Chapter 5 Signal Conditioning

Chapter 5

Chapter 5

Signal Conditioning

Signal Conditioning

Measurement and Instrumentation / EEE 2223 Measurement and Instrumentation / EEE 2223

A major concern in transducer desin is sim!ly identi"yin some material that A major concern in transducer desin is sim!ly identi"yin some material that chanes in a !hysical !arameter bein sensed#

chanes in a !hysical !arameter bein sensed#

 $he am!litude and linearity o" the transducer out!ut sinal ar

 $he am!litude and linearity o" the transducer out!ut sinal are usually not "irst%e usually not "irst% order concerns in transducer desin& but t

order concerns in transducer desin& but they are o" considerable concern inhey are o" considerable concern in data ac'uisition system#

data ac'uisition system#  $here"or

 $here"ore& sinal conditionine& sinal conditionin to increase amplitudeto increase amplitude and and improve linearityimprove linearity

is usually necessary in data ac'uisition systems# is usually necessary in data ac'uisition systems#

2 2

Signal Conditioning

A major concern in transducer desin is sim!ly identi"yin some material that A major concern in transducer desin is sim!ly identi"yin some material that chanes in a !hysical !arameter bein sensed#

chanes in a !hysical !arameter bein sensed#

 $he am!litude and linearity o" the transducer out!ut sinal ar

 $he am!litude and linearity o" the transducer out!ut sinal are usually not "irst%e usually not "irst% order concerns in transducer desin& but t

order concerns in transducer desin& but they are o" considerable concern inhey are o" considerable concern in data ac'uisition system#

data ac'uisition system#  $here"or

 $here"ore& sinal conditionine& sinal conditionin to increase amplitudeto increase amplitude and and improve linearityimprove linearity

is usually necessary in data ac'uisition systems# is usually necessary in data ac'uisition systems#

Signal Conditioning

3 3

Temperature Sensor Curve Temperature Sensor Curve

 $herm

 $hermistoristor $herm$hermocou!leocou!le

( ($)$)

1. Signal-Level and Bias Change

Example

1-Principles o !nalog Signal

Conditioning

An 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#

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

#



). *mprove Linearity

Principles o !nalog Signal

Conditioning

 $hermistor resistance +ersus tem!erature is hihly nonlinear and usually has a neati+e slo!e#

). *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#

. 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#



Principles o !nalog Signal Conditioning

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

/. 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#

7inal%le+el and bias chane

<inearization

3#

7inal con+ersion

=ilterin

>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#

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#

Solution

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

ommon !assi+e circuits that can !ro+ide

some o" the re'uired sinal conditionin

o!erations are:

1#

)i+ider circuit

Bride circuit

( "ilters

ivider Circuit

ivider Circuit

 $

 $o !ro+o !ro+ide con+ersion ide con+ersion o" resistano" resistance intoce into a +oltae +ariation#

a +oltae +ariation#

.)

.) C sC su!u!!l!ly +y +ololtataee (

(1&1&((22 C C didi+i+idder er rresesisistotorrss

Either (1 or (2 can be the sensor hose Either (1 or (2 can be the sensor hose resistance +aries ith some measured resistance +aries ith some measured +ariable#

 $he most "re'ue

 $he most "re'uently !er"ormed ty!e o" ntly !er"ormed ty!e o" sinal conditionin is asinal conditionin is a le+el or am!litude chane#

le+el or am!litude chane#

A chane in sinal le+el may re'uire either attenuation or A chane in sinal le+el may re'uire either attenuation or

am!li"ication o" the in!ut sinal# am!li"ication o" the in!ut sinal#

!ttenuation

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

netork#

!mpliication

!mpliication re'uires an acti+e de+ice such as a transistor or an re'uires an acti+e de+ice such as a transistor or an op-amp

op-amp##

It is common !ractice to

It is common !ractice to use both attenuation and am!li"ication "oruse both attenuation and am!li"ication "or sinal%conditionin !ur!oses in many electronic test

sinal%conditionin !ur!oses in many electronic test

instruments& such as electronic multimeter and oscillosco!es in instruments& such as electronic multimeter and oscillosco!es in order to !ro+ide multi!le ranes#

order to !ro+ide multi!le ranes#

1 1

ivider Circuit in Signal-level

ivider Circuit in Signal-level

change

Bridge Circuit

Bridge Circuit

3"heatstone Bridge4

3"heatstone Bridge4

 $

• $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

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 

23

Bridge Circuit

.a .b

 $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.

• 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

•

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#

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#

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#

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 (*#

Lead Compensation ,

applications9 a &ridge circuit may &e located at some distance rom the sensor %hose resistance changes are to &e measured.

sensor

ire 3 is the po%er lead& has no in"luence on the

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#

ire 1 e5!osed to the same en+ironment and chanes

E""ecti+ely& both (3 and (* are identically chaned& and thus (3(2 C (1(* shos that no chane in the bride null occurs#

;

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

;

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#

A closed%loo! system can e+en be constructed

that !ro+ides the bride ith a sel"%nullin

ability#

33

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

1#

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#

3#

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,.#

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#

*1

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#

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#

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

Circuit or Lo%-Pass 0ilter

Circuit or $igh-Pass 0ilter

Circuit or Band - Pass 0ilter

(es!onse o" the band%!ass RC _"ilter#

A band%!ass RC _{"ilter can be made "rom}

l

=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

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 >

% <o%!ass "ilter

% Because the lo%!ass "ilter blocks hih "re'uencies and

!asses only the lo "re'uencies to the ne5t system#

%  $hus& the "ilter the measurement sinal o" O1 k0z to

!ass and block the unanted sinal at 1 M0z#

1# Dulses "or a ste!!in motor are bein transmitted at 2,,,

0z# )esin a "ilter to reduce -,%0z noise but reduce the !ulses by no more than 3 dB#

2# A 2%k0z data sinal is contaminated by -,0z o" noise# 3# A sinal%conditionin system uses a "re'uency +ariation

"rom -k0z to -, k0z to carry measurement in"ormation#

 $here is considerable noise at 12, 0z and at 1 M0z# )esin a band!ass "ilter to reduce the noise by ,P#

*

?ther examples %hen

ilters are needed.

*# A "re'uency o" *,, 0z !re+ails aboard an

aircra"t# )esin a tin%$ notch "ilter to reduce

the *,,%0z sinal#

?ther examples %hen

ilters are needed.

; An o!%am! is a circuit consist o" resistors& transistors& diodes& and ca!acitors#

 It re'uires bi!olar !oer su!!lies K.s and %.s& ith res!ect to round#  e concerned only ith its in!ut and out!ut sinals#

1

?p-!mp Characteristics

; $he out!ut +oltae& .out is !lotted +ersus the di""erential in!ut +oltae

4.2%.16#

; hen .2 is much larer than .1& the out!ut is saturated at @.sat#

 hen .1 is much larer than .2& the out!ut is saturated at K.sat#

 $here is a narro rane o" di""erential in!ut +oltae& . here out!ut

chane "rom K.sat to @.sat#

 =or most o!%am!s& this in!ut +oltae rane is less than a mili+olt&

3

;

 $he in!ut im!edances are +ery hih&

ty!ically e5ceedin 1M& hereas the

out!ut im!edance is +ery lo& ty!ically less

than 1,, #



The op amp inverting ampliier.

7ummin !oint

•

.oltae "olloer

•

In+ertin am!li"ier

% 7ummin am!li"ier

•

Non%in+ertin am!li"ier

% )i""erential Instrumentation Am!li"ier

% )i""erential am!li"ier

% Instrumentation am!li"ier

8

 $he o! am! +oltae "olloer# $his circuit has unity ain but +ery hih in!ut im!edance#



Example 11

evelop an op-amp circuit that can

provide an output voltage related to

-1

ierential

Aost common

coniguration o

Example 1)

=iure belo shos a bride circuit "or hich (*

+aries "rom 1,,L to 1,2L# 7ho ho an

instrumentation am!li"ier could be used to !ro+ide

an out!ut o" , to 2#.#

-

An instrumentation am!li"ier includes +oltae "olloers "or in!ut isolation#

-to-* Convertor

1# Because sinals in !rocess control are most o"ten transmitted as a

current& s!eci"ically * to 2, mA& it is o"ten necessary to em!loy a linear +oltae%to%current con+ertor#

2# 7uch a circuit must be ca!able o" sinkin a current into a number o" di""erent loads ithout chanin the .toI trans"er characteristics#

Common measurement transmission in the

industrial environment

A sensor out!uts , to 1 .# )e+elo! a +oltae%to%current con+ertor so that this becomes , to 1, mA# 7!eci"y the

ma5imum load resistance i" the o! am! saturates at Q1,.# )etermine the +alue o" (3 and (* i" e ant to con+ert the , to 1. to * to 2, mA?

Example 1+

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At the recei+in end o" the !rocess%control transmission system& e o"ten need to con+ert the current back into +oltae# $his can easily be done by this circuit#

-