic test instruments& such as electronic multimeter and oscillosco!es in order to !ro+ide multi!le ranes# 1

Bridge Circuit 3"heatstone Bridge4  $o  $o con+ert im!edance im!edance +ariations +ariations into +oltae +ariations# +ariations#

2,

Bridge Circuit •

 $his circuit is used in sinal%conditionin a!!lications here a sensor chanes resistance  ith !rocess +ariable chanes#

•

 $he object labeled  D is a +oltae detector& used to com!are !otentials o" !oints a and b o" the netork#

•

Dotential o" !oint a ith res!ect to c : a

•

Dotential o" !oint b ith res!ect to c : &

•

hen there is zero di""erence and zero +oltae across the detector%it is called null (3(2 C (1(*

21

Example 5 I" a heatstone bride nulls ith (1C1,,, & (2C9*2 & and (3C,, & "ind the +alue o" (*# Assume the detector im!edance is in"inite# Anser : *21 

22

Bridge Circuit .a .b

 . C .a % .b

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Example /  $he resistor in a bride are i+en by (1C(2C(3C12,  and (*C121 # I" the su!!ly is 1, .& "ind the +oltae o""set# Assume the detector im!edance is in"inite# Anser : %21 m.

2*

Bridge Circuit •

hene+er heatstone bride is assembled and resistors are adjusted "or a detector null& the resistor +alues must satis"y the indicated e'uality F(3(2 C (1(*G#

•  $he

null is maintained& e+en hen the su!!ly +oltae dri"ts or chanes#

•

In modern a!!lications& the detector is a +ery hih%in!ut im!edance di""erential am!li"ier#

•

In the calibration !ur!oses and s!ot measurement instruments& a hihly sensiti+e al+anometer ith lo im!edance may be used#

2

6alvanometer detector •

In a im!edance ith lo im!edance& hen the bride is in unbalanced condition& current "los throuh the al+anometer detector causin a de"lection o" its !ointer# $his current is knon as o""set current#

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E5am!le 8 A bride circuit has resistance o" (1C(2C(3C2#,, k and (*C2#, k and a #,, . su!!ly# I" a al+anometer ith a ,#,% internal resistance is used "or a detector& "ind the o""set current# $o% to do it#  $he easiest ay to determine this o""set current is to "ind the  $he+eninHs e'ui+alent circuit beteen !oint a and b o" the bride 4ith the detector remo+ed6#

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Bridge 7esolution Bride resolution is here a minimum resistance chane that must occur be"ore the detector indicates a chane in o""set +oltae# It is an o+erall accuracy o" the instruments#

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Example 8 A bride circuit has (1C(2C(3C(*C12,#,% resistances and a 1,#,%. su!!ly# learly& the bride is nulled# 7u!!ose a 3% diit ).M on a 2,,%m. scale ill be used "or the null detector# =ind the resistance resolution "or measurement o" (*# Anser : >n a 2,,%m. scale& the ).M measures "rom ,,,#, to 1# m.& so the smallest chane is ,#1 m.& or 1,, J.#  $he smallest chane in resistance that can be measured is ,#,,*9  A bride o""set o" K1,,J. is caused by a reduction o" (*# it "ollo that a bride o""set o" %1,,J. ould be casued by an increase in (*#

2

Lead Compensation ,

in many process-control applications9 a &ridge circuit may &e located at some distance rom E""ecti+ely& both (3 and (* are changes identically chaned& and thus (3(2 C the sensor %hose resistance are to &e measured.

(1(* shos that no chane in the bride null occurs#

I" ire 426 chanes in resistance because o" s!urious in"luences& it introduces this chane into the (* le o" the bride#

sensor ire 3 is the po%er lead& has no in"luence on the bride balance condition #

ire 1 e5!osed to the same en+ironment and chanes by the same amount& but is in the (3 le o" the bride#

3,

Current Balance Bridge ;

In the !ast& many !rocess%control a!!lications used a "eedback system in hich the bride o""set +oltae as am!li"ied and used to dri+e a motor hose sha"t altered a +ariable resistor to renull the bride#



7uch a system does not suit the modern technoloy o" electronic !rocessin because it is not +ery "ast& is subject to ear& and enerates electric noise#

31

Current Balance Bridge ;

A techni'ue that !ro+ides "or an electronic nullin o" the bride and that uses only "i5ed resistors can be used ith the bride#



 $his method uses a current to null the bride#

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A closed%loo! system can e+en be constructed that !ro+ides the bride ith a sel"%nullin ability#

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Current Balanced Bridge

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Example : A current balance bride& as shon belo has resistors (1C(2C1,kL& (3C1kL& (*C,& and (C,L and a hih% im!edance null detector# =ind the current re'uired to null the bride i" (3 chanes by 1L# $he su!!ly +oltae is 1,.#

3*

ac Bridges 23 C 1*

3

ac Bridges

3-

Bridge !pplications 1#

2#

3#

Drimary a!!lication o" bride circuits in modern !rocess%control sinal conditionin is to con+ert +ariations o" resistance into +ariations o" +oltae#  $his +oltae +ariation is then "urther conditioned "or inter"ace to an !C or other system# It is thus im!ortant to note that the +ariation o" bride o""set is nonlinear ith res!ect to any o" the resistors#

38

Bridge !pplications *# I" a sensor has an im!edance that is linear ith res!ect to the +ariable bein measured& such linearity is lost hen a bride is used to con+ert this to a +oltae +ariation# # =iure belo shos ho . +aries ith (* "or a bride ith (1C(2C(3C1,, and . C 1,.# Note the nonlinearity o" . ith (* as it +aries "rom , to ,,L#

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3

Bridge !pplications -# I" the rane o" resistance +ariation is small and centered about the null +alue& then the nonlinearity o" +oltae +ersus resistance is small# 8# =iure shos that hen (* is beteen , to 11,L& then the +ariation o" . ith (*& on an e5!anded scale& is relati+ely linear# Am!li"iers can be used to am!li"y this +oltae +ariation#

*,

a6 Bride o""%null +oltae is clearly nonlinear "or lare% scale chanes in resistance# 4b6 0oe+er& "or small ranes o" resistance chane& the o""%null +oltae is nearly linear#

*1

7C 0ilter hat is a "ilter? A "ilter is a circuit that !asses a certain band o" "re'uencies hile attenuatin the sinals o" other "re'uencies# =ilter

Passive Filter

RC Filter 1. Low-pass filter 2. High-pass filter 3. Band-pass filter 4. Band-rejet filter

Active Filter

>!%Am!s 1ith Nain and =eedback

*2

Circuit or Lo%-Pass 0ilter

(es!onse o" the lo%!ass

RC "ilter#

Circuit or $igh-Pass 0ilter

(es!onse o" the hih%!ass

RC "ilter#

Circuit or Band - Pass 0ilter A band%!ass RC "ilter can be made "rom cascaded hih%!ass and lo%!ass RC "ilters#

(es!onse o" the band%!ass

RC "ilter#

Circuit or Band , 7e'ect 0ilter

l

*-

Comments o 0ilter =ilter !er"ormance is described in terms o" out!ut +oltae to in!ut +oltae 4.o/.in6 at di""erent "re'uencies and is e5!ressed as dB ; )< log 3o=i4  $he sinal source "or data ac'uisition system is a transducer#  $he ma5imum bandidth o" most transducers is 1, 0z# $hus& "ilterin noise "rom these transducer sinals is done usin lo%!ass "ilter# *8

Example 1< A measurement sinal ith am!litude *,%m. has a "re'uency o" O1k0z# $here is unanted noise at about 1 M0z# A "ilter is needed to eliminate the noise "rom the sinal# Identi"y the "ilter ty!e# hy?

!ns%er > % %

%