Circuits and methods for photoplethysmographic sensors

• US 10,219,711 B2
• Filed: 07/29/2016
• Issued: 03/05/2019
• Est. Priority Date: 07/29/2016
• Status: Active Grant

• Current Assignee: Fitbit, Inc. (Alphabet Inc.)
• Original Assignee: Fitbit, Inc. (Alphabet Inc.)
• Inventors: Wu, Anjian, Richards, Peter W., Yuen, Shelten Gee Jao
• Primary Examiner(s): Winakur, Eric
• Assistant Examiner(s): Liu, Chu Chuan (JJ)
• Application Number:
  US15/223,589
• Publication Number:
  US 20180028077A1
• Time in Patent Office: 949 Days
• CPC Class Codes: A61B 2562/0238 Optical sensor arrangements for performing transmission measurements on body tissue
• CPC Class Codes: A61B 5/02433 for infrared radiation
• CPC Class Codes: A61B 5/02438 with portable devices, e.g. worn by the patient
• CPC Class Codes: A61B 5/0255 Recording instruments specially adapted therefor
• CPC Class Codes: A61B 5/0261 using optical means, e.g. infrared light
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/681 Wristwatch-type devices
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence

Abstract

Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps for processing a photoplethysmographic (“PPG”) signal in a monitoring device that monitors a property of blood flow. In some embodiments, the processing includes obtaining a first digital signal representing a detected light signal having a non-pulsatile (e.g., DC) component and a pulsatile component (e.g., AC). An offset control signal is generated from an estimation of the non-pulsatile component and a second digital signal is generated after subtracting the offset control signal from the detected light signal and applying a gain to the subtracted signal. A reconstructed signal is generated that is calculated from the gain and one or more of (i) the first digital signal, and (ii) the second digital signal and the offset control signal.

Claims

1. A method for processing a photoplethysmographic (PPG) signal in a monitoring device that measures a property of blood flow, the method comprising:

causing one or more first light sources to emit light in the green wavelength spectrum during first periods of time;

causing one or more second light sources to collectively emit light in the red and infrared wavelength spectra during second periods of time different from the first periods of time;

obtaining a detected light signal from one or more detectors during the first and second periods of time, the detected light signal including pulsatile and non-pulsatile components;

obtaining a first digital signal representing the detected light signal;

generating, from the first digital signal, an offset control signal from an estimation of the non-pulsatile component;

generating a second digital signal by subtracting the offset control signal from the detected light signal to produce a subtracted signal and then applying a gain to the subtracted signal;

generating a reconstructed signal calculated from the gain, the second digital signal, and the offset control signal;

setting the offset control signal to zero while generating the reconstructed signal for each first period of time;

outputting data derived from portions of the reconstructed signal associated with the first periods of time as a heart rate; and

outputting data derived from portions of the reconstructed signal associated with the second periods of time as a blood oxygenation level.

2. The method of claim 1, wherein generating the offset control signal includes:

selecting a value for the offset control signal such that the second digital signal does not exceed a range threshold.

3. The method of claim 2, wherein generating the offset control signal includes:

selecting an initial value for the offset control signal;

determining that the second digital signal exceeds a range threshold;

selecting a subsequent value for the offset control signal; and

determining that the second digital signal does not exceed the range threshold.

4. The method of claim 1, further comprising:

constructing a digital representation over time of the detected light signal based on the data derived from the reconstructed signals.

5. The method of claim 4, wherein one or more of the second periods of time are consecutive and at least one of the offset control signals and the gains used to generate the reconstructed signal for the one or more consecutive second periods of time varies from one second period of time to the next second period of time.

6. The method of claim 1, wherein the first digital signal is generated by an analog-to-digital converter with sufficient resolution to measure, during the second periods of time, the non-pulsatile component but not the pulsatile component.

7. The method of claim 1, wherein the detected light signal represents the light emitted by the one or more first light sources or the one or more second light sources after interacting with a skin tissue, wherein the method further comprises:

operating a switching component to switch between the one or more first light sources and the one or more second light sources.

8. The method of claim 1, further comprising:

generating the detected light signal using one or more first photodetectors during the first periods of time,

generating the detected light signal using one or more second photodetectors during the second periods of time, and

operating a switching component to switch between the one or more first photodetectors and the one or more second photodetectors.

9. The method of claim 1, wherein the one or more first light sources are configured to emit light in a wavelength region of approximately 528 nm.

10. The method of claim 1, wherein the one or more second light sources are collectively configured to emit light in wavelength regions of approximately 660 nm and approximately 940 nm.

11. The method of claim 1, wherein:

the one or more first light sources are configured to emit light in a wavelength region of approximately 528 nm,

the one or more second light sources are collectively configured to emit light in wavelength regions of approximately 660 nm and approximately 940 nm.

12. The method of claim 7, wherein the operating the switching component to switch between the one or more first light sources and the one or more second light sources is based on a selected mode of operation of the monitoring device.

13. The method of claim 1, further comprising:

removing, from the first digital signal and prior to generating the second digital signal, a signal corresponding to a component of the detected light signal that is not the result of operation of the one or more first light sources or the one or more second light sources.

14. The method of claim 1, further comprising storing one or more of the first digital signal, the offset control signal, and the second digital signal in a memory in a time series data set.

15. The method of claim 8, further comprising:

operating an emitter switching component to switch between the one or more first light sources and the one or more second light sources.

16. A photoplethysmographic monitoring device, comprising:

one or more first light sources configured to emit light in the green wavelength spectrum;

one or more second light sources configured to collectively emit light in the red and infrared wavelength spectra;

one or more detectors;

one or more processors; and

a non-transitory, computer-readable medium storing instructions which, when executed, cause the one or more processors to;

cause one or more first light sources to emit light in the green wavelength spectrum during first periods of time;

cause one or more second light sources to collectively emit light in the red and infrared wavelength spectra during second periods of time different from the first periods of time;

cause a detected light signal to be obtained from one or more detectors during the first and second periods of time, the detected light signal including pulsatile and non-pulsatile components;

cause a first digital signal representing the detected light signal to be obtained;

cause an offset control signal to be generated from an estimation of the non-pulsatile component in the first digital signal;

cause a second digital signal to be generated by subtracting the offset control signal from the first digital signal to produce a subtracted signal and then applying a gain to the subtracted signal;

cause a reconstructed signal to be calculated from the gain, the second digital signal, and the offset control signal;

cause the offset control signal to be set to zero while generating the reconstructed signal for each first period of time;

cause data derived from portions of the reconstructed signal associated with the first periods of time to be output as a heart rate; and

cause data derived from portions of the reconstructed signal associated with the second periods of time to be output as a blood oxygenation level.

17. The photoplethysmographic monitoring device of claim 16, wherein the non-transitory computer-readable medium stores further instructions which, when executed, cause the one or more processors to:

operate a switch component to switch between the one or more first light sources and the one or more second light sources.

18. The photoplethysmographic monitoring device of claim 16, wherein the one or more detectors includes one or more first photodetectors and one or more second photodetectors and wherein the non-transitory computer-readable medium stores further instructions which, when executed, cause the one or more processors to:

generate the detected light signal using one or more first photodetectors during the first periods of time, and

generate the detected light signal using one or more second photodetectors during the second periods of time.

19. The photoplethysmographic monitoring device of claim 16, wherein the one or more first light sources are configured to emit light in a wavelength region of approximately 528 nm and the one or more second light sources, collectively, are configured to emit light in a second wavelength region of approximately 660 nm and a third wavelength region of approximately 940 nm.

20. The photoplethysmographic monitoring device of claim 16, wherein the non-transitory computer-readable medium stores further instructions which, when executed, cause the one or more processors to generate the offset control signal by selecting a value for the offset control signal such that the second digital signal does not exceed a range threshold.

21. The photoplethysmographic monitoring device of claim 20, wherein the non-transitory computer-readable medium stores further instructions which, when executed, cause the one or more processors to:

select an initial value for the offset control signal;

determine that the second digital signal exceeds a range threshold;

select a subsequent value for the offset control signal; and

determine that the second digital signal does not exceed the range threshold.

22. The photoplethysmographic monitoring device of claim 16, wherein the non-transitory computer-readable medium stores further instructions which, when executed, cause the one or more processors to:

construct a digital representation over time of the detected light signal based on the data derived from the reconstructed signals.

23. The photoplethysmographic monitoring device of claim 22, wherein one or more of the second periods of time are consecutive and at least one of the offset control signals and the gains used to generate the reconstructed signal for the one or more consecutive second periods of time varies from one second period of time to the next second period of time.

24. The photoplethysmographic monitoring device of claim 16, further comprising an analog-to-digital converter with sufficient resolution to measure, during the second periods of time, the non-pulsatile component but not the pulsatile component, wherein the analog-to-digital converter is configured to generate the first digital signal.

25. A non-transitory computer-readable medium storing computer-executable instructions for processing a photoplethysmographic signal in a monitoring device that measures a property of a blood flow, the computer-executable instructions, when executed, causing one or more processors to:

cause one or more first light sources to emit light in the green wavelength spectrum during first periods of time;

cause one or more second light sources to collectively emit light in the red and infrared wavelength spectra during second periods of time different from the first periods of time;

cause a detected light signal to be obtained from one or more detectors during the first and second periods of time, the detected light signal including pulsatile and non-pulsatile components;

cause a first digital signal representing the detected light signal to be obtained;

cause an offset control signal to be generated from an estimation of the non-pulsatile component in the first digital signal;

cause a second digital signal to be generated by subtracting the offset control signal from the first digital signal to produce a subtracted signal and then applying a gain to the subtracted signal;

cause a reconstructed signal to be calculated from the gain, the second digital signal, and the offset control signal;

cause the offset control signal to be set to zero while generating the reconstructed signal for each first period of time;

cause data derived from portions of the reconstructed signal associated with the first periods of time to be output as a heart rate; and

cause data derived from portions of the reconstructed signal associated with the second periods of time to be output as a blood oxygenation level.

26. The non-transitory computer-readable medium of claim 25, further storing computer-executable instructions that, when executed, further cause the one or more processors to:

select a value for the offset control signal such that the second digital signal does not exceed a range threshold.

27. The non-transitory computer-readable medium of claim 26, further storing computer-executable instructions that, when executed, further cause the one or more processors to:

select an initial value for the offset control signal;

determine that the second digital signal exceeds a range threshold;

select a subsequent value for the offset control signal; and

determine that the second digital signal does not exceed the range threshold.

28. The non-transitory computer-readable medium of claim 25, further storing computer-executable instructions that, when executed, further cause the one or more processors to:

construct a digital representation over time of the detected light signal based on the data derived from the reconstructed signals.

29. The non-transitory computer-readable medium of claim 28, wherein:

one or more of the second periods of time are consecutive and at least one of the offset control signals and the gains used to generate the reconstructed signal for the one or more consecutive second periods of time varies from one second period of time to the next second period of time.