Analog DevicesEVAL-HCRWATCH4Z传感器开发电路板和套件

The vital signs monitoring (VSM) watch, EVAL-HCRWATCH4Z, is a modular development, demonstration, and data collection platform for high performance vital signs monitoring applications based on Analog Devices, Inc. analog front ends and sensors. It is a wearable, battery-powered device which enables the continuous monitoring and on-demand spot check measurement of photoplethysmography (PPG), electrodermal activity (EDA, bioimpedance-based), skin temperature, electrocardiography (ECG, biopotential based), and motion/activity (based on a 3-axis accelerometer). It allows for synchronized, multiparameter data storage on internal memory for later data retrieval and offline analysis and/or live monitoring on a PC (Windows® OS) or Android or iOS-based device.

Overview

The VSM Watch is a modular development, demonstration, and data collection platform for high performance vital signs monitoring applications based on Analog Devices analog front ends and sensors. The platform optimized electrical and mechanical design hosts all the required circuits to sense, condition, digitize, process, store, and wirelessly transmit real-time, vital sign related data. This platform does the following:
•  Minimizes the risks associated with a new electronic design.
•  Minimizes the time to market for a new final product.
•  Facilitates the evaluation of a wide range of Analog Devices solutions in a single battery-powered wearable ecosystem.
•  Highlights and addresses the challenges associated with a wearable device.
•  Allows the developer to focus on other added value tasks such as algorithm development and the overall firmware for engineering and scientific research and validation.

This platform is NOT intended to do the following:
•  Act as the final product for a specific application
•  Demonstrate an application specific optimized design. Trade-offs have been made to allow platform flexibility.

This evaluation platform is for investigative work, only. The EVAL-HCRWATCH4Z is not a medically certified device.

The evaluation platform kit contains the following items:
•  The VSM platform (the watch)
•  Charging Cradle
•  A USB Type A to micro-USB cable
•  Firmware Debug Board

The USB cable is used for the following purposes:
•  Recharge the battery via the charging cradle
•  Upgrade the platform firmware
•  Download data stored in the internal flash memory for offline data analysis

OPTIONAL BLUETOOTH USB PC DONGLE (NRF52840)

The VSM Watch platform leverages an off-the-shelf Nordic BLE dongle for communicating wirelessly with a PC. For BLE connectivity, you may optionally purchase the Nordic nRF52840 dongle. You can find the nRF52840 here.

VITAL SIGNS MONITORED

PHOTOPLETHYSMOGRAPHY (PPG)
The ADPD4100 operates as a complete multimodal sensor front end, stimulating four LEDs on the VSM Watch and measuring the return signal on up to eight separate current inputs. Twelve time slots are available, enabling 12 separate measurements per sampling period. For more details on the ADPD4100, see the datasheet here.

MOTION AND ACTIVITY

Motion is sensed by the ADXL362, an ultra-low power, 3-axis, ±2 g/±4 g/±8 g digital output high resolution (1 mg/LSB) accelerometer. Its power consumption is 1.8 µA at 100 samples per second (SPS) and 3.0 µA at 400 SPS, while its motion activated wake-up mode only requires 270 nA. For more details on the ADXL362, see the datasheet here.

ELECTROCARDIOGRAPHY

The AD8233 is a 50 µA low noise single lead analog output biopotential front end. The AD8233 is connected to the electrodes hosted in the top and bottom surfaces of the watch. The configuration of this signal chain is like an ambulatory ECG device. For more details on the AD8233, see the datasheet here.

BIO-IMPEDANCE

Impedance is measured using the AD5940 Impedance AFE. Proper electrical contact between these two electrodes and the skin is critical for accurate and reliable long-term measurement. Note that these two electrodes are also used by the ECG measurement, which temporarily shorts them together. Therefore, the impedance measurement is not valid while the ECG measurement is ongoing. For more details on the AD5940, see the datasheet here.

SKIN AND AMBIENT TEMPERATURE

The skin temperature measurement is based on a thermistor. The thermistor used in the skin temperature measurement is thermally coupled to the bottom of the watch. This thermistor is connected to one of the analog inputs of the ADPD4100.

USE CASES

Basic modes of operation have been designed for the VSM Watch that are selectable by supplied Device Configuration Files:
•  High Performance PPG
•  Synchronized PGG with EDA
•  Synchronized PPG with ECG spot-check
•  High Performance ECG spot-check
•  Multi-wavelength PPG
•  Future Use Cases will continue to be developed

These modes of operation are intended to demonstrate the different types of configurations that are possible with the VSM Watch but are not specific to an end-application. The high configurability of the VSM Watch allows for the possibility to program a configuration that cannot be supported by the existing hardware, software, and firmware. Users can load these Use Cases as a known-good starting to point to explore measurements of interest before modifying the platform for their specific purpose.

EMBEDDED ALGORITHMS FOR EVALUATION

PEDOMETER
A pedometer algorithm takes raw data from the 3-axis accelerometer and outputs the steps taken.

AUTOMATIC GAIN CONTROL
The digitized output of the ADPD4100 is fed to this algorithm to ensure that the LED current and AFE gain is configured appropriately to maximize the usefulness of the Optical Signal. The default target is 70% of the allowable range for each LED (independently determined). This Gain Control is not optimized to achieve an ideal performance versus power. Further improvements can be made based on the end application’s requirements to achieve a better battery life.

HEART RATE MONITORING
This algorithm measures heart rate using PPG/ADPD signal while removing the motion-based interference. It operates on a single channel PPG/ADPD signal, together with 3-axis accelerometer data to produce the heart rate. The algorithm is provided as a pre-built Cortex-M4 library along with header file. It is designed to work with PPG and accelerometer data at 50Hz.

SIGNAL QUALITY INDEX
PPG signals collected via wearable devices are prone to noise sources and other artifacts that negatively impact the measurement accuracy of the sensor. The Signal Quality Index (SQI) algorithm gives a score (index) for each time window/segment of PPG data to determine if it is of a sufficiently high quality to be useful for other vital sign extraction or clinical diagnostic algorithms estimate of the heart rate. The SQI feature is supported for PPG signal frequencies ranging from 25-100Hz. The SQI score is a floating-point value between 0(poor signal quality) and 1(excellent quality).

ECG HEART RATE MONITORING
This algorithm measures heart rate from the ECG signal by detecting the QRS peak of the ECG signal. The algorithm is provided as a pre-built Cortex-M4 library along with header file. It is designed to work with ECG signal with ODR up to 200Hz.

Developer Resources

Arrow and Analog Devices have provided a rich set of technical collateral and user documentation, all available online at Arrow’s Github site. Follow the links below to the Github Repository of HW and SW collateral as well as the online Wiki site.

VSM Watch Github Repo

VSM Watch Wiki Page

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