Next Generation Biometric Sensing in Wearable DevicesC O L I N T O M P K I N S

D I R E C T O R O F A P P L I C AT I O N S E N G I N E E R I N G

S I L I C O N L A B S

C O L I N .T O M P K I N S @ S I L A B S . C O M

Wearables & 3rd Wave of Computing

DESKTOP COMPUTERS

LAPTOPS TABLETS

HANDSETS

INTERNET OF THINGS

CONNECTED HOME

SMART CITIES

WEARABLES

Next Generation Wearable Sensing

▪Other Biometrics include SpO2, Stress, Recovery, Hydration

▪High Performance optical sensing, EDA and ECG sensing

Today’sWearables

Next Gen Wearables

ENVIRONMENT BIOMETRICS

Heart Rate

MOTION

Ambient Light

BarometerUV Index

Temperature

Heart Rate++Skin TempOther Biometrics

Accelerometer Magnetometer

GPS

Gyroscope/Accelerometer+

Next Gen Motion Sensing

▪Next generation motion sensing▪Repetition counting

▪Exercise identification

▪ Form and technique

▪Coaching and guidance

▪Barometer▪Combined with motion or GPS location tracking

▪Ambient Temperature Sensing and Skin Temperature Sensing▪Difficult to implement

▪UV Index Sensing▪Recommend diffusor & cumulative UV exposure sensing

▪UV transparent overlay

Next Gen Environmental Sensing

Subdermal Tissue

Photoplethysmography Basics

▪ LED to Photodiode ~1.5 to 5 mm

▪Green LED Light is modulated by the blood flow causing capillary diameter changes

▪The green light is greatly attenuated by tissue

▪Green light works well ▪ Red and IR works for finger tip but not at

wrist Green LED Sensor PD

Optical Blocking is critical

Skin

100 us pulses repeated at 25 Hz

Attenuated and Modulated light

Photoplethysmography Examples

▪Stationary Subject▪Modulation is 0.5% RMS in this case

▪Very Clear, Low noise heart rate Signal ~75 BPM

▪Exercising Subject ▪Many motion artifacts

▪Needs special software and accelerometer signal to extract HR signal

Sample # (100 Hz)

HR Period

Modulation

Sample # (100 Hz)

Heart Rate Sensor System Design

▪Optical hardware design ▪Photodiode to LED spacing

▪Overlay

▪Modules solve many system challenges

LED

SKIN

Glass Overlay

Photodiode

Lens Lens

Clear Encapsulant

Blocking Plastic

Substrate

Clear Encapsulant

HRM Module3.7 x 7.0 x 1.1 mm

Discrete PD + 2 LED’s~5.06 x 4.26 mm

Improved PPG Overlay and Fit

▪Blocking Overlay▪Highest performance due to zero crosstalk

▪Minimize overlay thickness

▪Minimize module to overlay distance

▪Band and fit considerations▪ Sensor protrusion

▪ Fit and secure band

High Performance PPG Design

▪RAM & MCU memory and MIPS

▪Clock stability target <1%▪More important with FIFO/sample buffer

▪Avoids high MCU power for synchronizing

▪Synchronization between sensors▪Accelerometer interface supports data

synchronization

▪Higher performance optical sensors▪Multiple LED’s, spacing from sensor to LED to

make power/performance tradeoffs

▪Noise mitigation becomes more critical: ambient light cancellation and motion artifact cancellation

0

20

40

60

80

100

120

0 50 100 150 200 250

SNR

[d

B]

Integration Time [μs]

Optical Sensor ADC SNR

Power

Time

SleepSignal

Processing

System Power Considerations

▪Techniques to extend battery life▪High performance sensor system + good optical design enables more signal for

less light/power

▪ Shorter LED duration, integration time while maintaining high SNR results at lower power

▪ Sample buffer reduces CPU bus transaction times

▪Wireless radio update interval as long as practical

Wireless Transaction

HRM Signal Capture

PPG+ Enables New Biometrics

▪Heart Rate Variability (HRV)

▪Hydration

▪ SpO2

▪Recovery rate

▪ Sleep Quality

P u l s e O x i m e t r y

▪Electrocardiogram (ECG)▪ Sensing heart’s electrical signals

▪Chest strap most common tool for this today

▪Used for heartbeat waveform analysis

▪Bio-Impedance or tissue impedance sensing▪Most common method to analyze body fat

▪Can be used for hydration measurements

▪Alternate method of sensing heart rate

ECG & Bio-impedance

Heart Rate Measurement Algorithm Considerations

▪Motion cadence may equal heart rate

▪Algorithm uses multiple factors to determine HR

▪Algorithm affects accuracy of biometrics & dictates power consumption of HRM system

▪Tradeoffs for performance versus power consumption and cost of solution▪ Market segments have different requirements

▪ Factors affecting performance▪ Sensor/photodetector selection, optical design

▪ LED current dominates power consumption

▪ Algorithm performance variations and tuning vs particular activity

▪ Automatic motion detection as input to HRM algorithm

▪Reducing noise critical to performance▪ Band design very important to reduce motion noise

▪ Ambient light noise

▪ Accelerometer to cancel motion noise

Biometric Performance Optimization

Next Gen Wearable

Wearable 3.0 Motion Tracking

Accurate Heart Rate Monitoring

Accurate Calorie Estimate

Stress

Hydration

UV Index & Comfort Level

Training effect & fitness Level

Guidance and Coaching

Time for Questions