Chemiresistive sensors offer a cost-effective solution for measuring various gas concentrations in industrial control, HVAC systems, and health and safety applications. These sensors rely on heating elements to operate effectively, requiring precise measurement of the sensor's resistance while maintaining the proper temperature. Developers must balance design complexity with measurement accuracy to meet these requirements.
This article explores the properties of chemiresistive sensors and their role in different applications. It introduces the Integrated Device Technology (IDT) chemical gas sensor device and focuses on the specific requirements for using these sensors, along with analog design alternatives that support their operation. Finally, it presents a general MCU-based design methodology and discusses relevant boards and software for evaluating and developing gas sensor designs.
Accurate gas sensing is becoming increasingly important in both professional and everyday applications. Methane detectors are crucial in mining operations, while hydrogen measurements can alert users to battery issues. Accurate gas sensors can act as "electronic noses" in medical applications. In residential and commercial buildings, monitoring various gas levels can warn users about toxic gases and provide fire alerts.
Among available gas sensors, chemiresistive metal oxide sensors provide a cost-effective solution that delivers reliable results even in demanding environments. Changes in airborne gas concentration cause changes in the sensor's resistance, which can vary by several orders of magnitude within the operating range. This relationship between sensor resistance (R_S) and gas concentration (C) is represented by a simple formula involving two constants, A and α.
Equation 1: R_S = A * C^α
Equation 2: log(R_S) = log(A) + α * log(C)
Equation 2 shows a linear relationship between the logarithm of gas concentration and the logarithm of sensor resistance. However, this linearity is only maintained at lower concentrations, with less sensitivity at higher concentrations. IDT's SGAS701 hydrogen sensor and other chemical sensors exhibit this log-log relationship, although support circuits can introduce nonlinearities.
IDT offers a range of chemiresistive sensors for accurate gas measurements, including:
- Hydrogen using the SGAS701 sensor
- Volatile organic compounds (VOCs) using the SGAS707 sensor
- Flammable gases using the SGAS711 sensor
These sensors integrate a resistive element that heats the sensor to the optimal measurement temperature. For developers, the challenge lies in accurately measuring the sensor's resistance while maintaining the heating element at the correct temperature.
Analog front-end implementation considerations are essential when working with resistive devices like chemiresistive sensors. Developers can measure sensor resistance using various methods, such as placing the sensor in a voltage divider, driving it with a constant voltage source, or using a constant current source. Each method has its own trade-offs in terms of design simplicity and measurement quality.
For example, a simple voltage divider configuration (Figure 2) provides the simplest solution but may not be suitable for applications requiring accurate gas concentration measurements due to inherent limitations. The output voltage in a voltage divider will never reach the power supply value, and the resistor network limits the output voltage based on the following formula:
V_OUT = V_Bias * R_S / (R_FIXED + R_S)
By selecting an appropriate R_FIXED value, developers can achieve a useful voltage range between the sensor's baseline value (in air) and its full-scale response. Table 3 illustrates how different R_FIXED values affect the output voltage for various gas concentrations.
Another limitation of the voltage divider approach is nonlinearity. As the gas concentration increases, the linear log-log relationship between sensor response and gas concentration is lost, leading to smaller step changes in response. This makes the voltage divider method more suitable for gas detection rather than accurate quantitative measurements.
To improve accuracy, designers can use a constant voltage or constant current source for sensor excitation, eliminating the effect of R_FIXED on linearity. Constant voltage excitation allows for a linear logarithmic response using a simple analog front end, while constant current excitation ensures a completely linear relationship between the logarithm of gas concentration and the logarithm of sensor response over the entire operating range.
Heater driver circuits are also essential for maintaining the optimal operating temperature of the sensor. IDT's sensors require specific temperatures for best performance, and developers must ensure the heater drive circuit adjusts its output to maintain consistent sensitivity.
For constant voltage sources, traditional linear regulators like the Texas Instruments LM317 can provide suitable solutions. However, these solutions are susceptible to inaccuracies caused by ambient temperature changes or component variations. Constant current heater circuits offer a more flexible solution, allowing for better control over heater power and temperature.
IDT's SMOD7xx demonstration kit provides a dual constant current circuit design for SGAS701, SGAS707, and SGAS711 sensors. The SMOD7xx board combines constant current circuits with the respective sensors, along with a TI MSP430I2021 MCU and support circuitry. A stand-alone SMOD application program allows developers to explore gas sensing applications immediately, providing real-time insights into sensor resistance changes and application responses.
In conclusion, the ability to measure different gas concentrations is becoming increasingly important across a wide range of applications. Low-cost chemiresistive sensors from companies like IDT offer off-the-shelf solutions, but careful circuit design is essential to meet the unique requirements of these devices. By using a variety of techniques, designers can create gas sensing designs that balance circuit complexity and measurement accuracy to suit their specific needs.
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