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Academic Open Internet Journal |
Volume 15, 2005 |
PSPICE SIMULATIONS FOR PERFORMANCE TESTING OF SIGNAL CONDITIONING CIRCUIT
FOR TEMPERATURE SENSOR
A.V. Mancharkar And S.H.Behere*
Lecturer, Dept.of Electronic Science,
e-mail :
mancharkar_av@rediffmail.com
*Professor,Dept.of Physics,
Abstract - This paper
addresses performance of signal conditioning circuit for temperature sensor
viz. J-type thermocouple, as an example. In reality the circuit has to simulate
over the specified operation range. The output voltages are up to 3-digits
decimal point by experimenting several input voltage
values in the test circuit. The various input voltage values are used in
performing DC analysis. The low cost, fast simulation techniques for the
linearity and an accuracy of the circuit performance and reliability is
reported by using the PSpice test tools. The results
are explored with browsing output data facility and run probe analysis.
Keywords: SCC:
Signal conditioning circuit, DC analysis, simulation.
1. Introduction
The evolution of process control has seen the infusion of electronics technology into almost every facet because of low cost, reliability, miniaturization, and ease of interface. Using the signal conditioning circuits the direct transudation of any physical signal is converted into the required output signal. The specific type of signal conditioning depends, of course on the type of sensor employed as well as the nature of the specified output signal characteristic [1]. The electronic industry is getting progressively more and more efficiently at developing new products in wide ranges and variety of sensor applications to customer. We also see more and more products coming into the market with shorter and shorter product lives or some time they may fatal at times [2]. Hence, low cost circuit design, with an accurate, linear and faster testing technique is addressed.
In
the industrial processes temperature is a main factor. A variety of temperature
sensors are commercially available however signal conditioning plays an
important role in design and development of accurate circuit performance and
optimum reliability. Here, the product has to be practically designed first and
then tested for its intended results, but any degradation in the output signal
is found circuit parameters and values have to be replaced and circuit is
re-tested for improved performance and low drift in specified parameters over
the operating range. To complete one such process involves large amount of
time, higher cost and accurate component values. This critical problem of
product design and testing is simplified by the use of PSpice
simulations. This allows user to design, test, and perform various analysis to
optimize the accuracy, circuit performance and its reliability before any
product is actually made and transferred to the customer for their use [3]. The
signal conditioning circuit for J-type thermocouple temperature sensor is discussed
in this paper.
2. Simulation concepts
Electronic Simulation of circuit function is now a common practice in the design both of individual circuits and complete systems. The more of the circuit a designer can simulate, the faster the circuit can get into production and hence to market. PSPICE ( PC Version, Simulation Program with Integrated circuit Emphasis ) has become a common tool for analogue simulations and widely used, even for some mixed mode circuit designs [4].
Sophisticated instrumentation systems rely heavily on the application of a wide range of electronic circuits. Signal processing circuits constitute the essential link between the sensors and the final output equipment such as readout systems, computers and other devices. The signals obtained from sensors/transducers may be in analog or digital domain, very few instrumentation systems are fully digital by nature. Signal processing and conditioning is carried out in order to bring out the output signal to the desired description and standardized levels. In most cases, it is essential to see that the signal processing circuitry chosen preserves the desired functional relationship between the input and output signals and does not in any way impair the basic accuracy with which the measurement is carried out. The signal conditioner can vary in complexity with or without detectors, and filters. Alternately, they are termed as signal modifiers or signal processors. The output signal may be an analog or digital quantity [1].
PSPICE allow the designer to construct entirely new circuit without fabricating the actual circuit using hard wires, components and PCBs. The circuit performance and its reliability in any circuit to minimize failure can be tested. To meet the required standards of a circuit and hence quality instrument, the circuit analysis is performed. In case of any failure or problems observed, one can easily redesign it by modifying the very same circuit in few minute using highly sophisticated simulation tools. With adequate number of design and redesign iterations on a computer platform where it consumes only a small amount of time and no material. The design can be made robust. The simulated circuit can then be subjected to different analysis i.e. actual tests. The performance and reliability of circuit and instrument definitely shows results of up most levels. Thus it is the faster and low cost and less cumbersome process .
Dr.
Lawrence Negal at
To test and analyze the circuit performance first set the circuit for initial conditions such as zero and span adjustments using pots PT2 and PT1 as shown in the circuit diagram shown in Fig.1. Spice simulations and testing process is as follows.
· Drawing the circuit
· Selecting the type of analysis
· Simulation of the circuit
· Displaying the results of the simulation
The
3. Signal Conditioning Circuits
Measurement
systems handle various types of signals produced by the sensors.
These signals are
required to be conditioned for user friendly access. Following section describes the signal
conditioning circuit used for widely used temperature sensor viz. J-type
thermocouple.
3.1 Signal conditioning card for Thermocouple
The J-type
thermocouple is used frequently in industry because it is economical and has
high output in millivolts over its temperature range.
Due to relatively low voltage output associated with most thermocouples, amplification
circuit is used to increase sensitivity. The resulting output of this circuit
is used to drive or activate a readout device [7]. This card converts the
change in thermocouple voltage in mV into the corresponding change in output
voltage in mV.
Figure1: shows a
signal-conditioning card for a J type thermocouple. In this card, a +12 V is
used to apply excitation voltage voltage. Resistors
R4,R5,R6 and capacitors C3,C4 forms a low pass filter.
An Op-amp is used in non-inverting mode.
Fig.1 Signal Conditioning
Circuit for Thermocouple
For testing the card using PSpice
test tools the DC input voltage was swept in the range 0mV to 6mV in step of
0.1mV. While testing the card practically, initially the inputs of card were
shorted to the ground and PT2 is adjusted to get a voltage in mV, which
corresponds to room temperature. For testing this card, using potential divider
an input voltage in mV was applied to the card and output voltage was noted.
4.
Results and Discussions
The PSpice simulation of typical temperature sensor signal
conditioning circuit was also carried out successfully. The performance
dependence characteristic on input voltage in mVs is
shown in Figure2.
Fig.2
Figure3: shows variation of
output voltage with respect to changes in input voltage in mV which is practically applied.
It shows that as input voltage increases the output voltage also increases
and this change is linear.
Fig.3
This
paper reports that the results obtained after applying inputs to SCC using
PSpice test tools and the results obtained after applying
inputs practically are same. Since Spice is the industry standard for electronic
circuits simulations, it is used extensively for the design of other sensor
signal conditioning circuits such as pressure, flow, displacement, force,
vibration, etc. in sensor design and applications as the new product. Also
these simulation studies are uniformly may be applied for testing the circuit
performance and reliability under changing physical conditions under which
they are operating before actual circuit or product is made[8].
Hence it avoids the complex, cumbersome, cyclic testing procedures for a newly
developed product.
5.
Acknowledgements
The author (AVM)
wishes to thank the Management and Principal of the N.A.C.&
College, Ahmednagar and Head of the Department of Physics,
References
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And Instrumentation, Printice Hall of India,
[2] Ward A
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Chapman & Hall
[3] Walunj A K, Mancharkar A V and Shaligram
A D, PSpice Simulations for performance and reliability
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[4] Martin O¢ Hara, Modelling board-level DC-DC converters in SPICE (Electronic Product Design), July : 1998.
[5] Tunenga Paul W, SPICE: A guide to circuit simulation and analysis using PSPICE, Englewood Cliffs N.J. USA,Prientice Hall : 1998.
[6] Rashid M H, Microelectronic Circuits Analysis And Design, PWS Publishing Co : 1999.
[7] Rangan C S, Sharma G R, Mani V S
V, Instrumentation Devices and Systems,
Tata-McGraw-Hill Ltd,
[8] Walunj A K, Mancharkar A V and Shaligram A D, Temperature Sensors SCC PSpice performance and reliability testing due to resistive tolerances and effects, Proceedings of 7th NSPTS, University of Pune, 14-16Feb, 2000, p50.1-50.6.
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