Introduction
Signal conditioning of thermistor
Signal conditioning means manipulating an analogue signal in such a way that it meets
the requirements of the next stage for further processing.
Operational amplifiers(op-amps) are commonly employed to carry out the amplification
of the signal in the signal conditioning stage.
The signal conditioning equipment may be required to do linear processes like
amplification, attenuation, integration, differentiation, addition and subtraction. They
are also required to do non-linear processes like modulation, demodulation, sampling,
filtering, clipping and clamping squaring, linealizing or multiplification by another
function etc. the signal conditioning or data acquisition equipment in many a situation
be an excitation and amplification system for passive transducer.
It may be an amplification system for active transducer. In both the applications, the
transducer output is brought up to a sufficient level to make it useful for conversion,
processing, indicating and recording. Excitation is needed for passive transducers
because these transducers do not generate their own voltage or current. Therefore
passive transducers like strain gauges, potentiometers, resistance thermometers,
inductive and capacitive transducers required excitation from external sources. The
active transducers like techno generators, thermocouples, inductive pick ups and piezoelectric crystals. The thermistor constitute one arm or more than one arm of a
wheatstone bridge which is excited by an isolated DC source. The bridge can be
balanced by a potentiometer and can also be calibrated for unbalanced conditions.
Thermistor is a concentration of the term "Thermal Resistor". It is essentially a
semiconductor, which behaves as a resistor with a high negative temperature
coefficient of resistance. That is, as the temperature of the thermistor increases, its
resistance decreases. The temperature co-efficient is expressed in ohms per unit change
in degree Celsius (° C). thermistors with high temperature co-efficient of resistance are
more sensitive to temperature change and are therefore well suited to temperature
measurement and control.
CONTENTS
■ OBJECTIVE
■ EXPLAINATION
1. WHEATSTONE BRIDGE
2. THERMISTOR
3. OPERATIONAL AMPLIFIER
4. INSTRUMENTATION AMPLIFIER
■ METHODOLOGY
■ SCOPE
1. Wheatstone bridge:
Whetstone bridge is the most accurate method available for measuring resistances and
is popular for laboratory use. The circuit diagram of typical Wheatstone bridge is given
in figure
Rx
is the unknown resistance to be measured; R\, R 2 and R^ are resistors of known
resistance and the resistance of R 2 is adjustable. If the ratio of the two resistances in the known
leg (R 2 / R\) is equal to the ratio of the two in the unknown leg (R x / R 3 ), then the
voltage between the two midpoints (B and D) will be zero and no current will flow through the
galvanometer V g . R 2 is varied until this condition is reached. The direction of the current
indicates whether R 2 is too high or too low.
Detecting zero current can be done to extremely high accuracy (see galvanometer). Therefore, if
R\, R2 and R3 are known to high precision, then Rx can be measured to high precision.
Very small changes in R x disrupt the balance and are readily detected.
At the point of balance, the ratio of
R2 / R\ = R x / R 3
Therefore,
Alternatively, if R\, R 2 , and R 3 are known, but R 2 is not adjustable, the voltage difference
across or current flow through the meter can be used to calculate the value of R x , using
Kirchoff s Circuit laws (also known as Kirchhoff s rules). This setup is frequently used in strain
gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off
a meter than to adjust a resistance to zero the voltage.
In practical Wheatstone bridge, at least one of the resistance is made adjustable, to permit
balancing. When the bridge is balanced, the unknown resistance (normally connected at Rx)
may be determined from the setting of the adjustable resistor, which is called a standard resistor
because it is a precision device having very small tolerance.
Rx= (R2/R1). R3
APPLICATION OF WHEATSTONE BRIDGE:
A Wheatstone bridge may be used to measure the DC resistance of various types of
wire, either for the purpose of quality control of the wire itself or of some assembly in
which it is used.
For example, the resistance of motor winding, transformers, solenoids, relay coils and
resistance of thermistor, RTD also can be measured.
PRINCIPLE
Resistance of thermistor changes with change in temperature.
Resistance of thermistor decreases with increase in temperature while resistance of
thermistor increases with decrease in temperature. It is a thermally sensitive resistor.
FORMULA:-
RTI = RT2EXP[ P (1 / T1 - 1 / T2) ]
Where,
RT]= resistance of thermistor at absolute temp. Tl°k
RT2= resistance of thermistor at absolute temp. T2°k P= a constant depending upon the
material of the thermistor (Typically between 3500°k to 4500°k)
CONSTRUCTION:Thermistors are composed of sintered mixture of metallic oxides such as Manganese,
Nickel, Cobalt, Copper, Iron and Uranium. They are available in a variety of sizes and
shapes. They may be in the form of beads, roads and discs.
WORKING:A thermistor change in electrical resistance due to a corresponding temperature change
is evident whether the thermistor's body temperature is changes as result of conduction
or radiation from the surrounding environment or due to self heating brought about by
power dissipation within the device.
THEORY:- .Thermistor is a concentration of the term 'Thermal Resistor". It is
essentially a semiconductor which behaves as a resistor with a high negative
temperature coefficient of resistance. That is, as the temperature of the thermistor
increases, its resistance decreases. The temperature co-efficient is expressed in ohms
per unit change in degree celcius (° C). thermistors with high temperature co-efficient of
resistance are more sensitive to temperature change and are therefore well suited to
temperature measurement and control. Thermistors are available in a wide variety of
shapes and sizes. However, thermistor beads sealed in the tips of glass rods are most
commonly used because they are relatively easy to mount
TYPES OF THERMISTOR:1] PTC
2] NTC
NTC (Negative Temperature Coefficient)
It implies that the resistance of thermistor decreases with increases in its
temperature. These thermistors can detect changes in temperature, which could not be
observed with RTD's or Thermocouple circuits. NTC type thermistors mostly used in
industry. Resistance of thermistor is used in industry is 2.2 k Q. for temperature 30°C.
It's cost near about Rs. 10-15
SPECIFICATION:*Range(ºc): 50-300
*Accuracy :± 1
*Resolution: 0.01 ºC
*Scale: Non-Linear
*Thermister Constant: 4000ºK
OPERATIONAL AMPLIFIRES:
One type of amplifier, which finds its way into almost all points of measurement and
instrumentation system, is the operational amplifier. The word operational indicates
that the amplifier can perform mathematical operations like inversion addition,
subtraction, multiplication, division, integration and differentiation etc.
Properties of ideal operational amplifier are:
1. It should have an infinite input impedance.
2. It should have zero output impedance.
3. It should have an infinite gain (gain of the order of 105 to 109)
4. It should have flat response over a wide frequency range.
The operational amplifier consists of a very high gain amplifier with a negative
>
feedback. The gain of operational amplifier with negative feedback is determined by
feedback components and not by the internal amplifier circuitry.
APPLICATIONS OF OPERATIONAL AMPLIFIER:
Some of the important applications of an op-amp are:
1. Amplifiers
2. Active filters
3. Arithmetic circuits
4. Log and antilog amplifiers
5. Voltage comparators
6. Waveform generators
7. Precision rectifiers
8. Multipliers
9. Timers
10. Multivibrators
11. Regulated power supplies
Operational amplifier characteristics:
1. Input offset voltage : The input offset voltage is defined as the voltage that must be
applied to the input terminals to drive the output to zero. This is about 2mV for 741
amplifier. It should be understand thet the offset voltage changes with temperature.
2. Input offset current: just as a voltage offset may be required across the input to
make the output voltage zero, so a net current may be required between the inputs to
zero the output voltage. This current is called input offset current. This is equal to the
difference between the two input currents.
3. Input bias current: It is defined as the mean of the two input currents required to
make the output voltage zero.
4. Slew rate: it is the highest rate at which the output can change, it is expressed in
terms of v/jiS.
5. Unity gain frequency: in many cases, specifications include the frequency response
including unity gain frequency. This is the frequency at which the open loop gain of the
amplifier becomes unity. The low frequency gain is about 20,000 and falls to unity at
about 1MHz. the amplifier is said to have a 1 MHz gain bandwidth produt.
6. Common mode rejection ratio (CMMR): it is defined as the ratio of differential gain
to common mode gain. CMRR is infinite for ideal op-amp. Thus the output voltage
corresponding to the common mode noise is zero.
IDEAL CHARACTERISTICS OF OP-AMP:
1. Gain is maximum.
2. Input impedence should be infinite.
3. Output impedence should be zero.
4. CMRR should be infinite.
5. Bandwidth should be infinite.
TYPICAL VALUES OF OP- AMP:
1. Input offset voltage: ± 6mV.
2. Input offset current: ±200nAmp
3. Input bias current: ±7nAmp
4. Differential input resistance/impedence: 2m CI
5. Input impedence for 741 (FET op-amp): 1000 G Q
6. Output impedence: 75 Q.
7. Gain: 2 lakhs
8. Output voltage swing: 26Vpp
9. Supply current: 2.8 mAmp
APPLICATIONS:1. It is used in lab and medical purpose.
2. PTC type thermistors are used to protect the motor from overheating.
3. It gives very accurate reading at high temperature.
4. For measurement of level pressure, flow of the liquid, composition of gases, thermal
conductivity and vaccum measurement.
_____________________________________________
UA741
General-purpose single operational amplifier
UA741
General-purpose single operational amplifier
Features
■
Large input voltage range
■
No latch-up
■
High gain
■ Short-circuit protection
■
No frequency compensation required
■ Same pin configuration as the UA709
N
DIP
8
(plastic package)
Applications
■ Summing amplifiers
■ Voltage followers
■ Integrators
■ Active filters
■ Function generators
D
SO8
(plasticmicropackage)
Description
The UA741 is a high performance monolithic
operational amplifier constructed on a single silicon
chip. It is intended for a wide range of analog
applications.
The high gain and wide range of operating voltages
provide superior performances in integrator, summing
amplifier and general feedback applications. The internal
compensation network (6 dB/octave) ensures stability in
closed- loop circuits.
Pin connections
(top view)
2 - Inverting input 3 - Noninverting input 4 - VCC- 5 Offset null 2 6 - Output
7 -Vcc+
8 - N.C.
1/11
www.st.com
Schematic diagram
1 Schematic diagram
Figure 1. Schematic diagram
UA741
Absolute maximum ratings and operating conditions
UA741
2 Absolute maximum ratings and operating conditions
Table 1. Absolute maximum ratings
Symbol
Parameter
Value
Unit
Vcc
Supply voltage
±22
V
Vid
Differential input voltage
±30
V
Vi
Input voltage
±15
V
Output short-circuit duration
Infinite
Rthja
Thermal resistance junction to ambient SO-8 DIP8
125 85
Rthjc
Thermal resistance junction to case SO-8 DIP8
40
°C/W
°C/W
41
ESD
Tstg
HBM: human body model(1) DIP package SO package
500 400
V
MM: machine model(2)
100
V
CDM: charged device model(3)
1.5
kV
-65 to +150
°C
Storage temperature range
1. Human body model: a 100 pF capacitor is charged to the specified voltage, then discharged through a 1.5kfl! resistor between two pins of the device.
This is done for all couples of connected pin combinations while the other pins are floating.
2.
Machine model: a 200 pF capacitor is charged to the specified voltage, then discharged directly between two pins of the device with no external
series resistor (internal resistor < 5 n). This is done for all couples of connected pin combinations while the other pins are floating.
3.
Charged device model: all pins and the package are charged together to the specified voltage and then discharged directly to the ground through
only one pin. This is done for all pins.
4.
4. INSTRUMENTATION AMPLIFIER:
The low level signal output of electrical transducers often need to be amplified before
further processing. This is done by the use of instrumentation amplifier. The important
features of instrumentation amplifier are as follows.
1.
Selectable gain with high gain accuracy and gain linearity.
2.
Differential input capability with high gain common mode rejection.
3.
High stability of gain with low temperature co-efficient.
4.
low DC offset and drift errors referred to input.
5.
low output impedance.
The input amplifiers A[ and A2 act as input buffers with unity gain for common mode signals
ecm and with a gain of (1+2R2/Ri) for differential signals. A high input impedance is ensured
by the non-inverting configuration in which they operate. The common mode (cm) rejection
is achieved by the following stage which is connected as a differential amplifier. The
optimum common mode rejection can be obtained by adjusting R6 or R7 ensuring that
Ei - Ei R4 R6
The amplifier A3 can also be made to have some nominal gain for the whole amplifier by an
appropriate selection or R4, R5, R6 and R7.
The drift errors of the second stage add to the product of the drift errors of the first
amplifier and first stage gain. Hence, it is necessary that the gain in the first stage be
enough to prevent the overall drift performance from being significantly affected by the
drift in the second stage. The drift problem of instrumentation amplifier can be improved if
amplifiers Ai and A2 have offset voltages, which tends to track the temperature.
The gain of an instrumentation amplifier can be varied by changing R{ alone. A high gain
accuracy can be obtained by using precision metal film resistors for all the resistance.
Figure shows a simplified differential instrumentation amplifier using a transducer bridge. A resistive
transducer, thermistor, whose resistance changes as a function of some physical quantity such as
temperature is connected in one arm of the bridge and is denoted by (Rr ± A R), where RT is the
resistance of the thermistor and delta R is the change in the
Generally, resistors Ra, Rb and Rc are selected so that they are equal in value to the transducer
resistance RT at some reference condition. The bridge is balanced initially at a desired reference
condition. However, as the temperature changes, the resistance of the thermistor also changes
causing the bridge to unbalance (Va 4- Vb). The output voltage of the bridge can be expressed as a
function of the change in the resistance of the thermistor.
Let the change in resistance of the thermistor be delta R. since Rb and Rc are fixed resistors, the
voltage Vb is constant. However, voltage Va varies as a function of the change in thermistor
resistance. Therefore The negative sign in this equation indicates that Va<Vb because of the
increase in the value of delta R.
The output voltage Vab is of the bridge is then applied to the differential instrumentation amplifier
composed of three op-amps (see the figure). The gain of the basic differential amplifier is (-Rf / Rl);
therefore the output voltage VO of the circuit
V o = Vab (-Rf / R j ) = A R ( V ^ ) . R f
2 (2R + A R) Rl
Generally, the change in the resistance of the thermistor delta R is very small, therefore, we can
approximate (2R + delta R) 21 2R. Thus the output voltage
V0 =RF AR- (Vdc) Ri 4R
The equation indicates that V0 is directly proportional to the change in the resistance delta
R of the thrmistor.
METHODOLOGY
1.
The wheatstone bridge is balanced initially at a desired reference condition.
2.
As the physical quantity to be measured changes, the resistance RT
(resistance of thermistor) will also change by A R.
3.
Due to change in Rt, the bridge is unbalanced. Hence ,
Va^Vb
4.
The bridge output voltage Vab is then applied to a differential
instrumentation amplifier consisting of three op-amps as shown in fig.
5.
The expression for the output voltage of the instrumentation amplifier is
V()= Vab (-Rf/Rl)
Where,
-Rf/Rl : gain of the instrumentation amplifier
6. This output is then applied to an indicating meter which indicates the value of
the quality being measured.
Scope:
In industry, the actual process of control panel is situated far away. So output of
sensor is sends to the control panel. Due to atmospheric condition, signal at the
control panel will not the desired signal.
Therefore this type of signal conditioning of thermistor using instrumentation
amplifier can be used in industry.
References:
1.
electrical and electronic measurements : A.K.Sawheny
2.
electronic instrumentation : Kalsi
3.
electronic instrumentation : Khedkar
4.
op amps and linear integrated circuits: RamakantGaikwad
5.
linear integrated circuits: J.S.Katre
6.
www.google.com
7.
www.wikipedia.com