Compact Digital Humidity Sensor IC Extends Battery Life and Reduces Design Complexity 2012/11/09 Silicon Laboratories
Analog
In the emerging “Internet of Things” era, the traditional approach to humidity measurement using large-footprint legacy discrete circuits or bulky humidity sensing modules consuming 4-20 mA is no longer suitable to meet system designers’ demands for smaller, lighter and lower-power end products.
A compact, single-chip digital humidity sensor IC provides a more appropriate solution for meeting the requirements of applications, such as portable weather stations, nebulizers, data-loggers for goods and assets in transit, smart-phone/feature-phone accessories and remote environmental sensing nodes.
To better understand the benefits of using digital humidity sensors, let’s consider the following topics:
Background
Humidity is a measure of moisture in air or other gases. There are several ways to express humidity measurements:
If the moisture content exceeds the saturated vapor pressure, condensation occurs, and the moisture content is reduced to the saturated vapor pressure. Dew point (the temperature at or below which condensation or fog starts to form as a gas cools) is also used as a measure of the absolute moisture content of air.
Numerous techniques are available for measuring relative humidity, ranging from simple mechanical indicators using spring-loaded fabrics to highly complex and expensive analytical instruments, such as chilled-mirror optical hygrometers. In general, measuring humidity, whether it is relative humidity, dew point, absolute humidity or equivalent wet bulb temperature, is not an easy task.
According to the National Physical Laboratory (UK), humidity is a relatively difficult quantity to measure in practice and can be measured in an uncontrolled environment with an uncertainty of, at best, ±3 percent. Because RH is highly temperature-dependent, we need to know the precise temperature of the air to accurately establish its relative humidity. As little as 0.2 °C variation in temperature can cause a 1 percent error in RH.
Discrete resistive and capacitive relative humidity sensor devices have filled the gap between mechanical and optical RH sensing for many years. They are used in conjunction with discrete temperature sensors, such as thermistors and resistance temperature detectors (RTDs), to establish RH and dew point.
Resistive sensors use a polymer membrane that changes conductivity according to absorbed moisture.
Capacitive relative humidity sensor devices employ a polymer dielectric between the capacitor plates and measure RH 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. Figure 1 shows a typical discrete component sensor schematic.
Discrete solutions are smaller and easier to calibrate than mechanical systems, but they require many supporting components to linearize, calibrate and convert the RH values. This approach requires added board space, higher power and labor-intensive production line calibration of each unit before it is shipped to a customer. Furthermore, discrete sensor components cannot be reflow-soldered for high-volume assembly. These discrete sensors also suffer from poor accuracy, high lot-to-lot variation, significant hysteresis and severe sensor drift over temperature and time. This adds complexity to the production testing and calibration and often means that regular calibration is required in the end application throughout the product’s lifetime.
Single-Chip Digital Relative Humidity Sensor ICs
An emerging sensing solution combines relative humidity and temperature sensors directly on a single-chip CMOS IC with a digital I2C interface. Because both sensors are in close proximity on the same monolithic die and at the same temperature, the RH reading is always more accurate than that from a discrete solution.
A prime example of a single-chip sensing solution is Silicon Labs’ Si7005 digital relative humidity and temperature sensor. The Si7005 sensor measures humidity with a polymer film on the surface of the die, and temperature is measured by an on-chip diode band-gap circuit. The only external components required are a pair of bypass capacitors. Each Si7005 relative humidity sensor is factory calibrated, so customer calibration is unnecessary. The sensor is packaged in an industry-standard 4 mm x 4 mm QFN package with a small opening to expose the moisture-sensing polymer film. A version with a low-profile protective cover is also available as an ordering option. The cover provides added protection against soldering flux, dust, chemicals and other pollutants during the sensor’s lifetime, as well as added protection during reflow soldering. Table 1 summarizes the benefits of using a monolithic solution, such as the Si7005 humidity sensor, versus a legacy discrete approach.
Conversion from capacitance to RH and fine tuning of the RH accuracy is achieved by:
Further nonlinearity and temperature compensations are performed to achieve RH with typical ±3 percent accuracy. These nonlinear and temperature coefficients, independent of device and manufacturing lot, are provided by Silicon Labs.
Si7005 hardware, software and firmware optimizations, when combined with the protective filter/cover, provide the following benefits over traditional legacy discrete, hybrid and MCM solutions:
High integration – Measured humidity and temperature values are converted to a digital format by the on-chip signal conditioning circuitry and analog-to-digital converter (ADC). No external signal conditioning or conversion to digital is required to output a voltage or frequency. The bill of materials (BOM) consists of only two bypass capacitors versus the dozens of components that might be required to implement the same functionality with a discrete sensor. The Si7005 humidity sensor has a much smaller footprint and minimal height and weight compared to discrete sensors, modules or hybrids/MCMs. The result is lower total solution cost, less design effort, less space and weight, higher reliability and faster time to market.
Plug-and-play ease of use . Digital output and factory calibration eliminate the need to calibrate the Si7005 relative humidity sensor, making each sensor interchangeable. No software/firmware changes or recalibration is required to switch from one unit to another. The host handles the final linearization and temperature compensation, but the algorithm uses fixed values that do not vary from chip to chip. No time or labor is spent adjusting each unit on the production line, which makes production rework and field servicing more convenient.
Protective cover – The addition of an optional factory-installed protective cover, as shown in Figure 2, makes the Si7005 humidity and temperature sensor very robust and easy to use. This low-profile hydrophobic/oleophobic membrane protects the sensor before, during and after board assembly. It remains in place for the life of the product, protecting against liquids/condensation and particulates, such as dust. Measurement sensitivity is unaffected by the presence of the cover.
Powering the Si7005 Humidity Sensor from an MCU Port Pin for Low-Power Operation
How to power the sensor from a microcontroller port pin:
Si7005 power requirements at Vdd = 3.3 V using typical Si7005 data sheet Idd values:
• Device self-heating < 0.1 °C
The total energy consumed for the above temperature and humidity conversion under typical conditions is:
If a temperature and humidity conversion is performed once, twice or four times per minute, this energy of 96 μJoules equates to the following average currents:
Battery Selection for Power-Sensitive Portable Systems
Choosing the appropriate battery is an important aspect of portable humidity sensing system design. A battery's characteristics vary depending on internal chemistry, current drain and temperature. Batteries are classified into two broad categories:
Appendix 1 lists key battery features, and Appendix 2 details general battery types.
When operated at two conversions per minute, the Si7005 relative humidity sensor will consume 8.76 mA-hours (mAh) of battery life per year of operation, which is only a small fraction of the capacity of typical batteries. Depending on the type of battery chosen, the following additional considerations apply.
The Si7005 Vdd range is 2.1 to 3.6 V. However, for best conversion accuracy, Vdd should not go below 2.3 V. When using AA batteries, two batteries in series are recommended. The life of a 3000 mAh AA alkaline battery would be reduced to 2000 mAh if discharge is limited to 1.15 V, which is still very adequate.
While limited in capacity, coin cells are a good choice for powering up the Si7005 humidity and temperature sensor because the battery voltage does not go out of range for the sensor. For a coin cell battery with only 230 mAh of capacity, the Si7005 will consume a small portion of the battery life when a GPIO is used to power it down between conversions as previously described.
In the case of primary lithium and secondary batteries, the voltage of a fresh battery is too high for the Si7005 sensor and would require a series regulator.
Conclusion
The market demand for low-power portable humidity sensor devices is increasing, driven by new portable and mobile systems for the emerging “Internet of Things”. Single-chip digital humidity sensor ICs, such as Silicon Labs’ Si7005 device, are the ideal sensor choice for such systems. The Si7005 is a compact, high-performance relative humidity and temperature sensor that minimizes component count and BOM cost. On-chip calibration, a digital I2C host interface, an optional protective cover, and a choice of cost-effective evaluation boards and development kits enable developers to implement feature-rich portable humidity sensor systems quickly and easily. These systems are capable of achieving a minimum of 5 to 10 years product operation and battery life. The AA battery is a good choice for applications that are not space constrained. Where battery size is limited, coin cells are a good choice and provide years of service.
Appendix 1. Key Battery Features for Consideration
Voltage – It is important to know the nominal, minimum and maximum voltage of the battery. For example, a 1.5 V alkaline battery will discharge from 1.5 V to 0.9 V.
Silicon Labs invests in research and development to help our customers differentiate in the market with innovative low-power, small size, analog intensive mixed-signal solutions. Silicon Labs' extensive patent portfolio is a testament to our unique approach and world-class engineering team. Patent: www.silabs.com/patent-notice
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