Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor (CN0234)
The circuit shown in Figure 1 is a single-supply, low power battery operated, portable gas detector using an electrochemical sensor. The Alphasense CO-AX Carbon Monoxide sensor is used in the example.
Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of many toxic gases. Most sensors are gas specific and have usable resolutions under one part per million (ppm) of gas concentration. They operate with very small amounts of current, making them well-suited for portable, battery powered instruments.
The circuit shown in Figure 1 uses the ADA4505-2, dual micro-power amplifier, which has a maximum input bias current of 2 pA at room temperature and consumes only 10 μA per amplifier. In addition, the ADR291 precision, low noise, micropower reference consumes only 12 μA and establishes the 2.5 V common-mode pseudo-ground reference voltage.
Figure 1. Low Power Gas Detector Circuit
Total power consumption for the circuit shown in Figure 1 (excluding the AD7798 ADC) is approximately 110 μA under normal conditions (no gas detected) and 460 μA under worst- case conditions (2000 ppm CO detected). The AD7798 consumes approximately 180 μA when operational (G = 1, buffered mode) and only 1 μA in the power-save mode.
Because of the circuit’s extremely low power consumption, two AAA batteries can be a suitable power source. When connected to an ADC and a microcontroller, or a microcontroller with a built-in ADC, battery life can be from over six months to over one year.
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