Integrated AFE solution for noninvasive stroke volume monitoring

Vital signs monitoring (VSM) devices can acquire various electrophysiological signals, such as electrocardiogram (ECG), photoplethysmography (PPG), and bioelectrical impedance (Bio-Z). These signals reflect multiple aspects of human physiological status and are widely used in health monitoring, disease prevention, and auxiliary treatment. Impedance cardiography (ICG) is an important electrophysiological signal detection technique. It measures stroke volume (SV) by tracking changes in the instantaneous mean thoracic impedance, thereby evaluating the cardiac hemodynamic function of the human body. This article introduces the basic principles and measurement methods of ICG, as well as the functional features of related solutions based on ADI's highly integrated analog front-end (AFE) chips.

Impedance cardiography (ICG) is a noninvasive technique for assessing cardiovascular function by measuring variations in thoracic bioelectrical impedance associated with cardiac activity. Changes in blood volume during the cardiac cycle lead to corresponding impedance variations, from which the impedance cardiogram and its first derivative (dZ/dt) can be derived. The dZ/dt waveform contains distinct characteristic points that reflect key physiological events of the cardiac cycle, enabling the extraction of important hemodynamic parameters such as stroke volume (SV) and left ventricular ejection time (LVET). Due to its noninvasive nature, simplicity, and capability for continuous monitoring, ICG has been widely applied in vital signs monitoring (VSM) and cardiovascular assessment.
 
The impedance cardiogram, also known as ICG signal, is obtained using bioelectrical impedance (Bio-Z) technology and reflects changes in blood volume. By measuring the changes in the impedance of human tissues to a weak alternating current, the cardiac impedance signal reflecting blood volume variations can be obtained. In medical environments, the widely used four-electrode method is employed to measure human bioimpedance. The principle of this method is to apply an excitation current through one pair of electrodes, while extracting the voltage using another pair of electrodes.
 
In this bioimpedance measurement application, ADI's highly integrated analog front-end (AFE) solution can be adopted. It offers several advantages. First, the transmit channel integrates a sinusoidal current source, capable of delivering an AC excitation current across a wide frequency range from 16 Hz to 500 kHz, with a current amplitude range from 16 nA_rms to 1.28 mA_rms. Both frequency and current are fully configurable. In addition, its flexible input/output multiplexer (mux) supports either bipolar or tetrapolar electrode measurement structures. The receive channel features high input impedance, low noise, and a high common-mode rejection ratio (CMRR). It also provides programmable gain, configurable low-pass/high-pass filters, and two integrated high-resolution analog-to-digital converters (ADCs) (20-bit), enabling synchronous I_Q signal acquisition. Second, for applications requiring high absolute impedance accuracy, such as bioimpedance analysis/spectroscopy (BIA/BIS) or body impedance measurement in automated external defibrillators (AEDs), the ADI MAX30009 offers multiple calibration options. By connecting an external precision resistor to the dedicated four-wire calibration port, the highest accuracy can be achieved. Alternatively, its internally trimmed precision resistors also provide excellent calibration performance.
 
ADI's bioimpedance analysis application solutions are powered by high-precision, highly programmable impedance-to-digital converters, which enable intracellular detection for various bioimpedance applications with an electroimpedance spectrometry capability. An integrated digital engine enables maximum design flexibility for various applications. Body composition analysis ranges from body fat content measurement to advanced core body hydration analysis. ADI provides fully integrated analog front ends that can safely excite weak electric current flows through the body and measure voltage to calculate body impedance accordingly.
 
The bioimpedance measurement system composed of MAX32666, MAX20356, and MAX30009 is a complete bioimpedance measurement solution. It employs the highly integrated and programmable power management IC MAX20356 to manage the power rails, and communicates with the bioimpedance AFE MAX30009 via a low power Bluetooth®-enabled microcontroller unit (MCU). The transmit channel of the AFE uses an internal direct digital synthesizer (DDS) and digital-to-analog converter (DAC) to generate a sine wave sweeping voltage with adjustable frequency. This voltage is converted into a current stimulus through a bias resistor and applied to the human body. The response signal is obtained by the AFE receive channel through measurement at the input pins. In ICG measurements, the commonly used excitation signal frequency range is 20 kHz to 200 kHz, with an excitation current rms value of about 0.12 mA, corresponding to a human body AC impedance range of 0.1 Ω to 0.4 Ω. In this application, an excitation signal of 65.536 kHz and 256 µA is applied to the human body for impedance measurement.
 
The raw impedance signal is first processed sequentially through a 50 Hz notch filter, a median filter, and a low-pass filter to obtain a relatively clean impedance signal. The signal is then differentiated once to obtain the ICG signal. The peak points of the ICG signal are detected using the differential threshold method, and the resulting values are used to analyze changes in blood volume.

ADI's MAX30009 is a complete low-power, high-performance bioimpedance (Bio-Z) analog front-end (AFE) for measuring a range of physiological conditions in wearable applications. It is designed to provide high performance for fitness, wellness, and clinical applications, with ultra-low power consumption to help extend battery life. The Bio-Z receive channel has electrostatic discharge (ESD) protection, electromagnetic interference (EMI) filtering, internal lead-biasing, DC leads-off detection, DRVN lead-off detection, and ultra-low power lead-on detection during standby mode. The Bio-Z receive channel also has high input impedance, low noise, high common-mode rejection ratio (CMRR), programmable gain, various low-pass and high-pass filter options, and two high-resolution analog-to-digital converters for simultaneous I and Q acquisition.
 
The Bio-Z transmit channel has a sine-wave current generator to drive AC currents into the body with a wide frequency range of 16Hz to 500kHz and a wide magnitude range of 16nA_RMS to 1.28mA_RMS. The transmit channel can also operate in sine-wave voltage and H-bridge modes. The flexible input/output MUX allows for both bipolar and tetrapolar measurements with multiple sets of electrodes.
 
For measurements requiring high absolute impedance accuracy such as bioimpedance analysis/spectroscopy (BIA/BIS) and automated external defibrillator (AED) body impedance, the MAX30009 offers several calibration options. An external precision resistor can be connected to the four-wire calibration port for the highest accuracy. Internal trimmed resistors also provide high accuracy. The PLL-based timing subsystem allows for a wide range of fine-tuned stimulus and sampling frequencies, and can be synchronized with other ADI biosensors for simultaneous data collection. The MAX30009 is available in a 2.03mm x 2.03mm, 25-bump wafer-level package (WLP), operating over the -40°C to +85°C temperature range.
 
The MAX30009 can be applied in wearable fitness, wellness, and medical devices, multifrequency body composition analyzers, non-invasive hemodynamic monitors, and automated external defibrillators. It also features optimized performance to accurately detect respiration rate, galvanic skin response/electrodermal activity, bioimpedance spectroscopy, body composition and fluid analysis, and impedance cardiography and plethysmography.

ADI's MAX20356 is a highly integrated and programmable power-management solution designed for ultra-low-power wearable applications. It is optimized for size and efficiency to enhance the value of the end product by extending battery life and shrinking the overall solution size. A flexible set of power-optimized voltage regulators including multiple buck converters, a buck-boost converter, and linear regulators provide a high level of integration and the ability to create a fully optimized power architecture. The quiescent current of each regulator is ultra-low to extend battery life in always-on applications.
 
The MAX20356 includes a complete battery-management solution with battery seal, charger, power path, and fuel gauge. Both thermal management and input protection are built into the charger. The device also includes a factory-programmable button controller with multiple inputs that are customizable to fit specific product user requirements.
 
The MAX20356 is a low-noise, 1.5W buck-boost converter provides highly efficient and clean power conversion required for the LEDs used in optical heart-rate systems such as PPG and SPO₂ measurements. The MAX20356 is also equipped with a nano I_Q fast transient LDO, optimized for use in analog front-end (AFE) sensors. The MAX20356 is configurable through an I²C interface that allows for programming various functions and reading the device status, including the ability to read temperature and supply voltages through the monitor multiplexer. The MAX20356 is available in a 63-bump, 0.5mm pitch, 3.71mm x 4.48mm wafer-level package (WLP) and operates over the -40°C to +85°C extended temperature range. It can be applied in wearable devices and the Internet of Things (IoT).

ADI's MAX32666 is a low-power ARM Cortex-M4 with FPU-based microcontroller with Bluetooth 5 for wearables. Generation UB MCUs are built to run complex applications demanded by battery-powered and wirelessly connected devices.
 
DARWIN is a new breed of low-power microcontroller architecture built to thrive in the rapidly evolving Internet of Things (IoT). They are smart, with the biggest memories in their class and a massively scalable memory architecture. They run forever, thanks to wearable-grade power technology. They are durable enough to withstand the most advanced cyberattacks. DARWIN microcontrollers are designed to run any application imaginable - in places where you would not dream of sending other microcontrollers.
 
Generation UB microcontrollers are designed to handle the increasingly complex applications demanded by today's advanced battery-powered devices and wirelessly connected devices, while providing robust hardware security and Bluetooth® 5 Low Energy (Bluetooth LE) radio connectivity.
 
The MAX32666 UB class microcontroller is an advanced system-on-chip built on the DARWIN architecture, featuring an Arm® Cortex®-M4 with FPU CPU for efficient computation of complex functions and algorithms with integrated power management. It also includes the newest generation Bluetooth 5 LE radio with high throughput (2Mbps) and ADI's best-in-class hardware security suite trust protection unit (TPU). The device offers large on-board memory with 1MB flash and 560KB SRAM, which can be configured as 448KB SRAM using error correction coding (ECC). Split flash banks of 512KB each support seamless over-the-air upgrades, adding an additional degree of reliability. Memory scalability of data (SRAM) and code (flash) space is supported by two SPI execute-in-place (SPIX) interfaces.
 
The MAX32666 supports multiple high-speed interfaces including HS-USB, secure digital interface (SD, SDIO, MMC, SDHC, and microSD™), SPI, UART, and I²C serial interfaces, and an audio subsystem supporting PDM, PCM, I²S, and TDM interfaces. An 8-input, 10-bit ADC is available to monitor analog inputs from external sensors and meters. The device is available in 109-bump WLP (0.35mm pitch) and 121-bump CTBGA (0.65mm pitch) packages. It can be applied in connected home, gaming devices, hearables, industrial sensors, payment/fitness/medical wearables, and telemedicine.

ICG signals are one of the important biosignals for assessing cardiovascular health, closely related to cardiac function and hemodynamics. Therefore, accurate acquisition and analysis of ICG signals are crucial for heart health monitoring. ADI's MAX30009 is a dedicated bioimpedance AFE device that can generate various stimulation signals, such as sinusoidal sweeps, and accurately measure bioimpedance through quadrature demodulation. It can be used to acquire ICG signals to enable detailed analysis of cardiac activity. It also supports synchronized measurements with other biosensors, making it advantageous over traditional solutions in wearable cardiovascular monitoring applications. It is therefore an ideal choice for building high-precision ICG signal measurement systems.