Datasheet

DATASHEET
SMT172
last update
juni 1, 2015
reference
V10.0
page
4/9
Understanding the specifications
Sampling Noise
From the theory of signal processing it can be derived that there is a fixed ratio between the sensor’s signal
frequency, the sampling rate and the sampling noise. The sampling rate limits the measurement accuracy to:
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t
p
t
s
Note:
The above mentioned error T
err
is NOT related to the intrinsic accuracy of the sensor. It just indicates how
the uncertainty (standard deviation) is influenced when a microcontroller samples a time signal.
Sensor noise
Each semiconductor product generates noise. Also the SMT172 sensor. The lower limit of the noise is
determined by the flicker noise of the sensor, where further averaging will no longer reduce it. So the
measured noise of the sensor of course depends of the measurement time. The noise of the sensor is
about 0.002 °C when measuring over 3.6 ms (8 periods). But when measuring over about 1 s period this
sensor noise will be better than 0.0004 °C.
Package induced error
When applying high stress package materials, extra errors will occur and therefore system designers
should be aware of this effect. The TO-18 package has the minimum package induced errors. All other
packages can have a slightly bigger error on top of the error in the specifications but based on the recent
measurements on the plastic versions TO92, SOIC and TO220 the error will be less than 0.35 °C (-10 °C
to 100 °C) and 1 °C over the temperature range of -45 °C – 130 °C.
Long-term drift
This drift strongly depends on the operating condition. The measured hysteresis in a thermal cycle (TO-18
packaged samples) is less than 0.01 °C over the whole temperature range. Even at extreme condition
(TO-18 samples heated to 200 °C for 48 hours), the drift is still less than 0.05 °C over the whole
temperature range.
Where T
err
= measurement uncertainty (= standard deviation of the sampling noise)
t
s
= microcontrollers sampling rate
t
p
= period of the sensor output
t
m
= total measurement time, an integer number of t
p