Test & measurement


determination of the ohmic resistance in a liquid. To achieve this measurement, two inert electrodes, placed parallel to each other, are immersed into the liquid to measure the resistance using ac. Through this process, the mobility, particle density, and oxidation state of the electrolyte can now be estimated so as to allow conclusions to be made about the concentration of the solution.


pH MeasureMent pH measurement is based on the principle of a half-cell reaction, which takes place on the electrode membrane and has a direct dependence on the H+ ion concentration. From this potential difference results a voltage, which has a linear relationship with the pH value. One major problem of pH measurement is that pH sensors have a very high series resistance, which places high demands on the analysis electronics.


Figure 5. Circuit for pH, temperature, and conductivity measurements with the ADuCM355.


diagram with the key components of the ADuCM355 is shown in Figure 4. It controls electrochemical and biological


electrocHeMical iMpedance analysis Electrochemical impedance analysis is a measurement in which the impedance of the electrochemical cell or the sensor is measured over an entire series of different frequencies. By means of the changing impedance over the different frequencies, the wear of the sensor can be measured and the signal chain accordingly automatically adapted. The decrease in sensor accuracy over time (a few days to weeks) is problematic here. This can strongly negatively affect the overall accuracy of the various measurements. This is the case, for instance, with continuous glucose measurement (CGM). Because the measurements are critical to health, the accuracy of the sensor must be continuously checked. An example circuit can be seen in Figure 3. The previously described medical


measurements differ greatly in terms of requirements and parameters, and thus the respectively used measurement methods. Besides that, a temperature measurement for compensation must additionally be performed to enable temperature calibration. Multiple sensors must be used in part for complementing or increasing the accuracy. In a discrete design, all of these measurements require a large circuit board area and have a high power consumption. These days, especially in medical technology,


small, power-saving, and low cost solutions are being sought so that they can be inserted into wearable and usable devices. ADI developed the ADuCM355 for exactly these design challenges.


aducM355—one size Fits all One solution that can unite all measurements is the ADuCM355. This highly integrated chip is composed of a power-saving analogue front end (AFE) and a microcontroller that assumes the housekeeping and security functions such as the cyclic redundancy check (CRC). A block


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sensors with an extremely low power consumption. The ARM Cortex-M3 processor technology-based chip has current, voltage, and resistance measurements. It has, in addition to a 16-bit, 400 kSPS multichannel, SAR ADC with input buffers, an integrated antialiasing filter (AAF), and programmable gain amplifier (PGA). The transimpedance amplifier (TIA) in the current inputs have programmable gain and load resistors to accommodate different sensor types. The AFE also contains amplifiers that were specifically developed for the potentiostat for maintaining a constant bias voltage with respect to an external electrochemical sensor. The respective input channel can be selected via an input mux upstream of the ADC. These input channels include three external current inputs, several external voltage inputs, and internal channels. Two of the three voltage DACs are dual-output DACs. The first output on the DAC controls the noninverting input of a potentiostat amplifier, whereas the other one controls the noninverting input of the TIA. The third DAC (sometimes referred to as a high speed DAC) is designed for the high performance TIA for impedance measurements. The output frequency range of this DAC is up to 200 kHz. The ARM Cortex- M3 processor also has a flexible multichannel direct memory access (DMA) controller, which supports two independent serial peripheral interface (SPI) ports, a universal asynchronous receiver/transmitter (UART), and an I2C communication peripherals. A series of communication peripherals can be configured for specific applications as required. These peripherals include UART, I2C, two SPI ports, and general- purpose input/output (GPIO) ports. The GPIOs can be combined with the universal timers to form a pulse width modulated (PWM) output.


FurtHer MeasureMents Most of the sensors for the described measurements can be operated directly via the


ADuCM355 inputs. This works, for example, with potentiostat measurements such as the glucose measurement. In contrast, more accurate measurements such as the conductivity or the pH need an extended signal chain, and thus an external chip such as the LTC6078. It increases the input impedance to adapt it to the high output impedance of the sensors and thus enable accurate readout. Apart from the measurements previously described, measurement of the temperature is necessary so that the fluctuations of the sensor can be compensated for. The extended measurement principle is shown in Figure 5. By means of the larger signal chain, the ADuCM355 can read out both voltage and current. With the circuit shown, impedances in the range of less than 100 Ω to 10 MΩ can be detected. The large measuring range enables coverage of the complete impedance spectrum required in medicine. A high dynamic range is especially important for conductivity measurements so that various concentrations can be measured.


conclusion While the measurements for diverse liquids have impedance measurement as the underlying principle, they still differ from one another. For example, different sensors must be connected to record different parameters. To cover this versatility on one hand and accommodate the growing trend toward small, power-saving devices on the other hand, a smart solution is urgently needed. The ADuCM355 not only meets all these requirements but also takes on its role as a Swiss Army knife for impedance measurements in medicine. This IC, in fact, enables - in addition to fluid measurements - other impedance measurements in medicine, for example, body fat analysis or skin impedance. In addition, due to its versatility, it can also measure electrochemical


gases such as CO or CO2 with the right sensors. This makes the ADuCM355 an all-in-one package for impedance measurements.


Analog Devices www.analog.com January 2022 Instrumentation Monthly


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