Revisiting Humidity Sensor Design

Many design engineers have found through application trialand error that traditional integrated sensor solutions often exhibit pooraccuracy, unstable behavior around the measuring range limits andunsatisfactory chemical resistance against contaminants. Additional limitationscommonly encountered include lack of dew formation resistance, inadequatelong-term stability and failure during load spikes. There also tend to beissues with the digital I²C protocol and interface, as all pointsoften do not correspond to the standard. (I²C refers to theinter-integrated circuit, which is a multi-masterserial single-ended computerbus used to attach low-speed peripherals to a motherboardor embedded system.)

Toaddress these issues, our goal as a sensor developer was to create a miniaturepolymer sensor combinedwith a matching ASIC-optimized for a specific application-on a common carriersubstrate. This type of  connectiontechnique is economical when using standard technologies like printed thicklayer substrate and wire bonding, and then connecting to the target system viabrazed SIL contact strips or standard TO sockets.

Ahumidity sensor device created in this way offers the advantage of an optimumperformance for the user over an entire application range, even at the lowerrange limit at around 0 percent relative humidity as well as at extreme highhumidity and dew formation. This approach also results in lower manufacturingcosts, as the individual components and ASIC, as well as the sensor and carriersubstrate, can be tested before assembly. This translates into lower costs forend-users such as system designers.

ASICFunctionality

The ASIC carries out the entire signal processing ofthe physical parameters of humidity and temperature, which are made availablevia an I²C interface as digital values. Temperature measurementtakes place on-chip with an accuracy of better than 0.2C and a resolution of0.02 K.

The entire operating voltage range from 2.7 to 5.5 Vis supported. The average current consumption in the "standby" mode is around 1µA and at measuring rate of 1 Hz, it is approximately 22 µA, making the sensor suitablefor battery-operated applications.

The ASIC consists of the internal module cap digital converter (14bits), a polynomial signal processor, a coefficient memory for the calibratedvalues and the digital I²C-interface.

The humidity measurement is similarly accurate: The correction algorithmsimplemented in the ASIC use quadratic polynomials to ensure computationalcorrection of offset, gain and linearity behaviour, as well as temperaturedrift. Therefore, the application window is extremely wide from 0 to 100percent relative humidity in the temperature range of -40 to 125C with amaximum dew point of 80C.

Constructionand Dimensions

The ASIC is mounted together with the polymer sensor element on amechanically robust thick film ceramic carrier with dimensions of 5 x 5 mm. Dueto the small thermal mass, a fast response is achieved. Both the sensor and theASIC are bonded.

Up to the active layer of sensor element, the structure isenvironmentally resistant, protected with a glass-filled globe top. SMDcapacitors are integrated on the module, eliminating the need for additionaldecoupling capacitors. The solderable connection pins in the grid of 5.0 mmalso accommodate commercial connectors, making the humidity sensor an interchangeable,calibrated functional module.

All materials are optimized for minimum water absorption, thus leavingthe microclimate in the environment undisturbed. With these materials, theapplication temperature range of -40 to 125C is well covered.

At the I²C compatible interface, the processed physicaldimensions of relative humidity and temperature are transmitted as numericaldigital values. Therefore, it is not necessary to do any further settingthrough the application software. The digital interface corresponds to the I²Cstandard (up to 400 kHz clock rate) and can be used together with other I²Ccomponents connected at the same bus. In addition to the fixed allottedaddress, a second address can also be defined. Therefore, simultaneousoperation of up to 126 sensors is possible on the same I²C bus.

Martin Friedrich ismanaging director at Hygrosens. He has been actively involved in the area ofindustrial instrumentation for more than 20 years; [email protected].ServoFlo is the U.S. distributor for Hygrosens products, www.servoflo.com