“Gas detection instruments are widely used in various applications ranging from household air quality measurement equipment to industrial toxic gas detection solutions. The history of the application of electrochemical gas sensors can be traced back to the 1950s, when electrochemical sensors for oxygen monitoring were developed. One of the first applications of this technology is a glucose biosensor, which is used to measure glucose hypoxia. In the following decades, the technology has been developed, the sensor has become miniaturized and can detect a variety of target gases. Many of these instruments use electrochemical gas sensors. This sensor technology requires a dedicated front-end circuit for biasing and measurement.
Gas detection instruments are widely used in various applications ranging from household air quality measurement equipment to industrial toxic gas detection solutions. The history of the application of electrochemical gas sensors can be traced back to the 1950s, when electrochemical sensors for oxygen monitoring were developed. One of the first applications of this technology is a glucose biosensor, which is used to measure glucose hypoxia. In the following decades, the technology has been developed, the sensor has become miniaturized and can detect a variety of target gases. Many of these instruments use electrochemical gas sensors. This sensor technology requires a dedicated front-end circuit for biasing and measurement.
Using built-in diagnostic features (such as impedance spectrum or bias voltage pulses and slopes), you can check the health of the sensor, compensate for aging or temperature-induced accuracy drift, and estimate the remaining life of the sensor without user intervention. This feature allows each edge node to replace smart and accurate sensors. The integrated ultra-low power microcontroller directly biases the electrochemical gas sensor and runs the on-board diagnostic algorithm.
Figure 1: Typical electrochemical gas sensor signal chain
Basic knowledge of electrochemical gas sensors
The circuit shown in Figure 2 shows how the electrochemical gas sensor is connected to the potentiostat circuit, and how to bias and measure it. Common 2-lead, 3-lead and 4-lead electrochemical gas sensors can be used interchangeably. The integration of this signal chain significantly reduces the cost, size, complexity and power consumption of sensor nodes.
Figure 2: The schematic circuit diagram of the connection between the electrochemical gas sensor and the potentiostat.
The basic principle of electrochemical gas detection is that the target gas is oxidized or reduced at the electrode to generate a current, and the target gas can be detected by measuring this current. The most common sensors have two or three electrodes. Some sensors also have a fourth electrode. In the 3-electrode configuration, each electrode is called a working electrode (WE, also called a detection electrode (SE)), a reference electrode (RE), and a counter electrode (CE), respectively. The figure above is a simplified schematic diagram of this electrochemical cell.
The target gas enters the sensor chamber through the porous working electrode and diffuses into the electrolyte (most commonly acid), where it is oxidized or reduced. The current generated by this reaction is then detected by the external potentiostat circuit and converted to the corresponding voltage level. It is often necessary to apply a continuous or pulsed bias voltage to the sensor electrodes to ensure optimal performance. For a 3-electrode sensor, a bias voltage is applied between RE and WE. The reaction at CE is equal but opposite to that between RE and WE. If a reduction reaction occurs at WE, an oxidation reaction occurs at CE.
Figure 3: Electrochemical gas sensor-simplified diagram
Application of electrochemical gas sensor and calculation of related parameters
The data sheet of the gas sensor specifies the bias voltage required for normal electrochemical operation of the sensor. Bias voltage refers to the voltage difference between RE and SE/WE. This differential voltage is set by the output of the low-power digital-to-analog converter (LPDACx). LPDACx has two outputs: a 12-bit resolution output (VBIASx) and a 6-bit resolution output (VZEROx). The VBIASx output of LPDACx is internally connected to the non-inverting terminal of the power amplifier (PA). Externally, VBIASx must be connected to the AGND pin through a 100 nF capacitor. The output of the PA amplifier is directly connected to the CE of the sensor. The feedback to the inverting terminal of the PA amplifier comes from the RE pin of the sensor; therefore, the VBIASx voltage determines the RE pin voltage.
The VZEROx output of LPDACx is internally connected to the non-inverting terminal of the low-power transimpedance amplifier LPTIAx. Do not use this pin as a voltage source for external circuits. The electrochemical gas sensor itself is only connected to the ADuCM355 through the REx, CEx and SEx terminals, and the optional fourth terminal can be used for the diagnostic electrode (DEx), as shown in Figure 2.
Use the following formula to obtain the effective bias voltage of the sensor:
VBIAS_EFF = VVBIAS C VVZERO
It is recommended to set the VZERO voltage to 1100 mV, and then set the VBIAS voltage according to the sensor bias voltage value in the sensor data sheet.
Depending on the sensor type, the bias voltage may also be negative. The following formula explains how to configure the positive and negative bias voltage of the DAC.
When the required bias voltage is positive (12-bit output ≥ 6-bit output),
VVBIAS = 0.2 V + (LPDACDAT[11:0] × 0.54 mV) +0.54 mV
VVZERO = 0.2 V + (LPDACDAT[17:12] × 34.38 mV)
When the required bias voltage is negative (12-bit output VVBIAS = 0.2 V + (LPDACDAT[11:0] × 0.54 mV)
VVZERO = 0.2 V + (LPDACDAT[17:12] × 34.38 mV)
LPDACDAT is the data output control register of the low-power DAC.
0.54 mV is approximately 1 LSB of a 12-bit DAC.
34.38 mV is approximately 1 LSB of a 6-bit DAC.
The sensor’s detection/working electrode (WE) is connected to the LPTIAx through the inverting input pin SEx. LPTIAx has programmable load resistance (RLOAD) and programmable gain resistance (RTIA). The current flowing into/out of the SE electrode of the sensor reflects the target gas in the atmosphere around the sensor. The sensor data sheet uses “current/ppm” to express this amount. The LPTIAx amplifier converts the current into a voltage, which is then buffered and measured by an analog-to-digital converter (ADC). Choose the RTIA resistor value to maximize the ADC input range of ±900 mV. The RTIA value is calculated using the following formula:
0.9 V is the ADC input range.
Sensitivity is defined as nA/ppm.
Max_Range is the maximum range of the sensor, in ppm.
The microcontroller can calculate the current flowing into/out of the SEx pin and determine the ppm level of the target gas.
Single-chip electrochemical measurement system based on ADuCM355
ADuCM355 is an on-chip system that can control and measure electrochemical sensors and biosensors. The device is an ultra-low-power mixed-signal microcontroller based on the Arm® Cortex™-M3 processor with current, voltage and impedance Measurement function. It is very suitable for the design of electrochemical gas detection system, as well as food quality, life science and biological sensing analysis.
Figure 4: Simplified functional block diagram of ADuCM355
ADuCM355 provides a means to overcome the challenges of electrochemical gas detection technology. The two measurement channels not only support the most common 3-electrode gas sensors, but also support 4-electrode sensor configurations. The fourth electrode can be used for diagnostic purposes or as a working electrode for the second target gas in a dual gas sensor. Any potentiostat can also be configured to sleep mode to reduce power consumption while maintaining the sensor bias voltage, thereby reducing the stabilization time that the sensor may require before normal operation. The analog hardware accelerator module supports sensor diagnostic measurements, such as electrochemical impedance spectroscopy and chronoamperometry. The integrated microcontroller can be used to run compensation algorithms, store calibration parameters, and run user applications. ADuCM355 also considers EMC requirements when designing, and is pre-tested to comply with the EN 50270 standard.
If the application does not require an integrated microcontroller, you can use the front-end version-AD5940
Sensor health diagnosis and life expectancy
Different manufacturers and electrochemical gas sensors for different target gases have different lifetimes. Information about life expectancy can be found in the sensor manufacturer’s data sheet. However, the actual life span strongly depends on storage and working conditions. The longevity of electrochemical gas sensors and the need for regular calibration are the most challenging aspects of this type of sensor. Therefore, people hope to be able to monitor the health of the sensors directly in the instrument.
The ADuCM355 has a built-in waveform generator and a discrete Fourier transform (DFT) module. The impedance spectrum measurement can be achieved by applying an AC signal sweep to the counter electrode. This measurement can show the quality of the charge transfer between the electrodes, thereby effectively detecting the aging of the sensor electrolyte. Laboratory tests show that there is a good correlation between the impedance and sensitivity of the sensor.
Other methods of detecting the health of sensors include pulse testing and ramp testing. These tests apply a voltage pulse or ramp above the bias voltage to test the sensor responsivity and charge transfer, respectively.
All these measurement results combined with the algorithm running on the ADuCM355 help to improve the accuracy, performance and life of the electrochemical gas sensor. In order to achieve this level of intelligent diagnosis and prediction, it is necessary to obtain the characteristics of a large number of sensors through tests (such as accelerated aging).
Arduino wireless development platform based on ultra-low power ARM Cortex-M3 processor
External temperature and humidity sensors are provided on the sensor board. It is connected to ADuCM355 through the I2C interface. The performance of most electrochemical sensors changes with temperature and humidity, so these effects need to be compensated. In addition, it is worth mentioning that the circuit uses 3-lead electrochemical gas sensors (CE, RE, WE) for testing. However, it can also support 4-lead (CE, RE, WE1, WE2) and 2-lead sensors (CE and WE). The four-lead sensor has a variety of electrode configurations. The fourth electrode can be used as an additional diagnostic electrode (DE). Some sensors can detect two gases. In this case, the fourth electrode is configured as a working electrode (such as a CO and H2S combined sensor).