Oil, gas & renewables


OPERATIONAL AMPLIFIER FOR ELECTROCHEMICAL SENSORS


This article from Tom Au-Yeung, product application engineer at Analog Devices, discusses the applications of an operational amplifier for electrochemical gas sensors such as ethanol and carbon monoxide (CO). It also discusses the desired performance to provide optimum results for accurate measurement of ethanol and CO with the lowest power consumption for portable devices.


Figure 1. A MAX40108 block diagram of an electrochemical sensor. E 50


lectrochemical gas sensing elements require constant bias to operate correctly and accurately, which potentially consumes a tremendous amount of power. Normal power management systems attempt to


keep everything shutdown when in idle or sleep mode. However, electrochemical sensors require tens of minutes or even hours to stabilise. Hence, the sensing element and its bias circuitry must constantly be in the always- on power state. Furthermore, the required bias voltage is often quite low for connection to a 1 AA battery cell for consumer applications. The MAX40108 is a low power, high precision operational amplifier (op amp) that operates with a power supply voltage as low as 0.9 V, which was specifically designed for instrumentation type applications. In addition, the device features rail-to-rail inputs and outputs and consumes only 25.5 µA typical supply current and 1 µV typical zero-drift input offset voltage over time and temperature, making it an ideal device for a wide variety of low power applications for consumer products such as ethanol and CO gas sensors.


OVERVIEW Figure 1 shows the block diagram of an electrochemical sensor such as ethanol or CO. The system consists of a low voltage op amp that operates directly from a 1.5 V AA/AAA battery, providing bias current to the electrochemical sensor while the rest of the system is in sleep mode to save power consumption. The first op amp, U1, powers the


electrochemical cell’s reference electrode. The second op amp, U2, is configured as a transconductance amplifier, converting the sensor’s current output to voltage output to be digitised by a microcontroller after being amplified. This is done by the MAX44260, U3, which is a 1.8 V, 15 MHz low offset, low power, rail-to-rail input/output (I/O) op amp. ES is the electrochemical sensor.


Figure 2. The ethanol sensor SPEC 3SP_Ethanol_1000 package 110-202. October 2023 Instrumentation Monthly


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