Multifunction biosensor/vital signs monitor for wearables, from ams

September 30, 2015 // By Graham Prophet
Sensor and mixed-signal IC specialist ams (Graz, Austria) has applied optical detection techniques to produce a sensor capable of accurate heart rate measurement; the company says its device is the first “total solution for 24/7 heart rate measurement for wearables”.

AS7000 is the first in what is to be a family of health/fitness solutions from ams, for wearable devices. The AS7000 solution incorporates a highly integrated optical sensor module accompanied by software to provide the highest accuracy optical heart rate measurements (HRM) and heart rate variation (HRV) readings, backed by opto-mechanical design-in support. The 24/7 reference is to indicate that ams intends the device for products that are worn continuously, rather than (for example) only during periods of exercise.

In a 6.1 x 4.1 x 1.0 mm package, ams presents the AS7000 as the first complete integrated health and fitness solution for wearables; it may enable devices such as fitness bands as well as sports and smart watches to replace cumbersome, uncomfortable electro-cardiogram (ECG) chest straps as the source of HR measurements in fitness and health monitoring applications. The device delivers a processed output in beats per minute, updated once per second.

The module includes the LEDs, photo-sensor, analogue front end (AFE) and controller as well as application software required to implement an accurate optical HRM/HRV fitness band product. The module also accommodates skin temperature and skin resistivity measurements by providing interfaces to external sensors.

The operation of the AS7000 is based on photoplethysmography (PPG), an HRM method which measures the pulse rate by sampling light modulated by the blood vessels, which expand and con-tract as blood pulses through them. Unlike existing optical AFEs, which produce raw PPG readings, the AS7000 integrates a digital processor which implements algorithms developed by ams. These convert the PPG readings into digital HRM and HRV values.

ams observes that deriving accurate and reliable data from the raw measurements in this domain is far from simple. The variation in returned light that is due to expansion and contraction of blood vessels is, in effect, a small AC signal superimposed on a large DC components: around two orders of magnitude difference. Motion, including that due to the heartbeat itself, introduces a further “error” source of motion artifacts, that can be removed by adding an accelerometer (external to the AS7000).

At its introduction, the AS7000 outputs heart rate data; software can follow trends in the HRM data to derive HRV parameters. Skin temperature and resistivity (GSR, galvanic skin resistance) parameters will be added (via software), and ams also has a feasibility study looking at blood pressure monitoring. This can, in principle, be derived from the transcutaneous optical measurement, the company says, but is an even more challenging signal processing task. The same is true for SPO2, or (reflective) oxygen saturation measurement, and that, too, is on the company’s roadmap for the product range.