SENSORS & SENSING SYSTEMS u ANALOG DEVICES
Operational Amplifier for electrochemical sensors
Tom Au-Yeung, product application engineer, Analog Devices, discusses the applications of an operational amplifier for electrochemical gas sensors such as ethanol and carbon monoxide (CO). He also discusses the desired performance to provide optimum results for accurate measurement of ethanol and CO with the lowest power consumption for portable devices.
E
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 0.9V, which was specifically designed for instrumentation type applications. In addition, the device features rail-to- rail inputs and outputs and consumes only 25.5µA typical supply current and 1µV 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 1.5V 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 1.8V, 15MHz low offset, low power, rail-to-rail input/output (I/O) op amp. ES is the electrochemical sensor.
ETHANOL SENSOR EVALUATION In this ethanol sensor evaluation, the sensor used is the SPEC 3SP_Ethanol_1000 package 110-202 shown in Figure 2.
This SPEC ethanol sensor generates a current 16 February 2024 Irish Manufacturing 14
Figure 1: A MAX40108 block diagram of an electrochemical sensor
Figure 2: The ethanol sensor SPEC 3SP_Ethanol_1000 package 110-202
proportional to the volume of the captured gas. It is a three-electrode device: WE, RE, and CE. WE: Working electrode. This WE is biased at 0.7V and used for sensing the gas vapour. RE: Reference electrode. This RE provides a stable electrochemical potential of 0.6V bias voltage in the electrolyte, which is not exposed to the gas vapour. CE: Counter electrode (CE). The CE conducts when a gas is present. The level of conduction is proportional to the concentration of gas, which can then be electrically measured by the system. In this gas sensor evaluation, the gas particles need to be physically in contact with the SPEC sensor. In other words, the ethanol sensor is basically measuring only the gas that is present at the exact location of the sensor itself. Therefore, to detect gases such as ethanol and CO accurately and effectively, place the sensors where the concentration of gas is expected to diffuse to the location. In this experiment, a cotton swab was
dipped in an ethanol solution and placed right in front of the SPEC sensor. Figure 3 depicts the capture of the ethanol
vapour as shown in a blue curve. The green curve is the current consumption of the entire system including the microcontroller, which is 90mA typical. However, the current consumption of the
MAX40108 itself is a mere 25.5µA at VDD = 0.9V and TA = 25°C as shown in Figure 4. When in idle mode, the microcontroller
wakes up every 10 seconds to monitor the ethanol vapour. When the vapour is present, the microcontroller starts measuring the vapour concentration as shown in the blue curve. The red line shows the AA battery voltage at approximately 1.5V, and the yellow line is the CE voltage. To see the effect of the ethanol sensor’s
response to the vapour concentration, the cotton swab was moved farther away from the sensor. The result was captured as shown
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