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IETE Journal of Research
ISSN: 0377-2063 (Print) 0974-780X (Online) Journal homepage: http://www.tandfonline.com/loi/tijr20
A Simple Fast, Accurate, Cheap Transducer/Sensor
Linearizer Using Prom
Umesh Kumar
To cite this article: Umesh Kumar (1983) A Simple Fast, Accurate, Cheap Transducer/
Sensor Linearizer Using Prom, IETE Journal of Research, 29:2, 88-89, DOI:
To link to this article: http://dx.doi.org/10.1080/03772063.1983.11452952
Published online: 10 Jul 2015.
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A noyel digital hardware set-up is postulated for transforming the
characteristics of a transducer into linear output using a PROM lookup table. The method has been actually tested on a thermistor and
found to yield Tery accurate results.
MAJORITY OF TRANSDUCERS/SENSORS produce OUtput voltages which
have a nonlinear relationship with the measurand. Several important examples include the thermocouple, thermistor (for measurement
of temperature); measurement of electron mean energy in a laser
gas discharge (Lawson & Lucas 1965); pirani pressure transducer;
infra-red sensor etc. There is an ever-increasing use of such nonlinear transducers in the fields of research and industry for measurement of different parameters, e.g. flow, velocity, pressure and temperature. Thermisto" and thermocouple are in general use for the measurement of temperature. It is, therefore, extremely important and imperative to have a method to process the nonlinear voltages and form an
input/output characteristic such that it linearizes the output voltage
with the corresponding values of the measurand.
A number of different techniques have been proposed in recent
years to achieve the linearization of the transducer characteristics.
One method is to use an electronic circuit incorporating diodes to
switch in selected resistances at pre-determined voltages. Another
way is to use an analogue method of linearization (Weir 1974) which
requires modifications to a dual-slope integrating digital voltmeter.
Broughton (1974) has presented a technique for lhe analysis and design
of six one-thermistor temperature transducer circuits by expressing
the output/input voltage ratio as a linear fractional transformation.
One more method is to use digitallinearization involving the use of
an analogue-to-digital converter (ADC) connected to a computer.
A simplified version employs a microprocessor unit or a pocket programmable calculator which is interfaced to an ADC. The system is,
however, still expensive and is not suitable for linearizing transducers
with highly nonlinear response. The simplest and the best method is
to use a set of look-up tables.
The aim of this article is to present a fast, accurate, cheap and easy
method for use with linear and nonlinear transducers/sensors. It
can be readily incorporated in a simple instrument and this simple
solution can be commercially utilized. The attractive and salient
features and virtues are its versatility, much lower component count,
cheap price, low power consumption, easy straight-forward and direct
fabrication and the excellence and accuracy of linearization. In fact,
the whole logic can be readily manufactured on a single integrated
circuit (IC) chip with its associated lower size, weight and cost, high
reliability, increased miniaturization and direct on-the-line production
possibility in bulk in a manufacturing environment. The system
discussed is built around a PROM.
The circuit configuration for achieving the linearization of transducer/sensor characteristics using hardwired logic is shown in the block
diagram in Fig. 1.
Manuscript received 1982 February 27; revised 1982 September 27;
finally revised 1982 November 13.
Fig. 1. Hard wired logic for transducer/sensor linearization.
The principle of action of the circuit configuration together with the
important characteristics and function of each block is described as
The transducer/sensor and amplifier stage generates an analogue
voltage which is applied to the high speed, dual slope A/D convertor.
The ADC 1103 is a very fast successive approximation converter
packaged in a small module. It is a complete self-contained converter,
requiring only standard power supplies and the usual control signals.
The input range and output coding are user selected. It is specially
well suited for applications requiring high throughput rates with no
compromise in accuracy.
Depending on the magnitude of the voltage applied, a count is
stored in the segment length counter through dual-slope integration.
The chip 3814 is a Fairchild DVM logic 4i decade multiplexed BCD
output segment length counter. The stored count is directly proportional to the magnitude of the voltage applied. Let us say the stored
count is 40,000.
During this time interval of integration, the spillover logic has been
so designed to produce an output pulse each time the count advances
by one hundred. The pulse generated advances the memory address
counter. The chip 74161 is a synchronous presettable 4-bit binacy
counter for applications in high-speed counting designs. It is fully
programmable and the outputs may be preset to either level. Presetting is synchronous with the clock. The counter sequentially
addresses the PROM look-up table.
The PROM look-up table has been initially programmed and
prepared from an analysis and study of the actual and desired linear
transducer/sensor characteristics. The chip 1702A is a 256-word by
8-bit electrically programmable ROM. Initially all the 2048 bits of
the memory are in 0 state (output low). Information is introduced by
selectively programming 1's (output high) in the proper bit location.
The organization of the PROM for this application is shown in Fig. 2.
The chip has got a transparent lid which allows the user to expose to
UV light to erase the bit pattern. A new pattern can then be written
into the device. The circuitry is completely static.
The PROM look-up table outputs voltages in binary form propor-~
tional to the corrections to be applied to the transducer/sensor characteristics. These corrections have been stored at sequential address
locations in the look-up table. The correction voltage generated is
applied to a binary rate multiplier which outputs a frequency proportional to it.
The 7497 is a TTL MSI monolithic synchronous 6-bit binary rate
multiplier which can achieve 32 MHz typical maxill1_!lm operating
frequencies. The output frequency is equal to the inrut frequency
multiplied by the rate input M and divided by 64 :fou~ =M. fin/64
where, M= F.2s + £.24 + D.2l + c.22 + B.21 + ,oo. The necessary correction in frequency is applied to the clock frequency and
therefrom to the A/D converter.
• ~,
-r-+-+--+-----------t-+--t---,S SK 6 e
The digital inputs directly interface to TTL logic and operates from
a + 5V to + 15 V supply with only 20 mW dissipation. In this way,
an accurately linear characteristic can be obtained from the transducer I
sensor as desired.
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Do to
A nonlinear transducer of prime importance is a thermistor available for the measurement of temperature. The digitallinearizer proposed proved to be very successful in the applicationofaccurate temperature monitoring, as is evident from Fig. 3. For a thermistor, the
resistance obeys an exponential law
R =A exp (B/T) and its output voltage is
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11 v .. ~,
Fig. 2. A look up table PRO M organisation showing input and output
connections (INTEL).
I,A exp (B/T),
Lawson (P A) & Lucas (J). Electron Diffusion in Hydrogen at
Electric Fields and Low Gas Pressures. Proc. Phys. Soc. 85,
1965; 177-185.
Weir (K G). Analogue Linearized Technique for a Digital
Voltmeter. J. Physics : E :Scientific Instruments. 7, 1974; 507509.
Broughton (M B). Analysis and Design of Almost-Linear
One-Thermistor Temperature Transducers. IEEE Trans. IM-23,
I; 1974; 1-5.
Fig. 3.
Voltage vs. temperature characteristics for a thermistor.
The AD 7520 chip is a monolithic CMOS low cost, 10-bit multiplying D/A converter. The device uses advanced monolithic CMOS
and thin-film technologies to provide upto 10-bit differential linearity.
The circuit described trips a three-phase motor in the case of a phase
failure and inhibits its reverse rotation in the event of a phase sequence
reversal. The circuit detects phase failure within one cycle time.
use costly electromagnetic and thermal
relays to sense single phasing and phase-reversal conditions. The
Manuscript received 1982 July 31.
Department of Electrical Engineering,
1./.T., New Delhi IJO 016.
The digital linearizer unit proved to be very useful in linearizing
nonlinear transducer and is not restricted to the application described
in the text, but can be used with any type of transducer available.
The majority of the transducers available today can be linearized with
its capabilities. The complete unit can be implemented and realized
easily in micro-miniaturized integrated circuit (IC) form.
This approach has already amply demonstrated its capability and
is well within the range of modern technology.
where, 18 is the constant current passing through the thermistor, A
and B are constants and T is the absolute temperature. The above
characteristic in eqn. (1) as well as the linearized output available were
plotted in Fig. 3. The digitallinearizer gave temperatures in very good
agreement over the experimental range of 20-90°C with those given by
the mercury thermometer within 1°C. Thus, the accuracy attained
was very high with percentage error in the range of 1 per cent only.
circuit described is built around a single IC chip and it accomplishes
the same functions at a much lower component count and cost
(Fig. 1).
The low-voltage supply for circuit operation is derived from one of
the phase voltages while the other two phase voltages are stepped
down and shaped into square waves by INVERTERS ]-4 and capacitors
C 1 and C2 . Phase voltage VR is taken as a reference and the correct
phase sequence as VR-Vy--V8 . The retriggerable one shot (ROS)
is configured such that its B input being high, a negative transition