Search

Nowadays, efficient and remote health monitoring is becoming increasingly important, given both the ageing of population and the combined action of an increase in obesity level and cardiovascular diseases. The healthcare industry is becoming more reliant on new methods to monitor and treat patients. This, along with an increased interest in fitness and wellness, is calling for more affordable, precise and wearable health monitoring devices. In this context, photoplethysmography (PPG) appears to be a key technology allowing non-invasive monitoring of vital biological indicators such as the heart rate, the blood oxygen saturation, the respiration rate and the arterial blood pressure. A standard PPG system comprises pulsed LEDs synchronized with a photosensor and a processing chain. The LEDs diffuse light in the human skin. Processing the signal held by the diffused light allows the extraction of the vital parameters. Despite the great potential behind the PPG technology, the fairly large power burnt by the LEDs still represents a serious challenge towards truly continuous PPG operations, limiting its practical exploitation. State-of-the-art PPG sensors, both in academia and in commercial products, still follow a quite standard design paradigm. Indeed, they rely on off-chip photodiodes and relatively standard circuitry. The commercially available smartwatches and wearables fall short of meeting customer requirements in terms of reliability, precision and battery lifetime. In this regard, we should not expect any dramatic improvement unless there are fundamental changes in the PPG sensor technology. This is particularly true on the photosensor side, since its parasitic capacitance represents one of the limiting factor in terms of power/noise. Pinned-photodiode (PPD) are today the key ingredients of CMOS image sensors, thanks to the low dark current, low noise and large sensitivity operations. Several markets including security, scientific and medical imaging are relying today on this technology. The excellent performance of a PPD makes it particularly interesting for the PPG application. Indeed, the LED power can be reduced provided the noise floor is decreased proportionally. In this work a truly micropower PPG sensor combining an array of double transfer gates (TG) PPDs together with an ultra-low noise read-out chain is presented. Compared to conventional solution, this work achieves the same signal-to-noise ratio (SNR) at a significantly lower LED power. The PPDs array is implemented on the same chip together with the analog front-end including the analog-to-digital conversion. The full CMOS integration allows a dramatic reduction of the parasitic capacitance at the sense node leading to a larger conversion gain and a lower noise. This approach also provides higher miniaturization and lower cost compared to traditional solutions with off-chip PDs. The use of an array additionally enables spatial averaging leading to further noise reduction. Consequently, the LED power can be reduced dramatically. The chip, implemented in a 0.18 µm CMOS Image Sensor (CIS) process, features a total input referred noise of 0.68 e-rms per PPD, independently of the input light, and achieves a 4.6 µW total power consumption, including 1.97 µW LED power, at 1.38 bpm heart rate average error. Compared to the most recent state-of-the-art works this means more than an order of magnitude in power reduction.

The invention relates to a photoplethysmography (PPG) sensing device comprising - a pulsed light source, - at least one pixel to create photo-generated electrons, synchronized with said pulsed light source. It is mainly characterized in that each pixel comprises: - a pinned photodiode (PPD) having two electronic connection nodes, - a sense node (SN), to convert the photo-generated electrons into a voltage, and - a Transfer Gate (TGtransfer) transistor, having its source electronically connected to one electronic connection node of said pinned photodiode (PPD), and being configured to act as a transfer gate (TG) between said pinned photodiode (PPD) and said sense node (SN), allowing the photo-generated electrons to sink when the light is pulsed-off, the photo-generated electrons integration when the light is pulsed-on and the transfer of at least part of the integrated photo-generated electrons to said sense node for a read-out.

A Time-of-flight optical device and a 3D optical detector comprising a CMOS integrated circuit with an array of photosensitive pixels that are, at least in part, interconnected to form macro-pixels (180). Each macro-pixel (180) groups a plurality of individual pixels contributing their photocarriers to a common sense node SN through a plurality of transistors in parallel. Preferably the integrated circuit includes switched capacitor circuits arranged to combine the potential of the sense nodes SN of a plurality of macro-pixels, and/or to perform correlated double samplings in an energy efficient way. Each pixel has now an additional sink gate (194) between the pinned photodetector, PPD, potential well and a positive voltage source. By this additional sink gate (194), the storage well of the PPDs can be emptied without transferring the charge to the sense node. The value of the transfer gate voltage V_TG may be adapted such that the potential barrier is not lowered all the way down, but decreased to a value VB

The current trend towards embedding more and more light sensors in portable and wearable devices is calling for higher integration and reuse of the sensor interface electronics. In many applications, the light needs to be generated locally, which becomes the dominant source of power consumption. Power can hence be saved by making the sensor more sensitive to lower light. At low light the noise is totally dominated by the noise coming from the electronic readout chain. This paper shows how this noise can be minimized all along the readout chain, from the pixel to the ADC. It also shows that many low-light applications actually share the same readout architecture. The latter is made of the pixel including a source follower, a shared amplifier and a correlated double sampling (CDS) or correlated multiple sampling (CMS) stage that is key for eliminating the low-frequency noise and reducing the white noise for CMS. It is shown how each of these building blocks can be optimized for reaching a minimum input-referred noise. The design methodology is then illustrated by three applications, including a 0.5 electon(rms) CMOS VGA imager, a 2.6 mu W PPG sensor and a 10 mu W 1D time-of-flight distance ranging device.