Who is not familiar with Ohm’s law? For dc voltages, it states that the current through a conductor between two points is directly proportional to the voltage across those two points. In other words, the resistance of the conductor is constant independent of the current. For ac voltage, the situation completely changes and becomes more complex. The resistance becomes an impedance and is defined as the voltage and current ratio in the frequency domain. The magnitude or real part represents the ratio between voltage and current, where the phase or imaginary part is a measure of the phase shift between voltage and current.
There are many use cases in the medical industry where impedance measurement is being applied. This technology can be used in a wide range of applications such as retrieving certain human body parameters, detecting diseases, or analyzing fluids such as blood or saliva. Although these use cases have impedance measurement in common, each of them has its own set of key requirements.
Analog Devices developed a new impedance measurement family called AD594x. It is highly accurate and has several power modes to support either spot checks or continuous measurements. In this article you will learn about the features of this chip and its primary applications.
Introduction and Focus
Chips for impedance analysis are relatively new. Roughly 15 years ago, Analog Devices introduced the AD5933/AD5934, which were the first family of impedance analysis chips. The second generation, called the ADuCM350, was launched in 2015. Both families are selling in high volumes, but they are not always the best solution to support today’s newer applications. With the trend of wearable devices and battery-powered systems, the main challenges are meeting the required performance levels in an as small as possible form factor, and with very little power consumption. The AD594x has been developed to support today’s wearable market and to meet all key requirements including high accuracy, small size, and low power.
The AD594x (Figure 1) is a multifunctional impedance analyzer that is tailored for medical and industrial type applications. The analog front end is completely configurable and can be modified to support a wide range of different use cases, including electrodermal activity (EDA) or galvanic skin response (GSR), body impedance analyses, hydration measurement, and biochemical measurement. The focus of this article is on medical related applications, but the AD594x can also be used in industrial applications such a toxic gas analyses, PH measurement, conductivity, or water quality measurement.
Relative Measurement for EDA/GSR
A 2-wire measurement principle can be performed to measure relative impedance or impedance change. One target application for this is monitoring stress or mental health by electrodermal activity or galvanic skin response. Monitoring the mental state or stress is important, since stressful situations over time might result in chronic diseases such as diabetes, cardiac problems, or cancer. During mental changes or when people become stressed, the sympathetic nervous system of the human body activates the sweat glands in the skin. This effect increases the skin conductivity, which results in an impedance drop.
Skin impedance monitoring is a voltammetry measurement. An excitation signal is applied across the unknown impedance (the skin in this case) and the voltage across the impedance is measured. Alternated, the current through the impedance is measured and a DFT is performed on the ADC results to calculate the impedance change. Figure 2 shows the high level measurement principle of an EDA or GSR measurement. This measurement is performed with an excitation frequency close to dc. It is recommended to use a low frequency excitation rather than measuring with a dc voltage, to prevent polarization of the electrodes and eliminate harm to the human tissue. In general, an excitation frequency up to a maximum of 200 Hz is applied, because higher frequencies penetrate inside the body and won’t measure just at the skin surface. Depending on the location of the electrodes on the human body, changes in conductivity will vary with someone’s emotional or mental state.
There is no direct equation on impedance change vs. mental stress, so usually this measurement is performed in parallel with other measurements such as heart rate and/or heart rate variability. An algorithm is required to take the outcome of the various measurements to translate this into a measurement for stress levels. EDA/GSR technology for stress requires a continuous 24/7 measurement, and the AD59xx is designed for that. The power consumption is <80 μA at an output data rate of 4 Hz. EDA/GSR measurement is also used for applications such as sleep analyses.
4-Wire Measurement for Body Impedance Analyses
In medical applications, a popular type impedance measurement is bioimpedance analysis (BIA). BIA is a 4-wire measurement principle, and this configuration supports applications where absolute accuracy is required. The AD59xx is capable of supporting applications with a receive bandwidth up to 50 kHz and a signal-to-noise ratio (SNR) of 100 dB. One of the most common 4-wire BIA applications is body composition measurement, where the amount of fat free mass is measured. This setup, however, also can be used to monitor the amount of water in the human body or to measure cardiac behavior by bioimpedance spectroscopy. The measurement principles are all the same, however we can change the use case by changing the ac excitation frequency and the position of the electrodes on the human body. Figure 3 shows the setup of a 4-wire measurement principle. The unknown Z in this setup represents the human body. A voltage is pushed to the impedance under test to bring it to a certain common-mode potential, while an ac excitation voltage is applied and the response current is measured with a high speed transimpedance amplifier. The impedance finally can be calculated as Z = VCOMMON/I.
In the block diagram of Figure 3, you see that the impedance is isolated from the measurement front end by resistors and capacitors. The resistors limit the maximum current that can flow through the body. CISO ensures that no dc signal can be generated between the electrode and ground or to other electrodes. This is one of the requirements to achieve the medical safety standards, such as IEC 60601.
As previously mentioned, the electrode location on the human body, as well as excitation frequency, will represent the measurement that is performed. Low frequencies up to a few hundred Hz stay at the skin surface while higher frequencies penetrate deeper inside the body. For a healthy person, roughly 60% of their total body weight consists of water. One third of the total body water is extracellular fluid (ECF) and the rest is found inside the cell structure (intercellular fluids). Due to the electric model of the cell structure, ac up to 50 kHz will measure through the extracellular fluids. Higher frequencies go beyond the cells and can measure the intracellular fluid. Depending on the electrode location, the excitation frequency and the algorithm used to interpret the impedance measurement, body composition such as the percentage of total body fat, or the amount of body water (to measure dehydration) can be determined. The AD59xx is capable of supporting each of these applications. In some use-cases, single frequency excitation is used, where in other applications multiple frequencies or a frequency sweep is used. In addition, the frequency on the number of measurements can differ. Where body composition is measured either once per day or once per week, dehydration usually will be measured continuously. For continuous measurements, power dissipation is very critical and that is why the flexibility of the AD59xx has a big advantage.
Other applications of the AD59xx could be measuring respiration rate based on thoracic impedance, beat-to-beat cardiac output monitoring by transthoracic impedance measurement, or impedance measurement to estimate bladder volume.
AD59xx for Biochemical Measurements
Biochemical analysis is another application of the AD59xx. This technology uses amperometric/potentiostat type measurements on a sensor, which models a typical electrochemical cell. The sensor often is a test strip with reagent where you apply a sample of the material under test. Any analyte that can be oxidized or reduced is a candidate for amperometric measurement. For medical applications, various samples of human fluid can be analyzed such as blood, urine, or saliva. The system requires a (programmable) current source and a potentiostat amplifier. The simplest form of an amperometric measurement is done by applying a step response voltage across the sensor, which causes a chemical reaction. With a transimpedance amplifier, the current is measured as a reference for the reaction. In addition to the previously mentioned 2-wire technique, the AD59xx is also capable of supporting 3-, and 4-wire amperometric measurement techniques.
As the measurement technique is always the same, the test strip determines the sample under test. Blood glucose measurement is the most popular one and it is often used for the biochemical measurement of diabetic patients. In a 3-wire configuration, the electrochemical cell consists of a working electrode (WE) onto which the reaction takes place, a reference electrode (RE) that maintains the constant potential, and a counter electrode (CE) that supplies the reaction current. Figure 4 shows a block diagram of this configuration.
The potentiostat supplies a desired cell potential VCELL between WE and RE, and measures the reaction current between the WE and CE. One trend is to move away from a glucose spot check measurement, and use continuous glucose monitoring (CGM). The meter continuously measures blood sugar levels and sends data to an insulin pump. The pump then dispenses the required dose of insulin to the body. This artificial pancreas technology improves the lives of people with diabetes. Instead of a person watching blood glucose levels during the day, the system runs completely independent, without any human involvement. The AD59xx is ideally suited for this application because it has very high precision with ultra low power and it performs all the required safety checks. The system in Figure 4 is built with three main functions: a biochemical AFE, a microcontroller, and a dedicated power management chip. In the near future, the AFE will be integrated into the MCU to reduce overall board space.
In addition to diabetes, many other diseases, as well as drugs and hormones, can be tested with this technology. In industrial applications, this technology is mainly used for gas sensing and fluid analyses.
AD59xx Features and Key Specifications
The AD59xx is a high precision analog front end (AFE) designed for electrochemical-based measurement techniques, like amperomtric, voltametric, and impedance measurement. The front end has an ultra low power mode to support portable and battery-powered systems. In parallel to this, the chip has the capability to support high performance and diagnostic-based applications that mainly can be found in clinical and lab environments.
The AD59xx is designed around three main building blocks: an input receive signal-chain, a waveform generator plus transmit channel, and a sequencer with a discrete Fourier transform (DFT) engine to measure complex impedances. Depending on the use-case, the excitation loop with receiver channel can be configured differently. For applications that require a sensor excitation varying from dc up to 200 Hz, the low power DAC and low noise potentiostat amplifier can be used. For applications that require higher excitation frequencies, up to 200 kHz, the integrated high speed DAC is used. The DAC can generate sinusoid and trapezoid excitation waveforms. For each of the modes, low power or high speed, a dedicated transimpedance amplifier has been integrated. Each has a programmable transimpedance amplifier to support a wide range of sensors, which can be connected to the AFE. The output of the TIAs can be multiplexed into the second stage of the input receive channel. At this point, auxiliary channels also can be measured, such as external voltages and currents or internal diagnostic signals like supply voltage, die temperature, or reference voltage. The output of this multiplexer that functions as a channel selector is connected through a buffer, a programmable gain amplifier, and an antialiasing filter into a 16-bit, 800 kSPS successive approximation register (SAR) ADC.
Wearable electronics, point-of-care cloud connected systems, and the Internet of Things are terminologies that we run into almost every day. Sensing is a very important aspect of all of these systems, and impedance measurement is one of the more notable types of sensing. The AD59xx has been developed to meet the objectives of today’s needs. 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, and a small form factor opens a wide range of new markets and applications that were hard to address in the past. For portable and battery-powered systems in particular, this family of tiny devices brings huge advantages. The AD59xx family works seamlessly together with the AD8233, a single-lead ECG front end. Both chips can work in a master/slave configuration where impedance and ECG measurements can be performed by using the same set of electrodes to the human body. As processor, the ultra low power ADuCM3029 Cortex®-M3 is recommended in combination with the ADP5350 power management and Li-Ion charger device.
A mix of evaluation boards for the different use cases are available to simplify the design cycle and improve time to market.