The compact and flexible ADPD4000 Multimodal Sensor Front End revolutionizes wearable vital signs monitoring (VSM) devices. Most people are familiar with many wearable VSM devices including activity trackers and full-featured smart watches. At Analog Devices, we see wearables as a critical piece of today’s digital health initiatives moving healthcare from reactive to preventive and predictive care in an effort to improve wellness management.

The #1 goal of healthcare-oriented wearables is to provide continuous accurate biometric information, resulting in useful insights for healthcare professionals. Critical VSM parameters include heart rate, oxygen saturation, electrocardiography (ECG), blood pressure, and respiration rate. We believe a multi-parameter enabled wearable bridges today’s gap between consumer wearables and clinical patient monitors.
ADPD4000 efficiently measures, amplifies, filters and digitizes multiple biometric signals in a compact (2.1mm x 3.1mm) package, including:
1. Photometric measurement that enable PPG (photoplethysmography) sensing that can be used to derive heart rate, SpO2 and blood pressure.
2. Bio-potential measurement that enable ECG (electrocardiography) sensing
3. Bio-impedance measurement that enable GSR (galvanic skin response) and thoracic impedance (respiration rate) sensing
Many VSM analog front ends (AFEs) can measure one or two parameters, but typically, as the parameter count grows, so does the required number of AFEs. This results in both increased size and increased power consumption. In order to generate the personalized healthcare insights demanded by digital health initiatives, accurate VSM devices must be low power, small and light weight. ADPD4000’s unique architecture enables best-in-class SNR/Watt optical measurements at the same time as precision ECG and/or impedance and temperature measurements.

ADPD4000 enables true multi-parameter sensing
ADPD4000 can be programmed as an optical front-end measuring heart rate or oxygen saturation by using multiple wavelengths. In addition, different channels simultaneously measure bio-potential signals like ECG using external electrodes connected to the body. Moreover, bio-impedance can be measured with small excitation currents and the same high precision signal chain for signal processing. In all, up to 8 input channels support endless design flexibility and configurability including, for example, a simultaneous and synchronized ECG and PPG measurement to estimate complex bio-signals such as blood pressure.
In addition, configuration pre-programmability maximizes system design flexibility while offloading processor resources, which helps to reduce overall power consumption and footprint of the wearable. On chip oversampling and averaging improve measurement fidelity or ENOB (effective number of bits), and measurement results are stored in a 256-byte integrated FIFO register.

ADPD4000 makes accurate ECG measurements, even with dry electrodes
ECG measurement is nothing new in the clinical world. However, emerging digital health initiatives aim to solve declining global caregiver-to-patient ratios through at-home automated pre-screening of ECG measurements. High quality wet electrodes have a limited lifetime and can cause patient irritation. However, with patent pending IP, ADPD4000 provides accurate ECG measurements with dry electrodes which provide much greater patient comfort and electrode lifetime (14-21 days). Instead of a traditional ECG voltage input, the ADPD4000 measures electrical charge accumulated on a sensing capacitor. The charge measurement process reduces sensitivity to variations in the skin-to-electrode contact impedance. Figure 1 shows the applications circuit with inherent immunity to variations in the contact impedance.

ADPD4000 enables multi-wavelength PPG sensing to get richer optical insights from the body
Today’s VSM wearables typically measure PPG from a single wavelength green LED(s), but the ADPD4000 comes with 8 independently controllable LED drivers with adjustable output current per channel. Combined with a flexible timing engine, the device enables multi-wavelength measurements of, for example, heart rate, SpO2 (oxygen saturation) or more advanced hydration or blood pressure measurements. The receive signal chains have a programmable transimpedance amplifier followed by a dual-stage rejection block to cancel out ambient light interferers. The signal-to-noise ratio (SNR) of the transmit/receive signal chain can be fine tuned to trade-off power consumption and targeted signal quality using a unique multi-pulse architecture. It is this architecture that also enables ADPD4000 to achieve best-in-class SNR (Signal-to-Noise) per Watt measurements which drives extended battery life in wearable applications.

ADPD4000 utilizes the simplest method to measure bio-impedance
Next generation wearable systems will also measure skin conductance (or Galvanic skin resistance, GSR) which is an indicator of stress and other neurological conditions. The ADPD4000’s highly programmable GPIO can output excitation signals at various frequencies while the integrated transimpedance amplifier measures the current response to get relative body impedance measurements.
In summary, wearable device designers are challenged to make accurate biometric measurements while consuming minimal power in a small form factor. The ADPD4000 solves these challenges and enables the next generation of wrist-worn wearables, body patches and drug delivery systems. For each of these applications, performance, size, and power consumption are critical specifications. Creating a high performance multi-parameter wearable device becomes as simple as combining the ADPD4000 with a microcontroller and communications IC, a battery and electrodes. The ADPD4000, with its high-performance multi-channel sensor input flexibility, stimuli channels, digital processing engine, and timing control, meets all these requirements and beyond.