1. application circuit of electrochemical sensor
Electrochemical sensor is one of the most commonly used sensors in the industrial field. At present, it has been widely used in gas detection, water quality detection, biological analysis, food inspection and other products. The sensor uses the principle of converting chemical reaction into current and voltage, and can generate electrical signals proportional to the content of the measured substances. The electrochemical sensor is generally composed of electrodes, reactants, etc. it is a kind of consumable sensor. Therefore, in order to maximize its service life, the chemical reaction between its internal substances and the detected objects is very small and slow. Therefore, the electrical signal output by the electrochemical sensor is very weak. In practical applications, engineers need to design precise and stable circuits to drive, condition and collect electrochemical sensors.
In order to make the electrochemical sensor work, the circuit system that needs to be built is called the constant potential circuit. Taking the three terminal electrochemical gas sensor as an example, as shown in Figure 1, the circuits to be built include bias voltage source, potential holding, current to voltage, filtering, analog-to-digital conversion, etc., which are sent to MCU for data processing. The signal before the ADC circuit is the voltage signal converted by the TIA circuit, which changes in proportion to the gas concentration. According to the oxidation or reduction reaction of the sensor, the voltage converted by the TIA circuit may change forward or reverse. The components of the whole circuit include precision operational amplifier, precision resistor, high-performance ADC, etc. the circuit design of each part has its theoretical support, which requires engineers to have a high reserve of analog signal circuit knowledge.
Figure 1 conditioning circuit of three terminal electrochemical gas sensor
More importantly, the electrochemical sensor is sensitive to temperature and humidity, has poor response consistency, and decays with the change of service life, which leads to a high threshold for the use of the electrochemical sensor and a great difficulty in production and commissioning. This article will introduce a module product scheme with a very advanced technology in the electrochemical application industry. It is independently developed by Junlong technology and can be compatible with almost any field and all types of electrochemical sensors. It is very easy to integrate and develop, so that electrochemical sensor users from all walks of life can quickly realize the construction and production of their own products.
2. module level electrochemical circuit scheme
The product model of the electrochemical module scheme is mcum355, which is developed based on the platform chip aducm355 of ADI company. It integrates high-performance analog circuit, temperature and humidity sensor and processor, and reserves rich peripheral interfaces. It can be compatible with gas detection, water quality detection, biological impedance analysis, blood glucose measurement, food analysis and other applications. As shown in Figure 2, it is the physical drawing of mcum355. The module is in the form of a 2*2cm stamp hole module. It interacts with data through the serial port, which is easy to integrate and use. The module uses 3.3V power supply, the conventional working current is 5mA, and the current consumption is only a few UA under the condition of low power consumption or sleep.
Figure 2 physical drawing of mcum355 module product
3. principle and structure of the module
As shown in Figure 3, the board of mcum355 contains one aducm355 chip, one ltc6078 high resistance operational amplifier, one sht31 temperature and humidity sensor, and some necessary peripheral passive devices. Among them, aducm355 has very comprehensive internal functional resources. It includes analog circuits, including 2-channel low-power potentiostatic circuit, 1-channel high bandwidth potentiostatic circuit, high-performance ADC circuit, rich switch switching matrix and power management. In addition, it is also a Cortex-M3 core processor with 26MHz operating frequency. Therefore, it has all necessary resources for low-power MCU, such as various computing resources Various data interfaces, etc.
Figure 3 internal principle structure of mcum355
The ltc6078 on the board is a high resistance two-way operational amplifier of ADI company. One circuit is used as the interface circuit of pH electrode, and the voltage signal is directly sent to the ADC function channel in aducm355. The other circuit is used as the interface circuit of conductivity electrode to convert weak current into voltage, and then sent to the ADC channel. For the application of water quality detection, please refer to the following text for the specific connection method of the sensor.
Sht31 on the board is a precision temperature and humidity sensor. In the gas detection scene, temperature and humidity can be used as the basis for environmental index expansion and result correction. In addition, several precision resistors are set on the board for the internal analog circuit correction of aducm355 and as the reference resistors for the water quality measurement process.
The on-board core chip aducm355 contains two low-power potentiostatic circuits, which are the core supporting functional circuits of electrochemical sensors and can be used for all electrochemical gas sensors, blood glucose and food detection electrodes, etc. In addition, it also includes a high bandwidth potentiostatic circuit, which is generally used for the measurement of electrochemical impedance spectroscopy (EIS). EIS measurement has mature applications in battery analysis, biological characteristics analysis, etc. in addition to being used to assist in the analysis of measured substances, the most important thing is to predict the sensor life. The EIS measurement function is realized through this module product, without building a complex circuit structure. Other functional modules of aducm355 are functional circuits for acquisition and control. The internal functions of the chip are very rich. The specific functions are to switch the connection mode of analog circuits through the internal powerful switch matrix, so as to realize a variety of circuit functions.
In order to save the number of interfaces, this module product only opens a part of the necessary interface resources of aducm355, as shown in Figure 4, which is the external available interface resources of mcum355 product. If you are interested in the chip resources, you can download the information from the product page of aducm355 on ADI’s official website for detailed research. The module can be directly connected to gas sensors, water quality sensors, etc. the design of interface resources fully considers the characteristics of sensor types and their connection modes.
Figure 4 available external interfaces of mcum355
4. for gas detection
When a multi-functional electrochemical module is used to build a gas detection application, a three terminal electrochemical sensor is generally used, and its pins can be directly connected to the corresponding interfaces of the module. Mcum355 has two groups of exactly the same electrochemical channels, which can be connected to two three terminal electrochemical gas sensors, as shown in figure 5. Of course, this module also supports two terminal gas sensors, such as oxygen sensors. Just connect its current sensing electrode to the we pin, that is, only the current voltage conversion function circuit inside the module is used. In addition, another commonly used gas sensor is the PID or MOS principle, which outputs a single terminal voltage signal. At this time, it is only necessary to connect the output pin of the sensor to the AIN terminal of the mcum355 module.
Figure 5 connection scheme for gas detection application
Figure 6 shows the physical diagram of a functional demonstration kit for gas detection application, which is designed and developed by Junlong technology. The kit uses two electrochemical gas sensors and one PID sensor to measure three gas concentration indicators: oxygen, CO and TVOC, which is the maximum number of gas sensors that a single mcum355 module can support. The display screen of the Demonstration Kit is driven and controlled by a separate MCU, and interacts with the mcum355 module through serial port commands. The kit displays air temperature and humidity data from the sht31 sensor on the mcum355 board. The gas detection kit is powered by 5V USB, and only one LDO with 3.3V output is required to meet all power supply requirements.
Figure 6 Gas Detection Demonstration Kit
5. applied to water quality detection
When using mcum355 module products to build water quality detection applications, it is mainly based on a single module to measure water temperature, pH value, ORP value and conductivity. The sensor connection scheme is shown in Figure 7. The water temperature sensor is connected to electrochemical channel 1. The principle of the water temperature sensor is thermal resistance. This method uses the principle of indirect resistance measurement by its constant potential circuit. The pH electrode is connected to a special channel on the module, mainly because of its high resistance output characteristics. At the same time, the pH electrode can also reflect the ORP value of the measured liquid. For the conductivity electrode, the anode terminal is connected to the electrochemical 0 channel of the module, and the cathode terminal can be connected to the electrochemical 0 channel or a special high resistance channel, depending on the impedance range of the measured liquid. If it is a low resistance liquid (with strong conductivity), it can be connected to the we0 pin of the electrochemical 0 channel. If it is a high resistance liquid (with weak conductivity), it needs to be connected to the high resistance interface as shown in Figure 7.
Figure 8 is a demonstration kit for water quality application. Its structural principle is similar to that of the gas kit. It is designed and developed by Junlong technology. The types of sensors that water quality applications can support are not limited to the schemes mentioned in this paper. Any electrochemical water quality electrode sensor can be compatible, such as dissolved oxygen sensor.
Figure 7 connection scheme for water quality detection application
Figure 8 water quality inspection Demonstration Kit
6. AC impedance measurement and analysis
AC impedance measurement is also electrochemical impedance spectroscopy (EIS) measurement. It applies a small amplitude sine wave signal to the electrochemical sensor, and then measures its current response to obtain the impedance value. Because it is AC impedance, the measured result value has the property of phase angle, that is, it contains real part and imaginary part. In the actual measurement process, signals of different frequencies will be applied to the sensor to obtain a set of impedance data, that is, to form an impedance spectrum. The impedance spectrum is drawn into a coordinate curve, which can be used to analyze the working condition of the sensor electrode. Generally, with the aging of the sensor electrode, the impedance curve will have obvious deviation, as shown in Figure 9. In practical applications, EIS is of practical significance. For example, if the gas sensor is analyzed by EIS, the remaining working life of the sensor can be estimated. If the water quality electrode is analyzed by EIS, the electrode surface can be known whether it is polluted or corroded.
Figure 9 EIS measurement results and curve
In fact, EIS has been widely used in battery characteristic analysis, corrosion detection and other directions. Compared with previous electrochemical electrode analysis methods such as chronoamperometry and cyclic voltammetry, EIS results will obviously have more abundant information because of their frequency components, which makes EIS measurement have a very broad application and development space. The mcum355 module can conveniently realize the EIS measurement function. When the external sensor does not move, the switch matrix circuit inside the module will connect the sensor to the high bandwidth constant potential circuit, automatically run the EIS measurement process according to the built-in control program, and then directly output the impedance spectrum results, as shown in Figure 9. The EIS measurement of mcum355 can support the excitation signal frequency up to 200kHz.
7. easy to use development interaction mode
Mcum355 module reserves a four wire SWD interface, and users can directly edit the debugging control code. At the same time, the default firmware program of mcum355 supports the interaction of serial port instructions. Engineers only need to send instructions to the module according to the protocol rules, and they will get feedback information or measurement results.
After power on, the mcum355 module sends information to the user through the serial port. Taking the gas detection application as an example, the module first sends the basic configuration information of the two electrochemical sensor channels, and then sends the result of converting the sensor current measured by the two channels into voltage. By default, the sensor data is updated every second, as shown in Figure 10. The user can use the voltage data to convert to the gas concentration value. It should be noted that during the conversion process, users also need to use software algorithms such as calibration and temperature compensation correction to further improve the accuracy of the final measured concentration value. This is because the electrochemical sensor is very sensitive to ambient temperature and humidity, and its output characteristics are not ideal linear.
Figure 10 working data of module in gas detection application
First, the user can configure the five main parameters of the module potentiostatic circuit by inputting the serial port command. As shown in Figure 11, the functional structure model of a single potentiostatic circuit is shown. The configurable parameters include Vbias, vzero, rload, rtia and rfilter. Vbias and vzero are the voltages generated through the DAC functional circuit, which determine the voltage offset and measurement baseline of the electrochemical sensor connected to the circuit, Rload load resistance is related to the characteristics of the sensor itself. Rtia determines the magnification of the current to voltage circuit, while rfilter can change the response speed of the signal. The three resistors here are essentially accurate digital potentiometers.
Figure 11 configurable constant potential circuit parameters
The configuration parameters are sent to the mcum355 module through a group of hexadecimal codes, as shown in Figure 12. It is an example of the protocol and its specific description. The main body contains the set values of the above five parameters. As mentioned above, the mcum355 module has two sets of the same low-power potentiostatic circuits, so the instruction also contains the serial number of the target channel. In the protocol, the different codes of the five parameters represent the corresponding configuration values respectively. For specific configuration instructions, please refer to the user guide document of the mcum355 module product.
Figure 12 example and description of configuration parameter instruction code
After the user sends the configuration parameter command, mcum355 will feed back the newly updated configuration information, pause the operation, and prompt the user to reset the module, as shown in Figure 13. At this time, the configuration information has been stored in the built-in flash space of the module. The module needs to re run the program code, then read the latest configuration information in the flash, control the internal circuit to reach new parameters, and then start running the measurement program.
Figure 13 feedback after sending configuration parameter command to the module
When the mcum355 module is running in the measurement mode, the user can send the mode conversion command to it at any time. As shown in Figure 14, the user sends the command to convert the module to the EIS measurement mode. At this time, the module will immediately start the EIS measurement process of two electrochemical channels. In the default program firmware, the frequency points of EIS measurement are distributed from 100Hz to 200kHz, and the whole process of single channel measurement takes about 15s. In the EIS measurement mode, the module will continuously alternate the measurement process of the two channel sensors until the user sends a command to the module to return to the normal measurement mode. In general, it is recommended that the user only make one round of EIS measurement for the electrochemical sensor in a short time. Since the measurement process uses a small amplitude sine wave signal to disturb the sensor, two opposite processes of oxidation and reduction will occur alternately on the electrode, which will not affect the working state of the electrochemical sensor in a short time. If EIS AC disturbance signal is applied for a long time, the internal reaction of the sensor may be disordered, resulting in abnormal phenomena such as output saturation. It takes a long time for the sensor to return to normal.
Of course, in some applications (such as bio impedance analysis), the electrode needs to continue the EIS measurement process to obtain the measured data for analysis. In this case, the customer can directly use the EIS results output as shown in Figure 14.
Figure 14 module running in EIS measurement mode
This paper introduces the multi-functional electrochemical module mcum355, which is developed based on the platform chip aducm355 of ADI company. It can be used in the fields of gas detection, water quality detection, biological analysis and food inspection. It has the characteristics of high integration and ultra-low power consumption. For gas and water applications, this paper describes the connection method and working principle between mcum355 and the sensor in detail. Users can apply it to their own design and transition to other electrochemical sensor applications. Mcum355 module adopts serial port for instruction and data interaction, which is easy to develop and use, and can help users quickly build electrochemical products. Through the built-in EIS measurement function, it can realize deeper applications such as sensor life prediction and electrode analysis, and further expand the applicable fields of the module. For richer electrochemical function applications, the technicians of Junlong technology can assist customers to complete the customized development of the built-in function software of the module.
 synopsis of Junlong technology mcum355: https://www.cytech.com/knowledge/technical-articles/cytech-engineer/adi-highly-integrated-solution-gas-and-water-quality
 aducm355 product page: https://www.analog.com/cn/products/aducm355.html
Field Application Engineer
Macnica Cytech Ltd.