We are using an integrated a 2-in-1 PPG (Photoplethysmography) and ECG (Electrocardiography) bio-sensing AFE (Analog Front-End) chip designed for health and fitness devices. This chip enables sampling of these two bio signals simultaneously with high sensitivity and sampling rate. This novel simultaneous sampling technology allows to calculated blood pressure.
The ECG channel of the AFE supports two-electrode (Einthoven I) mode with right leg drive. The AFE integrates a programmable gain amplifier (PGA), and a 24-bit sigma-delta analog-to-digital converter (ADC) to sense and digitize the ECG signal. Extremely high input impedance (500 MOhm) facilitates the use of two dry electrodes, in order to optimize product form factor, electrode cost and superior signal quality.
The MAP watch uses a four electrode configuration to mitigate the noise coupling into the measurement. Two electrodes are located on the back side, furthermore the watch case acts as two additional electrodes. We are currently evaluating the design according to stringent AAMI EC-11 medical standard.
The AFE chip comprises of two LED drivers, a high-end current-mode front end, and a flexible timing control for PPG channel. The flexible timing control enables the users to control the device timing for different applications and to power down the device for power saving. The front end can sense up to four channels (two for signal and two for ambient) by time multiplexing. Both LED drive, and PPG input circuit is highly programmable to allow perfect signal quality in all conditions, including variety of skin color and environment lightning condition. The PPG channel has more than 80 dB Common Mode Rejection Ration (CMMR) at line frequency.
The AFE IC contains built-in heartbeat interval estimation for PPG signals to optimize power consumption for sleep heart rate monitoring. This feature allows the external MCU to stay in idle mode when motion artifact cancellation is not required.
We are using an ultra-low power IMU (Inertial Measurement Unit) which includes a 16 bit triaxial accelerometer + 16 bit triaxial gyroscope.
The sensor includes advanced features like, step detector step counter which are available in the lowest power mode too.
Provided information is processed by the MAP watch in order to able to detect special events like fall of the user. Moreover all movement data is forwarded to the cloud for further analysis.
We are using one of the EDA electrodes to measure body temperature. The shape and poisition of the electrode were designed in order to have optimal thermal contact with the body.
Right under the electrode we are using an ultra-small, high-precision, low-power CMOS analog temperature sensor with typical accuracy of ±0.05°C.
The raw temperature data is corrected by the central processor in order to reduce the dependency of external temperature, and heat generated by the MAP device.
Thermal mass of the sensor area is highly optimized allowing fast response to skin temperature change.
Electrodermal activity (EDA) is the property of the human body that causes continuous variation in the electrical characteristics of the skin. Many times it is called Galvanic Skin Response (GSR).
EDA is a common measure of autonomic nervous system activity. We are measuring the skin conductance in wide impedance range, with a safe IEC 60601-1 compliant constant current source.
The current source has 0.2% initial accuracy. Measured voltage is fed to a 24 bit sigma-delta ADC. The low power ADC has internal 50/60Hz notch-filter, allowing precision measurements, end excellent common mode rejection.
The measurement sub-circuit is highly optimized for low power consumption, allowing continuous operation.