Single-supply, micropower toxic gas detector circuit design using electrochemical sensor

　　Circuit Functionality and Benefits : The circuit shown in Figure 1 is a single-supply, low power, battery-powered, portable gas detector using an electrochemical sensor. The Alphasense CO-AX carbon monoxide sensor is used in this example. Electrochemical sensors offer several advantages for instruments that detect or measure concentrations of a variety of toxic gases. Most sensors are designed for specific gases, have usable resolutions less than one part per million (ppm) of the gas concentration, and require very little operating current, making them ideal for portable, battery-powered instruments. The circuit shown in Figure 1 uses the ADA4505-2 dual-channel micropower amplifier, which has a maximum input bias current of 2 pA at room temperature and consumes only 10 A per amplifier. In addition, the ADR291 precision, low noise, micropower voltage reference consumes only 12 A to establish a 2.5 V common-mode pseudo-ground reference voltage.

　　The ADP2503 high efficiency, buck/boost regulator supports single-supply operation from two AAA batteries and consumes only 38 μA in power save mode. The total power consumption of the circuit shown in Figure 1 (excluding the AD7798 ADC) is approximately 110 μA under normal conditions (no gas detected) and approximately 460 μA under worst-case conditions (2000 ppmCO detected). The AD7798 consumes approximately 180 μA when operating (G = 1, buffer mode) and only 1 μA in power save mode. Due to the extremely low power consumption of the circuit, two AAA batteries can provide a suitable power supply. When connected to an ADC and a microcontroller or a microcontroller with a built-in ADC, the battery life can range from more than 6 months to more than a year.

　　Circuit Description: Figure 2 shows a simplified schematic of the electrochemical sensor measurement circuit. The operating principle of an electrochemical sensor is to allow gas to diffuse through a membrane into the sensor and interact with the working electrode (WE). The sensor reference electrode (RE) provides feedback to maintain a constant potential at the WE pin by changing the voltage on the auxiliary counter electrode (CE). The direction of the current on the WE pin depends on whether the reaction occurring is oxidation or reduction. In the case of carbon monoxide, oxidation occurs; therefore, the current flows into the working electrode, which requires the auxiliary counter electrode to be at a negative voltage relative to the working electrode (typically 300 mV to 400 mV). The op amp driving the CE pin should have an output voltage range of ±1 V relative to VREF to provide sufficient margin for different types of sensors (Alphasense Application Note AAN-105-03, Designing Constant Potentiostatic Circuits, Alphasense Corporation).

　　The current flowing into the WE pin is less than 100 nA per ppm of gas concentration; therefore, converting this current to an output voltage requires a transconductance-transimpedance amplifier with very low input bias current. The ADA4505-2 op amp has CMOS inputs with a maximum input bias current of 2 pA at room temperature, making it a good fit for this application. The 2.5 V ADR291 establishes a pseudo-ground reference voltage for the circuit, thus allowing single-supply operation while consuming very low quiescent current.

　　Amplifier U2-A draws enough current from the CE pin to maintain a 0 V potential between the WE and RE pins of the sensor. The RE pin is connected to the inverting input of U2-A; therefore, no current flows in it. This means that current flows out of the WE pin, which varies linearly with gas concentration. Transimpedance amplifier U2-B converts the sensor current into a voltage that is proportional to the gas concentration. The sensor chosen for this circuit note is the Alphasense CO-AX carbon monoxide sensor. Table 1 shows the typical specifications associated with this common type of carbon monoxide sensor.