Datasheet
DATASHEET
SMT172
last update
juni 1, 2015
reference
V10.0
page
3/9
Output Signal
According to tradition, the Smartec temperature sensors have a duty cycle (PWM) output that can be directly
interfaced with a microcontroller without the use of extra components. The output is a square wave with a
well-defined temperature-dependent duty cycle. In general, the duty cycle of the output signal is defined by a
linear equation:
where DC = Valid Duty Cycle
T = Temperature in °C
A simple calculation shows that, i.e. at 0°C, DC= 0.32 (32%); at 130°C, DC= 0.931 (93.1%).
Temperature is then derived from the measured duty cycle by:
(1)
The frequency of the output of the sensor is fixed and contains no temperature information. Only the duty
cycle contains temperature information in accordance to the formula given above. The output signal may
show a low frequency jitter or drift. Therefore most oscilloscopes and counters are not suited for verifying the
accuracy of these sensors. However, the duty-cycle value is guaranteed to be accurate within the values
specified for each type (housing).
A higher accuracy can be achieved when a second order conversion formula is used, an inaccuracy of
0.1 °C can be reached in the range of -20 °C to 80 °C (see page 6).
Valid Duty Cycle
A valid duty cycle in equation (1) is defined as the average of individual duty cycles from 8 consequent
output periods. This is due to the internal working principle of the SMT172 sensor. The difference of duty
cycle between individual periods within the 8 period output can be relatively large and also different from
sensor to sensor, but the averaged value (valid duty cycle) is very stable and precise.
Therefore a valid duty cycle is:
Where t
Hi
: time interval of high cycle
t
Li
: time interval of low cycle
DC
i
duty cycle of individual period i
DC
the final duty cycle
For improved noise performance, a measurement of multiples (N times) of 8 periods is recommended.
In words:
After each period the duty cycle has to be calculated and stored. The mean duty cycle has to be taken over 8
period or a multiple of 8 periods. This mean duty cycle is used to calculate the temperature.
Measurement always starts on the negative edge of the output signal.
DC
1
DC
2
DC
3
DC
4
DC
5
DC
6
DC
7
DC
8
Started from
any period