User's Manual

UMXXX
5. Second raw T output during calibration: T1_OUT, set T1 = T1_OUT
6. Current raw T output during calibration: T_OUT, set T = T_OUT
7. Current Temperature in Celsius degrees:
• T = T0deg + (T1deg-T0deg) * (T-T0)/(T1-T0)
HTS221 SYSTEM INTEGRATION
To get reliable and consistent measurements, the system design should maximize sensor
exposure to the external environment while minimizing error sources. See also application
note AN4722.
• Mechanical design: if there is one vent hole in the BlueNRG-2 sensor node housing, the
hole diameter should be maximized and the dead volume enclosed should be minimized;
two or multiple vent holes are preferable, in order to create a laminar airflow and minimize
the response time. Materials that absorb humidity should be avoided.
• Mechanical stress: any mechanical force applied directly or indirectly to the sensor may
affect the output. The BlueNRG-2 sensor node board should not be flexed/curved.
• Heat convection or temperature gradients on the board may affect the sensor Metal lines
and planes, such as the ground plane, should be kept far from the sensor. Milled slits further
increase decoupling. Insulation may be required to isolate the BlueNRG-2 sensor node from
convective and conducted heat.
• Light exposure may induce a change in temperature and humidity.
The internal heating element can effectively be used to speed up sensor recovery in case
of condensation. The heater control is not automatic; it must be switched on/off by the
firmware running on the host microcontroller. Humidity and temperature data should not be
read during the heating cycle. Valid data can only be read once the heater has been turned
off, after the completion of the heating cycle.
BlueNRG-2 sensor node has been designed for ultra-low power operation to enable a very
long battery life (3-10 years on a typical CR2032 coin battery, depending on the application).
Power and heat generated on-board is therefore very limited. Possible sources of
conducted heat, such as the BlueNRG-2 and the LED, have been placed as far as possible
from MEMS sensors to increase the accuracy of the measurements. Milled slits have been
used to further isolate HTS221 temperature sensor from neighboring components.
7. Digital MEMS microphone features
A MEMS microphone is a dual-die device. The first die is a MEMS (micro electromechanical
system) and it is basically a silicon capacitor; the second die is an ASIC (application specific
integrated circuit) that converts the polarized MEMS capacitance in an analog or digital