DATASHEET

SMT172

last update

maart 20, 2017

reference

V14

page

4

Therefore, a valid duty cycle is:

=

∑

=

Where

t

Hi

= time interval of high state

t

Li

= time interval of low state

DC

i

= duty cycle of individual period i

DC = the valid duty cycle

The specified accuracy and noise performance are based on a measurement of 8 periods. For improved

noise performance, measurement of multiples (N times) of 8 periods is recommended.

In other words:

After each period the duty cycle has to be calculated and stored. The mean duty cycle has to be taken over 8

periods 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.

Understanding the specifications

Sampling Noise

From the theory of signal processing it can be derived that there is a fixed ratio between the frequency of the

sensor output, the sampling rate and the sampling noise. The uncertainty of the temperature measurement is

determined by:

= 200

6

t

p

t

s

Note:

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 on the measurement time. The noise of the sensor is

about 0.002°C when measuring over 3.6ms (8 periods, 5V). When measuring over about 0.1s the sensor

noise is reduced to 0.0004°C.

Where T

noise)

t

t

t

t

p