“Precision Measurement of Ethanol and CO: Low-Power, High-Precision Operational Amplifiers Are a Must”
MAX40108, 25.5A low current consumption, small size features
Suitable for wearable, portable medical devices, and industrial IoT use
This article describes an operational amplifier for an electrochemical gas sensor for detecting gases such as ethanol or carbon monoxide (CO). It also explains how to accurately measure gases such as ethanol and CO while consuming as little power as possible in a portable device.
■ Electrochemical sensor must be left on to stabilize
Electrochemical gas sensing devices require continuous bias to operate accurately, which can lead to significant power consumption.
A typical power management system shuts down everything when the device is not being used or is in sleep mode.
However, since electrochemical sensors can require tens of minutes or even hours to stabilize, it is better to keep the sensing element and its bias circuit on continuously.
Additionally, in consumer applications the bias voltage required to connect to a 1 AA battery cell is usually very low.
The MAX40108 is a low-power, high-precision operational amplifier that operates from supplies as low as 0.9V and is designed for instrumentation applications.
The device features rail-to-rail inputs and outputs, a rated supply current consumption of only 25.5 A, and a rated zero-drift input offset voltage of 1 V over time and temperature.
Therefore, it is suitable for use in various types of low-power applications, including consumer products, such as ethanol and carbon monoxide (CO) gas sensor detection.
■ System Overview
Figure 1 shows a block diagram of an electrochemical sensor for detecting gases such as ethanol or CO.
The system uses a low-voltage operational amplifier that operates on a 1.5 V AA/AAA battery to provide bias current to the electrochemical sensor, while the rest of the system enters sleep mode to conserve power.
The first operational amplifier, U1, drives the reference electrode of the electrochemical cell.
The second operational amplifier, U2, is configured as a transconductance amplifier to convert the sensor's current output into a voltage output, which is then amplified and digitized by the microcontroller.
The microcontroller used is the MAX44260, which is U3. The MAX44260 is a 1.8V, 15MHz low-power rail-to-rail input/output (I/O) operational amplifier with low offset. ES stands for electrochemical sensor.
▲Figure 1. Block diagram of an electrochemical sensor using the MAX40108
You can see the circuit diagram of this electrochemical sensor by clicking the link below.
■ Ethanol sensor evaluation
The sensor used for ethanol sensor evaluation is SPEC 3SP_Ethanol_1000 Package 110-202 (Figure 2).
▲Figure 2. Ethanol sensor SPEC 3SP_Ethanol_1000 Package 110-202
This SPEC ethanol sensor generates a current proportional to the amount of gas captured. This sensor is a three-electrode (WE, RE, CE) device.
- WE(working electrode): WE is biased at 0.7 V and serves to detect gas.
- RE(reference electrode): RE provides a stable electrochemical potential of 0.6 V bias voltage to the electrolyte and is not exposed to gas.
- CE(counter electrode): CE conducts when gas is present. The level of conduction is proportional to the concentration of the gas, which the system measures electrically.
This gas sensor evaluation requires that the gas particles come into physical contact with the SPEC sensor. In other words, this ethanol sensor only measures gases present at the location where the sensor is placed.
Therefore, in order to accurately and effectively detect gases such as ethanol or CO, the sensor must be placed in a place where the gas concentration is diffused. For this test, a cotton swab was dipped into an ethanol solution, removed, and placed directly in front of the SPEC sensor.
Figure 3 shows the captured ethanol gas, indicated by the blue curve. The green line is the current consumption of the entire system including the microcontroller, rated at 90mA.
However, the current consumption of the MAX40108 itself is only 25.5 A at VDD = 0.9 V and TA = 25 C (Figure 4).
▲Figure 3. Performance of ethanol sensor
▲Figure 4. Current consumption across operating temperature range at various power supply voltages.
When in idle mode, the microcontroller wakes up every 10 seconds to monitor ethanol gas. If the gas is present, the microcontroller measures the gas concentration. This is shown in the blue curve in the figure. The red line is the AA battery voltage, which is about 1.5 V, and the yellow line is the CE voltage.
To see how this ethanol sensor reacts to gas concentrations, I moved the swab away from the sensor.
Figure 5 shows the results measured in this way. As expected, we can see that the blue curve representing the gas concentration is decreasing.
▲Figure 5. Performance of the ethanol sensor after moving the cotton swab soaked in gas away from the SPEC sensor.
■ Carbon monoxide sensor evaluation
Unlike ethanol, CO is a gas that can be harmful to humans and is produced when gasoline or even candles burn incompletely.
When conducting CO gas tests, proper ventilation is required for health and safety reasons. For this evaluation, CO gas was generated using a candle in a concave vessel and the CO gas concentration was captured using SPEC 3SP_ Ethanol_1000 Package 110-202, the same as in the previous test.
Figure 6 shows the capture of CO gas. The blue line is the CO gas concentration, and the green line is the current consumption of the entire system including the microcontroller, which is rated at 90 mA.
As with the ethanol test, the microcontroller wakes up every 10 seconds when in idle mode to monitor CO gas. When this gas is detected, the microcontroller measures the gas concentration.
This is the part indicated by the blue curve in the picture. The red line is the AA battery voltage, about 1.5V, and the yellow line is the CE voltage.
▲Figure 6. Performance of CO sensor adopting MAX40108
■ Conclusion
Accurately measuring gases such as ethanol or CO in consumer and industrial applications requires low-power, high-precision operational amplifiers that operate from supply voltages as low as 0.9 V.
The MAX40108 is designed to effectively capture and measure common gases such as ethanol and CO, featuring low current consumption of only 25.5A and an extremely small 8-ball WLP package measuring 1.22mm x 0.92mm.
Additionally, this amplifier product can further reduce power consumption by using shutdown mode, making it suitable for use in wearable devices, portable medical devices, and Industrial Internet of Things (IIoT) such as pressure, flow, level, temperature, and proximity sensing.
※ About the author
Tom Au-Yeung has been with Analog Devices for over 20 years. He has extensive experience in RF/wireless and analog technologies including mixers, amplifiers, power amplifiers, voltage controlled oscillators, ADCs, and DACs. He received his BSEE from California Polytechnic State University - San Luis Obispo and his Masters in Electrical Engineering from Santa Clara University.
