Datasheet

DATASHEET
SMT172
last update
juni 1, 2015
reference
V10.0
page
8/9
When measuring temperature of solid or liquid targets, it helps to have a good thermal contact between the
sensor and the target. This can be achieved with metals and thermal paste. When measuring air
temperatures, it is important to isolate the sensor from the rest of the measurement system, so that the
heating from the surrounding circuit components has only a small influence on the sensor temperature.
Self-Heating
All electronic circuits consume power, and all power becomes heat. Depending on the thermal resistance
to the environment and the related thermal mass on the heat path, this heat will cause an extra
temperature rise of the sensor die and will influence the final reading. Although the ultra-low power
consumption of SMT172 sensor minimizes this effect greatly, it is always important to take this into account
when designing a temperature measurement system. Design considerations like optimal thermal contact
with the environment and powering down the sensors whenever possible (see SMTAS08) are all useful
techniques to minimize this effect.
Thermal response time
The thermal response time of the temperature sensor is determined by both the thermal conductance and
the thermal mass between the heat source and the sensor die. Depending on the packaging material and
the immerging substances, this can vary in a wide range from sub-second to hundreds of seconds. The
following table illustrates the time constant (the time required to reach 63 % of an instantaneous
temperature change) of TO-18 packaged sensors.
Conditions of installation Time constant (s)
(TO-18)
In an aluminium block 0.6
In a bath filled with oil that is stirred
constantly
1.4
In air that moves at 3 m/s:
- Without heat sink
- With heat sink
13.5
5
In non-moving air:
- Without heat sink
- With heat sink
60
100
Supply voltage decoupling
It is common practice for precision analogue ICs to use a decoupling capacitor between Vcc and GND
pins. This capacitor ensures a better overall EMI/EMC performance. When applied, this capacitor should
be a ceramic type and have a value of approximately 100 nF. The location should be as close to the
sensor as possible.
It is advised to also add a series resistor of 220 Ohm, so a low pass filter is created. This will enhance the
EMC performance and it will limit the maximum current in case of faults or wrong connections.
SMT172
Vcc
Gnd
R = 220 Ω
OUT