# Forum: µC & Digital Electronics What is under the hood and how different temperature sensors work?

 Author: Mozo B. (Company: OMOS) (mozobata) Posted on: 2017-07-06 12:28

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There are a lot of different types of temperature sensors for various
applications. I am mostly interested in human body temperature
measurement.

After some reading I'vee been able to distinguish three groups of
sensors:

analog (like TMP36)
digital (like DS18B20:)here is the datesheet of
DS18B20:http://www.componentschip.com/details/Maxim-Integr...

infrared (like MLX90614)

While I understand the difference in measurement type/methodology,
accuracy and communication I am not sure how they work under the hood.
I've been trying to understand schematics in documentations but because
I haven't got lot of experience in that matter I failed.

analog sensors are using some kind of temperature variable
infrared sensors use infrared detection to determine the
temperature. This sensor is I guess the easiest to understand because
heat emits some kind of infrared and the point here is just to detect
how much of this light is emitted.

I have no idea how does digital works.

Can someone please give me maybe more complex explanation of this
sensors, the best would be with some references.

 Author: Lothar M. (lkmiller) (Moderator) Posted on: 2017-07-06 12:36

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Mozo B. wrote:
> I have no idea how does digital works.
As temperature is an analog value under the hood every temperatur has an
anlog frontend.

And after getting an analog temperature value out of this, there may
follow an AD converter with an interface to form a digital temperatur
sensor.

Thats all.

: Edited by Moderator
 Author: sleepy (Guest) Posted on: 2017-08-10 00:51

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Mozo B. wrote:
> This sensor is I guess the easiest to understand because
> heat emits some kind of infrared and the point here is just to detect
> how much of this light is emitted.

That ist correct, but Sensors which register midrange infrared radiation
are often based on thermoelements (Seebeck-effect), whereas contact
thermometers are based on the temperature dependency of the threshold
voltage of a silicon diode.
Thermoelements most often are made from other materials than silicon and
therefore are not easily integrated on a silicon die.

That makes the infrared sensitive variety more expensive.

But there are more differences.
IR thermometers measure the radiant power of the emitting surface wheras
contact sensors (NTCs, Diodes) measure the true temperature of the
object.
Therefore IR-thermometers in most cases are faster, but they can produce
large errors when measuring a surface with low emissivity e.g. a shiny
metal surface.

 Author: sleepy (Guest) Posted on: 2017-08-10 01:19

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P.S.:
Her ist a microphotograph of the Melexis IR-Sensor:
https://www.eevblog.com/forum/projects/unknown-dig...

In the middle you can see the thermopile wich consists of a large number
of thermoelements.
Because the output voltage of a thermoelement is very very small, many
of them are electrically connected in series in order to give a greater
voltage to the silicon based microchip which is located above the
thermopile.
That silicon die does all the voltage to digital conversion and also
handles the serial protocol with the outside world.

What one cannot see:
There also is a thermometer based on diode voltage drop integrated in
the silicon die.
This is necessary because thermoelements produce a voltage proportional
to the temperature difference of their "hot" and "cold" junction.
Therefore the integrated diode thermometer is necessary to compensate
for temperature fluctuations of the whole device.

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