Circuit design required for electrochemical sensors

Electrochemical sensors are one of the most common sensors used in industry today and are widely used in products such as gas detection, water testing, bioanalysis and food testing. This type of sensor uses the principle of converting chemical reactions into current and voltage to produce an electrical signal proportional to the amount of substance being measured.

Electrochemical sensors generally consist of electrodes, reactants, etc. In order to maximize the life of these consumables, the chemical reaction between the internal substance and the substance to be detected is very small and slow. and must be done. Therefore, the electrical signal output from the electrochemical sensor is very weak. This leads to the need to design highly accurate and stable circuits to drive, tune, and acquire electrochemical sensors.

The circuit system required to operate an electrochemical sensor is called a constant potential circuit. Taking a 3-terminal electrochemical gas sensor as an example, as shown in Figure 1, build circuits such as a bias voltage source, potential hold, current-voltage conversion, filtering, and AD conversion, and input them to the MCU for data processing. will be performed. The signal before the ADC circuit is the voltage signal converted by the TIA circuit, which varies in proportion to the gas concentration value. The voltage converted by the TIA circuit changes between positive and negative depending on the oxidation and reduction reactions of the sensor. Components that make up the overall circuit include precision op amps, precision resistors, high performance ADCs, and more. Each part of circuit design has theoretical backing, and engineers are required to have a high level of knowledge of analog signal circuits.

More importantly, electrochemical sensors are sensitive to temperature and humidity, have poor response stability, and decay over their lifetime. For this reason, not only difficult tuning is required when manufacturing electrochemical sensors, but also caution is required when using thermochemical sensors.

In this article, we will introduce a multifunctional module that Cytech, a Macnica group company, independently developed using ADI's cutting-edge analog ICs.

This module covers almost all fields and all types of electrochemical sensors and is very easy to integrate and develop, allowing users of electrochemical sensors in any industry to build and manufacture their own products. allows for rapid realization.

Multifunctional electrochemical module MCUM355

The MCUM355 is based on ADI 's platform-level chip ADUCM355. With built-in high-performance analog circuits, temperature/humidity sensors, processors, and a wide range of peripheral interfaces, this product is suitable for a wide range of applications such as gas detection, water quality testing, bioimpedance analysis, blood glucose measurement, and food analysis. Yes. As shown in Figure 2, the MCUM355 is 2 x 2 cm in size and has a serial interface for easy system integration. The module operates from a 3.3V power supply and has a typical operating current of 5mA. In addition, in low power consumption and sleep states, the current consumption is only a few uA.

Configuration and basic operation of MCUM355

As shown in Figure 3, the MCUM355 includes the ADUCM355, the LTC6078 high resistance op amp, the SHT31​ ​type temperature and humidity sensor, and the necessary peripheral passives. The ADUCM355 is comprehensively equipped with internal functional resources, including two low-power potentiostats, one high-bandwidth potentiostat, high-performance ADC circuitry, rich switching matrix, and power management. It is also a processor equipped with a Cortex-M3 core running at 26MHz, and has all the resources necessary for a low-power MCU, such as various arithmetic function blocks and various data interfaces.

​The LTC6078 installed in the module is an ADI high-resistance dual operational amplifier; one serves as the pH electrode interface circuit to directly input the voltage signal to the ADC in the ADUCM355, and the other serves as the conductivity electrode interface circuit to input the voltage signal directly to the ADC in the ADUCM355. The circuit is converted to voltage and input to the ADC. For water testing applications, specific sensor connection methods are provided later in this article.

​The SHT31 on the module is a precision temperature and humidity sensor that can be used in gas detection scenarios to expand environmental metrics and as a basis for calibrating results. Additionally, a large number of precision resistors are provided on the board to calibrate the ADUCM355 's internal analog circuits and as reference resistors for water quality measurement processes.

The ADUCM355 has two built-in low-power constant potential circuits that are the core of an electrochemical sensor, and can be applied to all electrochemical gas sensors, as well as electrodes for blood sugar level and food detection. Furthermore, it also has one built-in high-band constant potential circuit. This circuit is commonly used to measure electrochemical impedance spectroscopy (EIS), which has a proven track record in battery analysis and biological characterization. It can also be used to predict the lifespan of sensors.

The internal functionality of the ADUCM355 is thus very rich, with a powerful internal switching matrix that switches analog circuit connections to achieve a variety of circuit functions. By using the MCUM355, which is a modular version of the ADUCM355, EIS measurement functions can be realized without the need for complex circuit configurations.

In order to make the MCUM355 more compact, only a portion of the ADUCM355's interface is open, except for the interfaces required to directly connect gas sensors, water quality sensors, etc. Figure 4 shows the interface resources available from outside the MCUM355. Interface resources are designed taking into account the characteristics of sensor types and connection methods. Those interested in ADUCM355 chip resources can find detailed information, including data sheets, on the ADUCM355 product page on the ADI website. This module can be directly connected to gas sensors, water quality sensors, etc.

Application example 1: Gas detection

The MCUM355 allows you to build gas sensing applications using a 3-terminal electrochemical sensor and connecting its pins directly to the corresponding interface of the module. MCUM355 has two identical sets of electrochemical channels and can connect two three-terminal electrochemical gas sensors, as shown in Figure 5. Of course, 2-terminal gas sensors such as oxygen sensors can also be used by simply connecting the current detection electrode to the WE terminal, that is, by simply connecting the current-to-voltage conversion function circuit inside the module. Also, some gas sensors that are often used based on PID or MOS principles output a single-ended voltage signal, and in this case, only the output terminal of this sensor is connected to the AIN side of MCUM355. .

Figure 6 is a demo kit for gas detection designed and developed by Cytech. This kit uses two electrochemical gas sensors and one​ ​PID sensor to measure three gas concentration indices: Oxygen, CO and TVOC. This is the upper limit of gas sensors that can be supported by one​ ​MCUM355. The demo kit display is driven and controlled by another MCU that communicates with the MCUM355 through serial commands. The demo kit displays temperature and humidity data from the SHT31 sensor on the MCUM35. The gas detection demo kit is powered by 5V via USB, and a single​ ​LDO with a 3.3V output is sufficient to meet all power requirements.

Figure 6: Gas detection demo kit

Application example 2: Water quality inspection

When building a water quality testing application using MCUM355, one module is mainly used to measure four parameters: water temperature, pH, ORP value, and conductivity. The sensor connection method is shown in Figure 7, the water temperature sensor is connected to the electrochemical 1 channel. The principle of a water temperature sensor is the same as that of an RTD (resistance temperature detector); it uses the principle of a constant potential circuit to indirectly measure resistance. The pH electrode is connected to a dedicated channel on the module, primarily due to its high resistance output characteristics. On the other hand, pH electrodes can reflect the ORP value of the liquid being measured. For conductive electrodes, the anode side is connected to the electrochemical 0 channel of the module, and the cathode end can be connected to the electrochemical 0 channel or a dedicated high resistance channel. Depending on the impedance range of the liquid to be measured, use the WE0 terminal of the electrochemical 0 channel for low resistance liquids (high conductivity), or use the WE0 terminal of the electrochemical 0 channel for high resistance liquids (weak conductivity) as shown in Figure 7. A high resistance interface must be connected to the

Figure 8 is a demonstration kit for water quality applications with a similar construction principle to the gas kit designed and developed by Cytech. The types of sensors that can be used for water quality applications are not limited to the solutions introduced in this article, but water quality electrode sensors based on electrochemical principles can also be used, such as dissolved oxygen sensors.

Application example 3: EIS measurement

AC impedance measurements, also known as electrochemical impedance spectroscopy (EIS) measurements, involve applying a small amplitude sinusoidal signal to an electrochemical sensor and measuring its current response to obtain an impedance value. Since it is an AC impedance, the measured value has the property of a phase angle, and has a real part and an imaginary part. In the actual measurement, signals of different frequencies are applied to the sensor and a series of impedance data is obtained, i.e. an impedance spectrum is formed. The impedance spectrum is plotted as a coordinate curve and can be used to analyze the operating conditions of the sensor electrode. Generally, as the sensor electrode ages, there will be a significant change in the impedance curve, as shown in Figure 9. For example, EIS analysis of gas sensors can estimate the sensor's remaining operating life, and EIS analysis of water quality electrodes can determine whether there is dirt or corrosion on the electrode surface. EIS is extremely important in practical terms. play a role.

In fact, EIS is widely used for battery characterization and corrosion detection. Compared to previous electrochemical electrode analysis methods such as chronoamperometry and cyclic voltammetry, EIS results are clearly more informative due to the frequency content, making EIS measurements a very promising application. I can say it. Using MCUM355, you can easily implement EIS measurement functions.

When no external sensor is used, a switching matrix circuit inside the module connects the sensor to a high-bandwidth potentiostatic circuit and performs the EIS measurement process fully automatically according to the built-in control program. It then directly outputs the impedance spectrum as shown in Figure 9. MCUM355 supports EIS measurements up to a maximum excitation signal frequency of 200kHz.

MCUM355 development environment

MCUM355 has a 4-wire SWD interface that allows users to directly edit and debug control code. At the same time, the MCUM355 's default firmware program supports serial command interaction, allowing engineers to simply send commands to the module according to protocol rules and receive feedback and measurement results.

The MCUM355 sends information to the user through the serial port upon power-up. Taking a gas sensing application as an example, first send the basic configuration information of two electrochemical sensor channels, and then convert the sensor current measured by the two channels into voltage, as shown in Figure 10. Send. By default it updates every second.

Note that because electrochemical sensors are very sensitive to ambient temperature and humidity, and their output characteristics are not ideally linear, it is necessary to adjust the calibration during conversion to further increase the accuracy of the final concentration measurements. Please note that the user will need to provide software algorithms such as corrections and temperature corrections.

In the model of the potentiostat circuit functional configuration shown in Figure 11, the user can set the five main parameters of the potentiostat circuit of the module by entering serial commands. Vbias and Vzero are the voltages generated through the DAC function circuit that determine the electrochemical sensor's voltage bias and measurement baseline to the circuit. The Rload load resistance relates to the characteristics of the sensor itself, Rtia determines the amplification factor of the current to voltage circuit, and Rfilter can change the response speed of the signal. These three resistors act like a precision digital potentiometer.

Configuration parameters are sent to the MCUM355 via a defined hexadecimal coded protocol. An example of this protocol and its specifications is shown in Figure 12, and its body includes the settings for the five parameters mentioned above. As mentioned earlier, the MCUM355 has two identical sets of low-power constant potential circuits, so the instructions also include the serial number of the target channel. Different codes for each of the five parameters of the protocol represent the corresponding configuration value, and configuration instructions are provided in the user guide provided with the MCUM355.

When the user sends a configuration parameter command, the MCUM355 feeds back the just updated configuration information, pauses operation, and prompts the user that the module needs to be reset, as shown in Figure 13. At this point, the configuration information has been saved to the module's internal flash space, and the module must rerun the program code. It then reads the latest configuration information from flash, controls internal circuitry to reach the new parameters, and then begins execution of the measurement program.

When the MCUM355 is operating in measurement mode, the user can send a mode change command to the module at any time. As shown in Figure 14, when the user sends a command to change the module to EIS measurement mode, the module immediately starts the EIS measurement process for both electrochemical channels, and the default program firmware allows for single channel measurement. For this purpose, the frequency points for EIS measurement are distributed from 100Hz to 200kHz, and EIS measurement is started in approximately 15 seconds. In EIS measurement mode, the module continues to measure the two sensor channels alternately until the user commands the module to return to normal measurement mode. Generally, it is desirable to perform EIS measurements of electrochemical sensors only once in a short period of time. The measurement process uses a sinusoidal signal of small amplitude to disturb the sensor, causing two opposing processes of alternating oxidation and reduction on the electrodes, which in the short term can affect the operating state of the electrochemical sensor. There's nothing to give. If the EIS AC disturbance signal is applied for a longer period of time, the internal response of the sensor will be disrupted, causing abnormalities such as saturation of the sensor output, and it may take a long time to return to normal.

Of course, in some applications (such as bioimpedance analysis), the electrode requires a continuous EIS measurement process to obtain measurement data for analysis, in which case the user will receive the output EIS results shown in Figure 14. It can be used directly.

Summary

In this article, we introduced the multifunctional electrochemical module "MCUM355" based on ADI 's platform chip "ADUCM355". This module can be used for gas detection, water quality testing, bioanalysis, food testing, etc., and has high integration and ultra-low power consumption. Gas and water quality applications demonstrate how the MCUM355 connects and operates with sensors, allowing users to apply it to their own designs or move on to developing other electrochemical sensor applications. . The MCUM355 utilizes a serial interface for command and data exchange, making it easy to leverage for development and debugging, helping users quickly build electrochemical products. In addition, the built-in EIS measurement function enables more advanced applications such as sensor life prediction and electrode analysis, further expanding the module's uses.

Application example

・Gas detection

・Water quality inspection

・Bioanalysis

・Food inspection

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