Who doesn't know Ohm's law?

For DC voltage, it is expressed as the current through a conductor between two points is proportional to the voltage between the two points. In other words, the resistance of a conductor is constant regardless of the current flow. For AC voltage, the situation changes completely and becomes more complicated. Resistance becomes impedance, which is defined as the ratio of voltage to current in the frequency domain. The magnitude (or real part) represents the ratio between voltage and current, while the phase (or imaginary part) is the phase shift value between voltage and current.

There are many use cases for impedance measurements in the medical industry. The technology can be used for a wide range of applications, such as acquiring some specific human parameters, detecting diseases, or analyzing human fluids such as blood or saliva. While these applications have in common making impedance measurements, each application has its own set of key requirements.

ADI has developed a new impedance measurement chip called the AD594x family. The chip is very accurate and has multiple power modes for on-demand or continuous measurements. In this article, you will learn about the characteristics of this chip and its main applications.

The technique of making impedance measurements with chips is relatively new. About 15 years ago, Analog Devices introduced the AD5933/AD5934, the first family of impedance analysis chips. The second generation, the ADuCM350, was launched in 2015. These two series are still sold in large quantities, but they are not always the best solution for currently updated applications. As wearables and battery-powered systems become the trend, the main challenge is to meet the required performance levels in the smallest possible form factor with extremely low power consumption. The AD594x are designed to support today's wearable market and meet all key requirements including high accuracy, small size and low power consumption.

The AD594x (Figure 1) is a versatile impedance analyzer tailored for medical and industrial applications. The analog front end is fully configurable and can be modified to support a variety of different use cases, including electrodermal activity (EDA) or galvanic skin response (GSR), body impedance analysis, moisture measurement, and biochemical measurements. This article focuses on medical-related applications, but the AD594x can also be used in industrial applications such as toxic gas analysis, pH measurement, conductivity, or water quality measurement.

Figure 1. High-level functional block diagram of the AD594x

Relative measurement of EDA/GSR

The relative impedance (or the change in impedance) can be measured directly using the 2-wire measurement method. One target application is monitoring stress or mental health through galvanic skin activity or galvanic skin response. Mental status or stress monitoring is important because, over time, stress can lead to chronic diseases such as diabetes, heart disease or cancer. During changes in mental state or when people become stressed, the body's sympathetic nervous system activates sweat glands in the skin. This effect increases skin conductivity, which reduces impedance.

Skin impedance monitoring was measured by voltammetry. Apply an excitation signal across an unknown impedance (skin in this case) and measure the voltage across the impedance. Then measure the current through the unknown impedance. A DFT calculation is performed on the ADC result to obtain the change in impedance value. Figure 2 shows the overall measurement principle of EDA or GSR. The excitation signal frequency for this measurement is close to DC. Measurements with low frequency excitation (rather than DC voltage) are recommended to prevent electrode polarization and eliminate damage to human tissue. Typically, the maximum excitation signal frequency is up to 200 Hz, since higher frequencies penetrate into the human body and do not measure only the skin surface. Place electrodes in certain positions on the human body, and the conductivity will change with the person's emotional or mental state.

Figure 2. Measurement principle of EDA or GSR

There is no direct formula for the relationship between impedance changes and mental stress, so this measurement is usually done in parallel with other measurements such as heart rate and/or heart rate variability. An algorithm needs to be developed to translate the various measurements into psychological stress levels. The use of EDA/GSR technology for pressure monitoring requires continuous, all-weather measurements, for which the AD594x is designed. Power consumption is <80 μA at an output data rate of 4 Hz. EDA/GSR measurements can also be used for applications such as sleep analysis.

4-Wire Measurement Method for Human Impedance Analysis

In medical applications, impedance measurements are often used for bioimpedance analysis (BIA). BIA is a 4-wire impedance measurement that can be used in applications that require absolute accuracy. The AD594x receive bandwidths up to 50 kHz with a signal-to-noise ratio (SNR) of 100 dB. One of the most common 4-line BIA applications is body composition measurement to measure lean body mass. In addition, this setup can also be used to monitor water content in the human body or to measure cardiac behavior via bioimpedance spectroscopy. The measurement principle is the same, but we can achieve different applications by changing the AC excitation frequency and the position of the electrodes in the human body.

Figure 3 shows the principle of the 4-wire measurement method. The unknown quantity Z in this setup represents the human body. Apply an AC excitation voltage to the human body, superimpose a suitable common-mode voltage on it and measure it with a voltmeter, and use a high-speed transimpedance amplifier to measure the response current. The final impedance can be calculated by: Z = VM/I.

Figure 3. 4-Wire Measurement for Human Body Impedance Analysis

In the functional block diagram of Figure 3, it can be seen that the impedance is isolated from the measurement front end by resistors and capacitors. Resistors limit the maximum current that can flow through the body. CISO ensures that no DC signal is generated between the electrode and ground or other electrodes. This is one of the requirements to meet medical safety standards such as IEC 60601.

As mentioned previously, the electrode locations and excitation frequencies on the human body will represent the measurements performed. Low-frequency currents as low as a few hundred hertz only stay on the surface of the skin, while higher-frequency currents penetrate deep into the body. For a healthy person, water makes up about 60% of their total body weight. One-third of the body's water is extracellular fluid (ECF), while the rest is located within cellular structures (intercellular fluid). Given the electrical model of the cellular structure, AC currents up to 50 kHz allow extracellular fluid measurements. Higher frequency currents penetrate the cells, allowing measurements of the intercellular fluid. Based on electrode position, excitation frequency, and algorithms used to interpret impedance measurements, the composition of the body can be determined, such as total body fat percentage or body water content (a measure of dehydration). AD594x can support all these applications. In some applications, a single frequency excitation is used, while in other applications multiple frequencies or frequency sweeps are used. Additionally, the frequency of measurements may vary. For body composition, measurements are usually daily or weekly; for body dehydration monitoring, continuous measurements are usually required. For continuous measurements, power consumption is critical, so the flexibility of the AD594x is a huge advantage here.

Other applications for the AD594x include measuring respiratory rate based on thoracic impedance, monitoring beat-to-beat cardiac output using transthoracic impedance, or impedance measurements for estimating bladder capacity.

AD594x for biochemical measurements

Another application of the AD594x is biochemical analysis. This technique applies amperometric/potentiostat-type measurements to a sensor that acts as a model for a typical electrochemical cell. The sensor is usually a test strip with reagents on which a sample of the material to be tested is placed. Any analyte that can be oxidized or reduced may be used for amperometric measurements. In medical applications, various human fluid samples such as blood, urine or saliva can be analyzed. The system requires a (programmable) current source and potentiostat amplifier. The simplest way to measure current is to induce a chemical reaction by applying a step-response voltage to the sensor. Using a transimpedance amplifier, the measured current can represent the strength of the reaction. In addition to the aforementioned 2-wire techniques, the AD594x also supports 3-wire and 4-wire current measurement techniques.

The potentiostat provides the desired cell potential VCELL between WE and RE and measures the reaction current between WE and CE. There is a trend in blood glucose testing from on-demand testing to the use of continuous glucose monitoring (CGM). The meter continuously measures blood glucose levels and sends the data to the insulin pump. The pump then injects the desired dose of insulin into the body. This artificial pancreas technology improves the lives of people with diabetes. There is no longer a need for a dedicated person to observe blood sugar levels throughout the day, the system can operate completely independently without any human intervention. The AD594x is ideal for this application because it has very high accuracy and ultra-low power consumption, and can perform all required safety checks. The system in Figure 4 has three main functions: a biochemical AFE, a microcontroller, and a dedicated power management chip.

Figure 4. Functional block diagram of a 3-wire biochemical analyzer

In addition to diabetes, the technology can be used to test for many other diseases as well as drugs and hormones. In industrial applications, the technology is mainly used for gas detection and fluid analysis.

AD594x Features and Key Specifications

The AD594x is a high precision analog front end (AFE) designed for electrochemical-based measurement techniques such as current, voltammetry, and impedance measurements. The front end features an ultra-low power mode that supports portable and battery powered systems. At the same time, the chip is also capable of supporting high-performance and diagnostic-based applications, which are mainly used in clinical and laboratory settings.

The AD594x is designed around three main blocks: the input receive signal chain, the waveform generator and transmit channels, and a timing controller with a discrete Fourier transform (DFT) engine for complex impedance measurements. Depending on the application, the excitation loop and its receive channel can be configured differently. For applications that require the sensor excitation signal frequency to vary from DC to 200 Hz, a low-power DAC and a low-noise potentiostat amplifier can be used. For applications requiring higher excitation frequencies (up to 200 kHz), an integrated high-speed DAC is available. The DAC can generate sinusoidal and trapezoidal excitation waveforms. All modes (low power or high speed) have integrated dedicated transimpedance amplifiers. Each mode features a programmable transimpedance amplifier that supports various sensors connected to the AFE. The output of the TIA can be multiplexed to the second stage of the input receive channel. At this point, auxiliary channels such as external voltages and currents or internal diagnostic signals such as supply voltage, die temperature, or voltage references can also be measured. The multiplexer acts as a channel selector, and its output connects to a 16-bit, 800 kSPS successive approximation register (SAR) ADC through buffers, programmable gain amplifiers, and antialiasing filters.

in conclusion

Wearable electronics, cloud-based instant monitoring and RE, IoT are terms we see almost every day. Detection is one of the very important functions of these systems, and impedance measurement is one of the more important types of detection. The AD594x was developed to meet current demand goals. It is a high performance and flexible analog front end designed for impedance analysis, biochemical and electrochemical applications. The combination of high precision, ultra-low power consumption and small size opens up a variety of new markets and applications that have been difficult to achieve in the past. Especially for portable and battery-operated systems, this family of miniature devices brings huge advantages. The AD594x family works seamlessly with the AD8233 single-lead ECG front end. Both chips can operate in a master/slave configuration that shares electrodes connected to the body for impedance and ECG measurements. As for the processor, the ultra-low power ADuCM3029 Cortex™-M3 is recommended for use with the ADP5350 power management and Li-Ion charger device.

Evaluation boards for different use cases have been developed to shorten the design cycle and time to market.