CMOS Humidity Sensors

Humidity sensors are gaining more significance in diverse
areas of measurement and control technology. Manufacturers
are not only improving the accuracy and long-term drift of
their sensors, they are improving their durability for use in
different environments, and simultaneously reducing the
component size and the price.

Following this trend, Swiss-based Sensirion AG has introduced
a new generation of integrated, digital, and calibrated humidity
and temperature sensors using CMOS "micro-machined" chip
technology. The new product, SHT11, is a single chip relative
humidity and temperature multi sensor module with a calibrated
digital output which allows for simple and quick system
integration.

Conventional sensors determine relative air humidity using
capacitive measurement technology. For this principle, the
sensor element is built out of a film capacitor on different
substrates (glass, ceramic, etc.). The dielectric is a polymer
which absorbs or releases water proportional to the relative
environmental humidity, and thus changes the capacitance
of the capacitor, which is measured by an onboard electronic
circuit.


Weaknesses
Stefan Christian, product manager at Sensirion, lists three
primary weaknesses of conventional analogue humidity
sensors.

Poor long-term stability: Due to the relatively large
dimensions of the sensor elements (10-20 mm2), as well
as the aging of the polymer layer, current capacitive
sensors on the market exhibit varying degrees of sensitivity
to the same external influences. Therefore, the drift per
year, i.e., the yearly change in error tolerance of the sensor,
is becoming an important criteria for quality. The aging of
the metallic layer electrodes can also lead to errors in the
humidity signal.

Complicated calibration: Before use, capacitive humidity
sensors must undergo a complicated calibration process. For
this purpose, the end user must have complex and expensive
calibration and reference systems, as well as external electronic
components, such as memory components.

Analogue technology: Additional problems arise directly
from the analogue measurement principle, which links the
stability of the operating voltage inseparably to the sensor
accuracy. This problem can only be counteracted by
increased spending on electronics and inevitably leads to
higher integration costs.


By combining CMOS and sensor technologies, Mr. Christian
says Sensirion has released "undreamed-of synergy" with
this new standard. "The result is a highly integrated and
extremely small humidity sensor," the SHT11.




The device includes two calibrated microsensors for relative
humidity and temperature which are coupled to an amplification,
analogue-to-digital (ND) conversion and serial interface circuit
on the same chip.

A micro-machined finger electrode system with different
protective and polymer cover layers forms the capacitance
for the sensor chip, and, in addition to providing the sensor
property, simultaneously protects the sensor from interference.

This protection comes in "ways previously not achieved,"
says Mr. Christian. "Total coverage with condensation or
even immersion in liquid present no problems whatsoever.
One and a half year hardness tests have already shown
this result."

The temperature sensor and the humidity sensor together
form a single unit, which enables a precise determination
of the dewpoint without incurring errors due to temperature
gradients between the two sensor elements.

However Mr. Christian says the biggest technology leap
comes from the linkage of sensor elements with the signal
amplifier unit, the ND converter, the calibration data memory,
and the digital bus interface - all on a surface area of a few
square millimetres. The integration provides improved signal
quality and insensitivity to external disturbances (EMC).

High stabiIity performance
Mr. Christian points out several advantages of the close
linking of the various parts of the sensors. For example,
the signal amplification near the sensor allows the polymer
layers to be optimised not for the signal strengths, but
rather for long-term stability which is significant for
numerous applications.

The ND conversion, which is also performed "in place,"
makes the signal extremely insensitive to noise. A
checksum generated by the chip itself is used for
additional reliability.

Each sensor is calibrated in a precision humidity chamber
and the calibration coefficients are programmed into the
onboard memory. These coefficients are used internally
during measurements to calibrate the signals from the
sensors. The calibration data loaded on the sensor chip
"guarantees that Sensirion humidity sensors have identical
specifications and thus they can be replaced 100%," says
Mr. Christian.

More advantages
Other advantages include very short response times (4 sec
at lie), high precision (±2% to ±5% according to configuration),
low power consumption (<3µA standby), and small footprint
(7x5x3 mm).

The sensor chip can be connected directly to any
microprocessor system by means of the digital 2-wire interface.

Mr. Christian says Sensirion will position its new humidity
sensor in the automation, control, and building HVAC markets.
It could be used as the basis for a high precision "smart"
transmitter or in data logging applications where small
space is a premium. He thinks there may also be possibilities
in automotive markets, where it could be placed on surface
mount air conditioning sensor modules.

_____________________________________________________

For more information, contact Stefan Christian
e-mail at
Stefan.Christian@sensirion.com

Sensirion, Eggbuhlstr. 14, 8052 ZUrich, Switzerland
Website:
www.sensirion.com


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