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.

For instruments that detect or measure concentrations of multiple toxic gases, electrochemical sensors offer several advantages. Most sensors are designed for specific gases, with available 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 μA per amplifier. Additionally, the ADR291 precision, low-noise, micropower reference consumes only 12 μA and establishes a 2.5 V common-mode pseudo-ground reference.

The ADP2503 high-efficiency, buck/boost regulator operates from a single supply of two AAA batteries and consumes only 38 μA in power-saving mode.

The total power consumption of the circuit shown in Figure 1 (not including the AD7798 ADC) is approximately 110 μA under normal conditions (no gas detected) and approximately 460 μA under worst-case conditions (2000 ppm CO detected). The AD7798 operates with approximately 180 μA (G = 1, buffered mode) and only 1 μA in power-saving mode.

Due to the extremely low power consumption of the circuit, two AAA batteries can provide suitable power. When connected to an ADC and a microcontroller or a microcontroller with a built-in ADC, battery life can range from 6+ months to over a year.

Figure 2 shows the schematic diagram of the electrochemical sensor measurement circuit. Electrochemical sensors work by allowing 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 varying the voltage on the counter electrode (CE). The direction of current flow on the WE pin depends on whether the reaction occurring is oxidation or reduction. What occurs in the case of carbon monoxide is oxidation; therefore, current flows into the working electrode, which requires the 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 V REF to provide sufficient headroom for different types of sensors (Alphasense Application Note AAN-105-03, Designing 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 transimpedance amplifier with very low input bias current. The ADA4505-2 op amp has a CMOS input with a maximum input bias current of 2 pA at room temperature, making it ideal for this application.

The 2.5 V ADR291 establishes a pseudo-ground reference voltage for the circuit, thereby supporting single-supply operation while consuming very low quiescent current.

Amplifier U2-A draws enough current from the CE pin to maintain 0 V between the sensor's WE and RE pins. The RE pin is connected to the inverting input of U2-A; therefore no current flows in it. This means that the current flowing out of the WE pin varies linearly with gas concentration. Transimpedance amplifier U2-B converts the sensor current into a voltage proportional to the gas concentration.

The sensor chosen for this circuit note is the Alphasense CO-AX carbon monoxide sensor. Table 1 shows typical specifications associated with this common type of carbon monoxide sensor.

WARNING: Carbon monoxide is a poisonous gas that is dangerous at concentrations above 250 ppm; extreme caution should be used when testing this circuit.

The output voltage of the transimpedance amplifier is:

where I WE is the current flowing into the WE pin and R F is the transimpedance feedback resistor (shown as R8 in Figure 1).

The maximum response of the CO-AX sensor is 100 nA/ppm and its maximum input range is 2000 ppm carbon monoxide. Therefore, the maximum output current is 200 μA and the maximum output voltage is determined by the transimpedance resistance, as shown in Equation 2.

Powering the circuit with a 5 V supply creates a usable range of 2.5 V at the output of transimpedance amplifier U2-B. Selecting an 11.5 kΩ resistor for the transimpedance feedback resistor provides a maximum output voltage of 4.8 V, providing approximately 8% overrange capability.

Equation 3 shows the circuit output voltage as a function of ppm of carbon monoxide when the sensor uses a typical response of 65 nA/ppm.

Resistor R4 keeps the noise gain to a reasonable level. Choosing the value of this resistor is a trade-off between two factors: the magnitude of the noise gain and the settling time error of the sensor when exposed to high concentrations of gas. For this example, R4 = 33 Ω, which calculates the noise gain to be equal to 349, as shown in Equation 4.

The input noise of a transimpedance amplifier appears at the output as amplified by the noise gain. For this circuit, we are only concerned with low-frequency noise because the sensor operates at very low frequencies. The 0.1 Hz to 10 Hz input voltage noise of the ADA4505-2 is 2.95 μV pp; therefore, the noise at the output is 1.03 mV pp, as shown in Equation 5.

Since this is extremely low frequency 1/f noise, it is difficult to filter out. However, the sensor response is also extremely low; therefore this can be exploited by using a very low frequency low-pass filter (R5 and C6) with a cutoff frequency of 0.16 Hz. Even such a low frequency filter has a negligible impact on the sensor response time compared to the 30 seconds sensor response time.

An important characteristic of electrochemical sensors is their extremely long time constants. When first powered on, it may take several minutes for the output to settle to its final value. When exposed to a mid-scale step in target gas concentration, the time required for the sensor output to reach 90% of its final value can be between 25 and 40 seconds. If the voltage between the RE and WE pins changes drastically, it may take several minutes for the sensor output current to establish its final value. This also applies to the case where the sensor is powered periodically. To avoid long start-up times, P-channel JFET Q1 shorts the RE pin to the WE pin when the supply voltage drops below the JFET's gate-source threshold voltage (approximately 2.5 V).

Power this circuit with two AAA batteries or a 2.3 V to 5.5 V supply. Q2 provides reverse voltage protection, and the ADP2503 regulates the input supply to the 5 V required to power the sensor.