Health monitoring device including pinned photodiode

1. A photoplethysmography (PPG) sensing device, configured to output a signal comprising a DC component and an AC component, said device comprising:

a pulsed light source, adapted to be pulsed-on or pulsed-off,

at least one pixel for light-to-charge conversion to create photo-generated electrons,

each pixel being synchronized with said pulsed light source, characterized in that each pixel comprises:

a pinned photodiode comprising a well and having two electronic connection nodes,

a sense node, said sense node being a junction capacitance whose role is to convert the photo-generated electrons in the pinned photodiode well into a voltage, by the means of the conversion gain of said capacitance, and

a Transfer Gate (TGtransfer) transistor, having its source electronically connected to one electronic connection node of said pinned photodiode, and being configured to act as a transfer gate between said pinned photodiode and said sense node, 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.

2. The photoplethysmography (PPG) sensing device according to claim 1 , wherein each pixel further comprises:

a Sink (TGsink) transistor, having its source electronically connected to said pinned photodiode, and being configured to act as a transfer gate between said pinned photodiode and either a constant DC power source or a capacitance, allowing the photo-generated electrons to sink towards said constant DC power source or said capacitance when the light is pulsed-off and the photo-generated electrons integration when the light is pulsed-on.

3. The photoplethysmography (PPG) sensing device according to claim 2 , wherein each of said pixels comprises at least one further pinned photodiode, arranged in parallel with said pinned photodiode and connected to said sense node by means of a respective Transfer Gate (TGtransfer) transistor, each of said Transfer Gate (TGtransfer) transistors being configured to be operated synchronously.

4. The photoplethysmography (PPG) sensing device according to claim 3 , wherein each of said further pinned photodiodes is connected to said constant DC power source or said capacitance, as appropriate, by means of a respective Sink (TGsink) transistor, the gate of each of said Sink (TGsink) transistors being configured to be operated synchronously.

5. The photoplethysmography (PPG) sensing device according to claim 1 , comprising a plurality of said pixels, wherein said pixels are disposed as an array.

6. The photoplethysmography (PPG) sensing device according to claim 5 , further comprising a processor configured to average spatially outputs of the pixels.

7. The photoplethysmography (PPG) sensing device according to claim 1 , further comprising:

A first block (CDS 1 ) comprising:

a capacitance to store a value of the output signal of the photoplethysmography sensing device, when the pulsed light source is off and the sense node (SN) well is empty, and

a capacitance to store a value of the output signal of the photoplethysmography (PPG) sensing device, when the pulsed light source is off, and the photoplethysmography (PPG) sensing device is just subject to ambient light, and to store a value of the output signal of the photoplethysmography (PPG) sensing device when the pulsed light source is on.

8. The photoplethysmography (PPG) sensing device according to claim 1 , comprising a block comprising:

a capacitance to store a first value of the output signal of the photoplethysmography (PPG) sensing device, when the pulsed light source is on during a first pulse and the sense node well has been allowed to integrate for a predetermined time period, and

a capacitance to store a second value of the output signal of the photoplethysmography (PPG) sensing device when the pulsed light source is on during a second pulse, and the sense node well has been allowed to integrate for a predetermined time period.

9. The photoplethysmography (PPG) sensing device according to claim 1 , comprising a block (CDS) comprising:

a capacitance to store a value of the output signal of the photoplethysmography (PPG) sensing device, when the pulsed light source is off and the sense node well has been allowed to integrate for a predetermined time period, and

a capacitance to store a value of the output signal of the photoplethysmography (PPG) sensing device when the pulsed light source is on, and the sense node well has been allowed to integrate for a predetermined time period.

10. The photoplethysmography (PPG) sensing device according to claim 1 , wherein said sensing device is built in CMOS technology, and wherein each pixel is embedded in a system on chip.

11. The method of operating a photoplethysmography (PPG) sensing device, comprising:

pulsing a light source, adapted to be pulsed-on or pulsed-off;

detecting light from the light source with a pinned photodiode for light-to-charge conversion to create photo-generated electrons;

in a transfer phase, setting a Transfer Gate voltage of a transfer gate transistor to a value between a well potential of the pinned photodiode and a voltage applied to a Sink transistor to allow only photo-generated electrons exceeding an offset to be transferred to a sense node for read-out.

12. A method according to claim 11 wherein the Transfer Gate voltage is dynamically adapted.

13. A method according to claim 11 comprising a calibration step comprising:

A. setting the value of a DC power source to a predetermined value,

B. checking if a pixel response exceeds a predetermined threshold value, and

C. if the pixel response exceeds the said threshold value, repeating steps A and B until the corresponding pixel response is no longer exceeding the threshold.

14. The method according to claim 11 , comprising the steps of:

pulsing-off the light source, and

integrating charge in said pinned photodiode during a predetermined period of time;

transferring said charge to said sense node by means of said transfer gate and storing said charge in a first capacitor so as to generate a voltage corresponding to ambient light; then

pulsing-on the light source for at least said predetermined period of time,

integrating charge in said pinned photodiode during said predetermined period of time;

transferring said charge to said sense node by means of said transfer gate and storing said charge in a second capacitor so as to generate a voltage corresponding to ambient light mixed with the pulsed light source; then

subtracting the voltage corresponding to the ambient light from the voltage corresponding to the pulsing-on of the pulsed light source mixed with the ambient light to result in a voltage corresponding to a function of the detected light originating from the pulsed light source exclusively.

15. The method according to claim 11 , comprising:

pulsing-on a pulsed light source,

integrating charge in said pinned photodiode during a predetermined period of time;

transferring said charge to said sense node by means of said transfer gate transistor and storing said charge in a first capacitor so as to generate a voltage corresponding to the ambient light mixed with the light source; then

pulsing-off the light source, and

integrating charge in said pinned photodiode during said predetermined period of time;

transferring said charge to said sense node by means of said transfer gate transistor and storing said charge in a second capacitor so as to generate a voltage corresponding to ambient light; then

subtracting the voltage corresponding to the ambient light from the voltage corresponding to the pulsing-on of the light source mixed with the ambient light to result in a voltage corresponding to a function of the detected light originating from the pulsed light source exclusively.

16. The method according to claim 11 , comprising:

pulsing-on the light source,

integrating charge in said pinned photodiode during a predetermined period of time;

transferring said charge to said sense node by means of said transfer gate and storing said charge in a first capacitor so as to generate a voltage corresponding to the ambient light mixed with the light source; then

pulsing-on again the light source, and

integrating charge in said pinned photodiode during said predetermined period of time;

transferring said charge to said sense node by means of said transfer gate and storing said charge in a second capacitor so as to generate a voltage corresponding to ambient light mixed with the second sample of pulsed light source.

17. The method according to claim 11 , comprising:

pulsing-off a pulsed light source,

integrating charge in said pinned photodiode during a predetermined period of time; transferring said charge to said sense node by means of said transfer gate transistor and storing said charge in a first capacitor so as to generate a voltage corresponding to the ambient light mixed with the reset voltage; then

pulsing-on the pulsed light source, and

integrating charge in said pinned photodiode during said predetermined period of time.

18. The method according to claim 11 , wherein said photoplethysmography (PPG) sensing device comprises a plurality of pixels, said method further comprising a step of averaging spatially the outputs of each pixel.

19. The method according to claim 18 where the averaging of the pixels is performed in a passive switched-capacitor fashion through the connection of a plurality of capacitors leading to an electronic buffer.