ST LPS22HB, LPS25HB Application Note
August 2016
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www.st.com
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Application note
LPS22HB/LPS25HB digital pressure sensors:
hardware guidelines for system integration
Introduction
The purpose of this application note is to introduce guidelines for the hardware integration of
STMicroelectronics's LPS22HB and LPS25HB pressure sensors in the final customer's application.
Contents
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Contents
1 System integration .......................................................................... 5
2 Mechanical design rules ................................................................. 7
2.1 Sensor placement ................................ ................................ ............. 7
2.1.1 Exposition to the environment ............................................................ 7
2.1.2 Heat propagation ................................................................................ 8
2.1.3 Mechanical stress ............................................................................. 12
2.2 Sensor embodiment and housing .................................................... 13
2.3 Sensor protection ............................................................................ 15
3 Reference design: integration and housing on a personal
device ...................................................................................................... 16
4 Use case and configuration example for the LPS22HB .............. 18
4.1 Main device settings ........................................................................ 18
5 Use case and configuration example for the LPS25HB .............. 20
5.1 Main device settings ........................................................................ 20
6 Revision history ............................................................................ 23
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List of tables
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List of tables
Table 1: ODR configuration ...................................................................................................................... 18
Table 2: FIFO mode selection .................................................................................................................. 19
Table 3: ODR configuration ...................................................................................................................... 20
Table 4: Temperature resolution configuration ......................................................................................... 21
Table 5: FIFO coefficient filter ................................................................................................................... 21
Table 6: FIFO MEAN MODE configurations for different application scenarios ....................................... 22
Table 7: Document revision history .......................................................................................................... 23
List of figures
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List of figures
Figure 1: Pressure sensor system integration ............................................................................................ 5
Figure 2: Pressure sensor integration and embodiment reference ............................................................ 8
Figure 3: Pressure sensor integration and embodiment with vent channel ................................................ 8
Figure 4: Heating isolation implemented for protecting the sensor ............................................................ 9
Figure 5: Top view of the sensor housing: on the left a correct design with the heat isolation, on the right
a wrong design .......................................................................................................................................... 10
Figure 6: Sensor with a correct sensor placement on the PCB to get the appropriate isolation from heat
sources ..................................................................................................................................................... 10
Figure 7: Sensor with a bad wiring on the PCB ........................................................................................ 11
Figure 8: Sensor wiring with wrong placement on the PCB ..................................................................... 11
Figure 9: Bad configuration for mechanical stress (a) .............................................................................. 12
Figure 10: Bad configuration for mechanical stress (b) ............................................................................ 12
Figure 11: Good configuration for avoiding mechanical stress and reducing the dead volume (a) ......... 12
Figure 12: Good configuration for avoiding mechanical stress and reducing the dead volume (b) ......... 13
Figure 13: Example of good sensor embodiment and housing ................................................................ 14
Figure 14: Example of good sensor embodiment and housing with airflow channel ............................... 14
Figure 15: Example of a bad sensor embodiment and housing ............................................................... 15
Figure 16: Integration of the digital pressure sensor device in a sensor chamber with two vent apertures
.................................................................................................................................................................. 16
Figure 17: Device integration reference in a portable device ................................................................... 17
Figure 18: FIFO moving average filter scheme ........................................................................................ 21
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System integration
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1 System integration
The LPS22HB/LPS25HB pressure and temperature sensors' integration in application
systems such as portable devices like smartphones, wearable devices, weather stations or
industrial equipments shall be implemented without compromising the sensor
performances. The system integration can be done by looking at the main mechanical and
geometrical parameters and the factors that influence the sensor performance and thus
optimizing those.
The typical sensor integration scenario is described in Figure 1: "Pressure sensor system
integration" where the embodiment of the sensor has to be designed in order to get as
much as possible the correspondence between the pressure (Px) and temperature (Tx)
conditions of the environment under test, and (Ps, Ts) that represent the conditions around
the sensor sensing area, nearby the air inlet houses.
Figure 1: Pressure sensor system integration
Therefore, in order to get a reliable and consistent measurement, all the parameters
involved in the mechanical design must be dimensioned to get the maximum sensor
exposition to the external environment, to get a faster response time, in terms of pressure
and temperature, compatible with the required design specifications.
Every change in the condition under test must be reflected as a sensor consistent
measurement, also in the case of fast pressure and temperature variations. Therefore, the
integration design must guarantee the environment conditions matching with the sensing
area conditions not only in “steady-state” (static conditions) but also in dynamic conditions.
Deviations between the conditions under test and the conditions around the sensing area
are also influenced by heating sources, like other devices close the sensing area, the selfheating of the sensor. Changes in temperature are critical because not only the
temperature is influenced but, changes in temperature will also determine pressure
deviations and, as a consequence, a slower response of the system.
System integration
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Based on the considerations above, the design optimization consists of determining:
1. the placement of the sensor in the system
2. the sensor embodiment and housing
3. sensor protection from dust, water, or chemical solvent by a sensor chamber, in
presence of harsh environment
The above elements are further described in the following section of this document.
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Mechanical design rules
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2 Mechanical design rules
For the mechanical design, the main constraints and features to be considered are
described below, to provide a set of basic rules such as good design practices for a
successfull integration of the sensor in the final application context.
2.1 Sensor placement
The sensor placement has a direct impact on the sensor performance as follows, in terms
of sensor link to the environment, thermal propagation mechanism, and mechanical stress.
2.1.1 Exposition to the environment
To maximize the exposition with the environment where to measure pressure and
temperature, the sensor has to be placed in static and dynamic working condition.
In static conditions, or steady-state, after a change of the pressure and the temperature
environment and their stabilization, sensing conditions must be the same as the conditions
under test, or very close to the target value, depending on the application tolerance and
specifications.
In dynamic conditions, in the presence of fast changes of the conditions under test, the
sensor must be able to provide a reliable measurement output able to follow the dynamic of
the environment. At the end of the sensor integration design, the overall response time will
be modified, and the final performance shall match the target specifications. In general,
target is to avoid design with a response time lower than the product specifications. In
order to maximize the sensor performance in static and dynamic conditions after system
integration, depending on the design specifications the below guidelines are suggested,
with reference to Figure 2: "Pressure sensor integration and embodiment reference":
1. Place the sensor to get the best connection with the environment under test, as close
as possible to the vent aperture
2. Large dead volume will increase the response time, with a bigger contribute to the
pressure response time; therefore is recommended to minimize the volume, trying to
shape a tailored housing around the sensor geometry
3. Vent aperture should be as large as possible.
4. The depth of the vent aperture must be minimized.
As a reference for integration design, Figure 2: "Pressure sensor integration and
embodiment reference" describes an example of the above recommendations. In order to
maximize the environment connection and therefore to get a fast response time, the
volume around the sensor (dead volume) is minimized and the vent size aperture has the
same order of magnitude of the sensing area. A filter membrane protection has been
added, for protecting the sensor from water or harsh environment.
Mechanical design rules
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Figure 2: Pressure sensor integration and embodiment reference
A different implementation , more expensive but more efficient in terms of sensing
performances is the design with an air flow structure, described in Figure 3: "Pressure
sensor integration and embodiment with vent channel". The design with multiple vent
apertures is a more expensive solution, but providing , depending on the design
specifications, a faster response time.
Figure 3: Pressure sensor integration and embodiment with vent channel
It is useful to underline that the sensor can work properly even if it is placed in customer's
application system without considering any dedicated hole (vent aperture) unless that one
is not hermetically sealed. The design guidelines reported above are for getting out top
performances.
2.1.2 Heat propagation
The presence of heating sources near the sensor can deteriorate the performances by
modifying pressure and temperature measurement as well as generating thermal gradients
around the sensing area affecting the correct measurement in static and dynamic
conditions.
We report design guidelines for avoiding this effect, but, we remark that the increasing
temperature impacts on performances and is strongly attenuated by the embedded
temperature compensation of LPS22HB and LPS25HB devices.