Architecture

Individual pixels are aggregated into multiple Macro-pixel arrays, which in turn comprise a cluster. Typically a sensor will include up to four independent clusters whose output is averaged before conversion into a digital signal.

Our chip

SB140

The SB140 is a monolithic PPG sensor that enables high-fidelity PPG signal sensing with an ultra-low power consumption.

SB140

Characteristics

  • Photosensitive pixel array (2x32x32 pixels)
  • High quantum efficiency (QE) of >85%
  • Analog Front-End (AFE) for PPG applications
  • Integrated analogue-domain ambient light cancellation
  • Proximity Detection
  • Analog-to-Digital Conversion (ADC) with variable resolution up to 18 bits
  • 3 fully programmable LED drivers for applications such as oximetry
  • Internal and external clocking options
  • Autonomous sampling frequencies from 8 to 128 Hz
  • FIFO depth from 1 to 128 words
  • I²C interface up to 1 MHz
  • Over 90db SNR for DC light sensing
  • µA level consumption

As different light wavelengths are required depending the application, the SB140 has a high QE over a large spectrum

Key PPG challenges in the ear

Precision and reliability of measurements

Each biometric vital sign that can be detected through PPG requires different signal processing. This requires a raw signal with high fidelity to facilitate the extraction of the right information. The SB solution does this without applying special filters as the “filter approach” gives a better-looking signal but removes part of the information useful for accurate detection of signals such as the heart Interbeat interval (IBI) and Blood Pressure (BP)

Power requirement

PPG sensors’ power consumption is concentrated in two functional areas:

  • The Analog Front End (AFE)
  • Current to drive the illumination LEDs
Miniaturization

PPG modules usually consume a lot of space as they need three components:

  • The AFE
  • The photodiodes
  • The LEDs

Solution

High-Fidelity PPG Sensing

We have designed a high-fidelity PPG sensor in the form of a custom IC only 5.76 mm² in size, a fraction of any alternative, making it the ideal choice for incorporation into PPG-enabled earbuds.

Lowest Possible Power Consumption

As a result of our macro-pixel array’s extremely high sensitivity and a novel analog front end design, the amount of current needed to drive the LED sources in our PPG modules is greatly reduced.

Monolithic

Our custom IC integrates our advanced CMOS image sensor pixel array design, analog front end (AFE), and all interface and control circuitry into a single chip.

Multifunctional

The Senbiosys chip architecture allows operation in various modes, reducing the need for additional components:

  • PPG(Photoplethysmography)
  • ALS(Ambient Light Sensing)
  • Proximity Detection

White papers

Introduction to PPG

Every photon counts

Why the PPG signal fidelity is crucial in vitals extraction?

Earbud-Embedded Micro-Power mm-Sized Optical Sensor for Accurate Heart Beat Monitoring

Ring-Embedded Micro-Power mm-Sized Optical Sensor for Accurate Heart Beat Monitoring

FAQ

Can the PPG hardware be resistant to ambient light interference? Mechanism for ambient light suppression?

The PPG sensor is ambient light resilient. We measured 80dB ambient light rejection for an input dynamic ambient light oscillating at 100Hz.

The mechanism of ambient light suppression:
The sensor integrates light within two widows. During the first window, only ambient light is integrated (the LED remains off). During the second window, both the ambient light and the PPG light are integrated. The two samples are then subtracted in order to cancel ambient light. The time laps between the 2 samples can be set between 60μs and 140μs. Hence, all light variations slower than 60μs can be efficiently cancelled as soon as their intensity does not saturate the sensor. In case of high intensity ambient light (direct exposure to 100 klux) the sampling windows and the LED pulse can be reduced up to 2 μs in order to avoid saturation in such extreme light exposure.

Can ALS be resistant to infrared and flicker light sources?

The ALS can only be resilient to infrared if an infra-red blocker is applied on the optical window of the sensor.

The ALS performs a direct light to digital conversion of light sample integrated in window that can be set between 2 μs and 70 μs. The way to resist to Flicker light is to average multiple samples. This must be done at the MCU level, SB140 does not perform this averaging internally.

PPG use the highest sampling frequency, how to work with ALS?

ALS is an on-demand function. The PPG is continuous function. In order to perform an ALS the user must send a command that stops PPG for one sample and feeds and ALS measurement.

Please note that when the SB140 sensor performs PPG, it automatically senses ambient light and cancels it from the PPG samples. What is fed to the user at the output is the result of the subtraction of the ambient light from the PPG signal.

Is the design of PD and led center distance optimized?

The design of the PD and led are optimized based on optical simulations and experimental results. These optimizations are iterated for each body location and targeted form factor of
the module.

What about Led wavelength, PD spectral response curve and sensitivity?

Senbiosys chips are fully integrated imagers dedicated for PPG. There are no PDs. There is instead an array of integrated pixels implementing pinned photodiodes.

The curves below show the spectral response (quantum efficiency) of our optimized pinned photodiode pixel technology.

What about AFE signal to noise ratio (test method)?

The signal to noise ratio is measured by exposing the sensor to a DC light and measuring the noise at its output.

The following figure shows the measured SNR of SB220:

The maximum DC SNR goes close to 80dB on the raw data. Only the frequencies within the physiological range are interesting for PPG signal processing, that is why PPG signal is often low pass or band-pass filtered. When a digital filter is applied, the DC SNR reaches 88dB. The used filter is 2nd order Butterworth of a cut off freq. of 6Hz.

AFE power consumption, whether hardware synchronization port is reserved between multiple PD

Senbiosys chips are fully integrated imagers dedicated for PPG. There are no PDs. There is instead an array of integrated pixels implementing pinned photodiodes. The Senbiosys’ chips do not use external photodiodes.

The power consumption of SB220 at 122Hz sampling rate is 60μA.