Many design engineers have found through application trial
and error that traditional integrated sensor solutions often exhibit poor
accuracy, unstable behavior around the measuring range limits and
unsatisfactory chemical resistance against contaminants. Additional limitations
commonly encountered include lack of dew formation resistance, inadequate
long-term stability and failure during load spikes. There also tend to be
issues with the digital I2C protocol and interface, as all points
often do not correspond to the standard. (I2C refers to the
inter-integrated circuit, which is a multi-master
serial single-ended computer
bus used to attach low-speed peripherals to a motherboard
or embedded system.)
To
address these issues, our goal as a sensor developer was to create a miniature
polymer sensor combined
with a matching ASIC-optimized for a specific application-on a common carrier
substrate. This type of connection
technique is economical when using standard technologies like printed thick
layer substrate and wire bonding, and then connecting to the target system via
brazed SIL contact strips or standard TO sockets.
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A
humidity sensor device created in this way offers the advantage of an optimum
performance for the user over an entire application range, even at the lower
range limit at around 0 percent relative humidity as well as at extreme high
humidity and dew formation. This approach also results in lower manufacturing
costs, as the individual components and ASIC, as well as the sensor and carrier
substrate, can be tested before assembly. This translates into lower costs for
end-users such as system designers.
ASIC
Functionality
The ASIC carries out the entire signal processing of
the physical parameters of humidity and temperature, which are made available
via an I
2C interface as digital values. Temperature measurement
takes place on-chip with an accuracy of better than 0.2C and a resolution of
0.02 K.
The entire operating voltage range from 2.7 to 5.5 V
is supported. The average current consumption in the "standby" mode is around 1
µA and at measuring rate of 1 Hz, it is approximately 22 µA, making the sensor suitable
for battery-operated applications.
The ASIC consists of the internal module cap digital converter (14
bits), a polynomial signal processor, a coefficient memory for the calibrated
values and the digital I
2C-interface.
The humidity measurement is similarly accurate: The correction algorithms
implemented in the ASIC use quadratic polynomials to ensure computational
correction of offset, gain and linearity behaviour, as well as temperature
drift. Therefore, the application window is extremely wide from 0 to 100
percent relative humidity in the temperature range of -40 to 125C with a
maximum dew point of 80C.
Construction
and Dimensions
The ASIC is mounted together with the polymer sensor element on a
mechanically robust thick film ceramic carrier with dimensions of 5 x 5 mm. Due
to the small thermal mass, a fast response is achieved. Both the sensor and the
ASIC are bonded.
Up to the active layer of sensor element, the structure is
environmentally resistant, protected with a glass-filled globe top. SMD
capacitors are integrated on the module, eliminating the need for additional
decoupling capacitors. The solderable connection pins in the grid of 5.0 mm
also accommodate commercial connectors, making the humidity sensor an interchangeable,
calibrated functional module.
All materials are optimized for minimum water absorption, thus leaving
the microclimate in the environment undisturbed. With these materials, the
application temperature range of -40 to 125C is well covered.
At the I
2C compatible interface, the processed physical
dimensions of relative humidity and temperature are transmitted as numerical
digital values. Therefore, it is not necessary to do any further setting
through the application software. The digital interface corresponds to the I
2C
standard (up to 400 kHz clock rate) and can be used together with other I
2C
components connected at the same bus. In addition to the fixed allotted
address, a second address can also be defined. Therefore, simultaneous
operation of up to 126 sensors is possible on the same I
2C bus.
Martin Friedrich is
managing director at Hygrosens. He has been actively involved in the area of
industrial instrumentation for more than 20 years; martin.friedrich@hygrosens.com.
ServoFlo is the U.S. distributor for Hygrosens products, www.servoflo.com
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