CMOS humidity sensors provide digital accuracy
Humidity sensors are gaining significance in diverse areas of measurement and control technology. Manufacturers have improved accuracy, durability, and long-term drift of their sensors. Component size and price have also been reduced. Following this trend, Sensirion AG (Zürich, Switzerland) has introduced a new generation of integrated, digital, and calibrated humidity and temperature sens...
Humidity sensors are gaining significance in diverse areas of measurement and control technology. Manufacturers have improved accuracy, durability, and long-term drift of their sensors. Component size and price have also been reduced.
Following this trend, Sensirion AG (Zürich, Switzerland) has introduced a new generation of integrated, digital, and calibrated humidity and temperature sensors using complementary metal-oxide semiconductor (CMOS) "micro-machined" chip technology. The SHT11 is a single-chip relative humidity and temperature multi-sensor module with acalibrated digital output.
Conventional sensors determine relative air humidity using capacitive measurement technology. The sensor element is built from a film capacitor on different substrates (glass, ceramic, etc.). The dielectric is a polymer that absorbs or releases water proportional to relative humidity, thus changing the capacitance of the capacitor, which is measured by an onboard electronic circuit.
Stefan Christian, product manager at Sensirion, says conventional technology has three primary weaknesses: poor long-term stability, complicated calibration requirements, and the limitations of using analog technology.
To overcome these weaknesses, SHT11 includes two calibrated microsensors for relative humidity and temperature, which are coupled to an amplification, analog-to-digital (A/D) 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.
Additionally, the temperature sensor and humidity sensor form a single unit, which enables precise determination of dewpoint without incurring errors due to temperature gradients between the two sensor elements.
SHT11's biggest technology advance is the linking of sensor elements with the signal amplifier unit, the A/D converter, calibration data memory, and digital bus interface—on a surface area of a few square millimeters. I Integration provides improved signal quality and insensitivity to external disturbances.
Close linking provides advantages. Signal amplification near the sensor allows the polymer layers to be optimized for long-term stability, rather than signal strengths. A/D conversion, also performed "in place," makes the signal extremely insensitive to noise.
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. Calibration data loaded on the sensor chip guarantees that these humidity sensors have identical specifications and can be replaced 100%.
Other advantages include short response time (4 sec at 1/e), high precision (HVAC markets.