IC-Based Temperature Measurement Devices

August 15, 2017

In the last three blog posts, we have talked about a class of
temperature transducers that are ideally suited to the experimental
arena.  Both thermocouples and thermistors are easy to install and are
inexpensive enough to make large channel-count implementations
affordable.  The downside of both of them is that they require some
very specialized instrumentation equipment in order to get an accurate
measurement.

 

While there are a host of applications where thermocouples or
thermistors are appropriate, there are as many or more applications
where they are not.  For instance, it is hardly practical or cost
effective to install a fully cold-junction-compensated thermocouple
temperature sensing device inside the $30 thermostat that you use at
home to control your heating.  Furthermore, the $0.12 temperature
sensor in your car could hardly tolerate the size or cost constraints
that a thermistor circuit would demand.  What is often needed is a
low-channel-count (or one channel), inexpensive temperature sensor that
can be deployed by itself without the need for complicated support
equipment.

 

Indeed, the requirement for such a stand-alone temperature sensor was
discovered in our first discussion about thermocouples.  We needed a
method to measure the temperature of the isothermal terminal blocks
where the thermocouples connected electrically to the instrumentation.

 

Enter the IC-based thermal transducer.

 

As with everything else in the physical universe, the behavior of
integrated circuits is a function of temperature.  This, of course,
includes silicon based transistors.  In the early attempts to create a
constant-output voltage source (a very useful thing) out of integrated
circuits, a functional dependence of transistor output current on
temperature was discovered.  The form of the functional dependence is
described by the so-called ‘Ebers-Moll’ equation.  Any good
electronics handbook will contain this expression.  In this case, I
recommend what may arguably be the most famous book on electronics in
the western world; ‘The Art of Electronics’ by Horowitz and Hill.
(ISBN 0-521-37095-7).  Even the most salty electronics guys refer to
this book.  BTW, this book is a MUST for your engineering reference
book shelf.

 

 

Attempts to ‘build out’ or compensate for the temperature
dependence of transistors gave rise to techniques that produced, as a
by-product, a linear voltage output that varied with temperature.  The
IC temperature sensor was born.  We will halt the academic part of our
discussion at this point.  Suffice it to say that there may be as many
recipes to accomplish this task with transistors and discrete
electronic components as there are ways to cook eggs.  (I like to
cook….)

 

IC-based temperature sensors have MANY advantages:
1.      They can be fabricated on a single IC and packaged in very small
packages if needed.
2.      The require ONLY a source or power and no complicated compensation
circuitry.
3.      The output is usually highly linear and is presented as a scaled
voltage output.
4.      Because of the advances is mass-production of silicon devices, they
are relatively CHEAP, especially if you only need one or two of them at
a time.
5.      They are accurate down to +- (plus/minus) 0.01 degree C!.
6.      They require trivial operational power.

IC-based temperature sensors also have some disadvantages:
1.      They are not quite as sensitive to temperature changes as
thermistors.
2.      The do NOT typically have as wide a range as thermocouples or
thermistors.
3.      Because they are electronic packages, they have deployment
limitations based on environment.
4.      As with ALL temperature sensing devices, size, position and
configuration can and will affect response time and accuracy.

 

Bottom Line:

IC temperature sensor devices are best used when you have the need for
a stand-alone single point measurement.  They are often used on PCBs to
sense the temperature of a processor, chip, or terminal block
(thermocouple CJC).  There are many, many uses for IC based temperature
devices.  They are relatively cheap, require no additional circuitry,
and come in a multitude of packages.  You will probably NOT, however,
use them very often in the laboratory except for the measurement of
some reference or some static point in your system.

 

Good luck with your projects!

 

Wes

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