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Objective
We have to design a temperature meter using instrumentation amplifier. Which can measure the
temperature of any substance in specific range of temperature. The instrumentation system is
consists of three stage.
1. Input Stage.
2. Intermediate stage.
3. Output stage.
Instrumentation Amplifier
The most useful amplifier for measurement, instrumentation, or control is the instrumentation
amplifier. It is designed with several op-amps and precision resistors, which make the circuit
extremely stable and useful for accuracy is important. There are now many integrated circuits
available in single packages. Although this packages are more expensive than a single op-amp, when
performance and precision are required, the instrumentation amplifier is well worth the price, because
it’s performance cannot be matched by the average op-amp. Generally a transducer is used at the
measuring site to obtain the required information easily and safely. The transducer is a device that
converts one form of energy into another form. An instrumentation system is used to measure the
output signal produced by a transducer and often to control the physical signal producing it. Figure-1
shows block diagram of an instrumentation system.
Transmission lines
Physical quantity
To be measured
Input stage
Intermediate
stage
Transducer +
preamplifier
Figure:1
Instrumentation
amplifier
Block diagram of an instrumentation system
Output
stage
Indicator & automatic
process controller
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The input stage is composed of a pre-amplifier and some sorts of transducers depending on the
physical quantity to be measured. The output stage may devices such as meters, oscilloscopes, charts,
or magnetic recoders. In the block diagram transmission lines are used especially when the transducer
is at a remote test site monitoring hazardous conditions such as high temperatures. The transmission
lines permit signal transfer from unit to unit. The signal source of the instrumentation amplifier is the
output of the transducer. To amplify the low-level output signal of the transducer it can drive the
indicator or display is the major function of the instrumentation amplifier. The instrumentation
amplifier is intended for precise , low-level signal amplification where low noise, low thermal and
time drift, high output resistance , and accurate close-loop gain required . There are many
instrumentation amplifier , such as the µA725, ICL7605, and LH0036, that make a circuit extremely
stable and accurate . The requirements for instrumentation op-amps are more rigid than those for
general-purpose amplification. Such amplifier is called differential amplifier . such most
instrumentation system use a transducer in a bridge circuit. For our project we will consider a
simplified instrumentation amplifier system using a transducer bridge circuit.
Instrumentation Amplifier Using Transducer Bridge
Figure-2 shows s simplified differential instrumentation amplifier using a transducer bridge. A
resistive transducer whose resistance changes as a function of some physical energy is connected in
one arm of the bridge with a small circle around it and is denoted by (Rt±∆R), where Rt is the
resistance of the transducer and ∆R the change in resistance Rt. The bridge in the circuit of figure-2 is
dc excited but ac excited as well.
For the balance bridge at some reference condition,
𝑉𝑏 = 𝑉𝑎
or
𝑅𝑏(𝑉𝑑𝑐)
𝑅𝑏+𝑅𝑐
=
𝑅𝑎(𝑉𝑑𝑐)
𝑅𝑎+𝑅𝑡
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That is,
𝑅𝑐
𝑅𝑏
=
𝑅𝑡
𝑅𝑎
Circuit Diagram
RT±∆R
Resistive transducer
Va
Vb
Vo
Vab
Vab
Rm
R3=Rf
Indicating meter
Figure:2
Differential instrumentation amplifier using a transducer bridge
Generally, resistors Ra, Rb and Rc are selected to that they are equal in value to the transducer
resistance Rt at some reference condition. The reference condition is the specific value of the
physical quantity under measurement at which the bridge is balanced. The bridge is balanced initially
at a desired reference condition. As the physical quantity to be measured changes , the resistance of
the transducer is also changes, which causes the bridge is unbalanced (Va≠Vb). The output voltage of
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the bridge can be expressed as a function of the change in resistance of the transducer Let the change
in resistance of the transducer be ∆R. Since Rb and Rc are fixed resistor, the voltage Vb is constant.
Voltage Vo varies as a function of the change transducer resistance. According to the voltage-divider
rule we get.
𝑉𝑎 =
𝑉𝑏 =
𝑅𝑎 (𝑉𝑐𝑐 )
𝑅𝑎 +𝑅𝑇 +∆𝑅
𝑅𝑏 (𝑉𝑐𝑐 )
𝑅𝑏 +𝑅𝑐
The voltage Vab across the output terminal
𝑉𝑎𝑏 = 𝑉𝑎 -𝑉𝑏
=
𝑅𝑎 𝑉𝑐𝑐
𝑅𝑎 +𝑅𝑡+∆𝑅
–
𝑅𝑏 𝑉𝑐𝑐
𝑅𝑏 +𝑅𝑐
If 𝑅𝑎 =𝑅𝑏 =𝑅𝑐 =𝑅𝑇 =R , Then
𝑉𝑎𝑏 =
−∆𝑅(𝑉𝑐𝑐 )
2(2𝑅+∆𝑅)
The negative (-) sign in the equation indicates that 𝑉𝑎 < 𝑉𝑏 because of the increase in the value of
∆R.
The gain of the basic amplifier is (𝑅𝑓 /𝑅1 );
Therefore the output voltage Vo is
𝑉𝑜 = 𝑉𝑎𝑏
(-
𝑅𝑓
𝑅1
)=
(∆𝑅)𝑣𝑐𝑐 𝑅𝑓
2(2𝑅+∆𝑅) 𝑅1
The change in resistance of transducer ∆R is very small. we can approximate (2R+∆R) ≅ 2𝑅.
Thus the output voltage is
𝑅𝑓 ∆𝑅
𝑉𝑜 = 𝑉𝑐𝑐 𝑅
1 4𝑅
The equation indicates that Vo is directly propotional to the change in resistance ∆R
of the
transducer. Since the change in resistance is caused by a changed in terms of the units of the physical
energy. In our instrumentation amplifier we have used “Thermistor” as a transducer. Thermistors are
essentially semiconductor that behave as resistor. Usually with a negative temperature coefficient of
resistance. That is as the temperature of the thermistor
increases, the resistance decrease. The
temperature coefficient of the resistance is expressed ohms per unit change in degrees Celsius (˚C).
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Thermistor with a high temperature coefficient of resistance are more sensitive to temperature change
and therefore suited to temperature measurement or control .
Temperature Meter
Temperature meter is an electronic device which can measure can measure the temperature of
anysubstance in a specific range of temperature. The temperature meter can measure the temperature
inCelsius or Kelvin or Fahrenheit scale. The internal circuit which is designed by using
instrumentationamplifier converts temperature changes to a voltage and the indicting meter converts
the output voltage into temperature, such as
in Celsius or Kelvin or Fahrenheit scale. Then the
indicating meter can display the temperature of the substance.
Design Procedure of a Temperature Meter
The instrumentation amplifier shown in Fig-3 is the circuit diagram of a temperature meter because
the transducer in the bridge circuit is a thermistor and the output meter is calibrated in degree Celsius
or Fahrenheit. The bridge can be balanced at a desired reference condition, for instance 25˚C. As the
temperature varies from its reference value, the resistance of the thermistor changes and the bridge
become unbalanced. This unbalanced bridge in turn produce the meter movement. The meter can be
calibrated to read a desired temperature range by selecting an operating gain for the differential
instrumentation amplifier. The meter movement is dependent on the amount of imbalance in the
bridge, that is the change of ∆R in the value of thermistor resistance. The ∆R of the thermistor can be
determined as follow ;
∆R= (Temperature coefficient of resistance ) (final temperature –reference temperature).
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Circuit Diagram
+
Resistive transducer
RT±∆R
Va
Vb
Vo
Vab
Vab
°C
R3=Rf
Indicating meter
Figure:3
temperature meter circuit using instrumentation amplifier
Calculation of Temperature Measurement
I figure-2 if we put the value 𝑅1 =1kΩ, 𝑅𝑓 =4.7kΩ, 𝑅𝑎 =𝑅𝑏 =𝑅𝑐 =100 kΩ , 𝑉𝑐𝑐 =+5 V, op-amp supply
voltage= ± 15V, T=100 kΩ, reference temperature is 25˚C, temperature coefficient of resistance = 1kΩ/˚C or1%/˚C .
Now we can calculate the value of the output voltage at 0˚C and at 100˚C.
∆R =
−1𝑘Ω
˚𝐶
(0˚C-25˚C)
= 25 kΩ
Therefore , at 0C˚ we get
𝑉𝑜
=
4.7(103 ) 25(103 )
1( 103 )
400(103 )
(5)
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𝑉𝑜
= 1.47 V
Similarly at 100˚C,
∆R =
𝑉𝑜
−1𝑘Ω
˚𝐶
=
(100˚C-25˚C) =
-75 kΩ
4.7(103 ) (−75)(103 )
1( 103 )
400(103 )
(5)
𝑉𝑜 = -4.41 V
From this calculation we get the result that when Vo = 1.47 V the meter face can be marked as 0˚C ,
and when 𝑉𝑜 = -4.41 V , it can be marked as 100˚C . here at 25˚C , 𝑉𝑜 = 0 V; therefore a center-zero
meter is required . thus , using the resistance-temperature characteristics of the thermistor the meter
can be calibrated from 0˚C to 100˚C .
Thermistor with relatively higher resistance (Rt ≥ 1 𝑀Ω) and sensitivity (temperature coefficient of
resistance ≥ 3%/˚C) are best suited for remote measurement because the effect of transmission-line
resistance is negligible.
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Beside instrumentation amplifier we can produce an electronic temperature meter using practical
bridge amplifier.
Circuit Diagram
𝑅𝑡𝑟𝑎𝑛𝑠 = 𝑅𝑟𝑒𝑓 +∆R
I=
Bridge
excitation
voltage
current
I
𝑉𝑜 =-
∆𝑅
𝑅1 +𝑅𝑟𝑒𝑓
=I(∆R)
E
Figure: 4 Practical bridge amplifier with transducer
𝐸
𝑅1 + 𝑅𝑟𝑒𝑓
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Advantages
1. By temperature meter we can measure the temperature of any substance easily.
2. We can get the temperature in different scale such as Celsius or Kelvin or Fahrenheit
scale.
3. In temperature meter the resistive transducer is very sensitive with temperature therefore it
converts very little change in temperature into resistance change.
4. The operational amplifiers used in instrumentation system are precise special-purpose circuits
in which the electrical parameters such as offset, drifts, power consumption are minimized,
whereas input resistance, CMRR, and supply range are optimized. So it make the circuit
stable and accurate.
Limitations
1. This Temperature Meter using instrumentation amplifier cannot measure the
temperature of high range.
2. The operational amplifier are used are quite expensive.
3. Output voltage does not depends on 𝑉𝑐𝑐 . It can not exit the saturation voltage. So it
can not measure high range of temperature.
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Conclusion
The bridge circuit converts the resistance change of a transducer into a voltage change. An input
temperature change of 25°C to 100°C gives an output voltage change of 0 V to -4.41V. The
sensitivity of the temperature to voltage converter can be increase easily by increasing 𝑉𝑐𝑐 . The
maximum value of 𝑉𝑐𝑐 . Is set the maximum thermistors current to avoid the self heat. Therefore 𝑉𝑐𝑐 .
Has a maximum value of
𝑉𝑐𝑐 . = I(𝑅𝑇 +𝑅1 )
Reference Books
1. Op-Amp Linear Integrated circuits
(Ramakant A. Gayakward)
2. Operational amplifier and Linear Integrated circuits
(Robert F. Coughlin &Frederick F. Driscoll)