Relative humidity integrated on chip

11/7/2012 7:55 PM EST

The Internet of Things is changing many aspects of our lives and the products we design are being driving by constraints other than transistor count and performance. Many of the necessary pieces, such as sensors, may be located in areas that are difficult to get to, may have to run on very small amounts of power and in some cases scavenge the power that they need to operate. Many of these requirements can only be met by increased levels of integration – not just of the digital pieces, but of analog, RF and sensors. Cost is another issue and one of the biggest cost factors is related to the number of discrete pieces that make up a system.

A recent news release from Silicon Laboratories caught my eye. This is not an endorsement of the product in any way, but I like seeing new types of integrations. It is for a digital relative humidity and temperature “sensor-on-a-chip” solution. This new sensor combines a mixed-signal chip manufactured using standard CMOS fabrication techniques together with a new technique for measuring humidity using a polymer dielectric film. Traditional approaches to relative humidity sensing use discrete resistive and capacitive sensors, hybrids and multi-chip modules (MCMs). These all add significantly to the total solution cost.

Measuring relative humidity is not an easy task. This is because it is highly dependent on temperature. A variation of 0.2C can cause a 1% error in relative humidity. Capacitive relative humidity sensor devices employ a polymer dielectric between the capacitor plates and measure relative humidity by detecting the change in dielectric-constant (Εr) and capacitance caused by moisture absorbed in the porous polymer dielectric layer. An Εr of 3.0 to 4.0 would be a typical variation of dielectric constant as RH varies from 0 to 100 percent.

In this new chip, temperature is sensed by a precision bandgap referenced circuit on the die, and located much closer to the humidity sensor than would be possible using a discrete solution. Humidity is sensed by measuring the capacitance change of an industry-standard low-k dielectric layer applied to the surface of the die. The sensor is packaged in a 4 mm x 4 mm QFN package with a small opening to expose the moisture-sensing polymer film.

So, how does this stack up to the old ways? Silicon Labs recons that a traditional sensor of this kind would consume 4-20mA while their new chip draws a scant 240 to 320 μA. That is within the bounds of the power that can be supplied by a microcontroller (MCU) port pin each time a conversion is required.

Brian Bailey – keeping you covered

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