Circuits and methods for photoplethysmographic sensors

• US 9,743,838 B2
• Filed: 10/02/2015
• Issued: 08/29/2017
• Est. Priority Date: 10/02/2015
• Status: Active Grant

• Current Assignee: Fitbit, Inc. (Alphabet Inc.)
• Original Assignee: Fitbit, Inc. (Alphabet Inc.)
• Inventors: Richards, Peter W.
• Primary Examiner(s): Winakur, Eric
• Assistant Examiner(s): Liu, Chu Chuan (JJ)
• Application Number:
  US14/873,921
• Publication Number:
  US 20170095156A1
• Time in Patent Office: 697 Days
• Field of Search: 600310, 600323, 600336, 600340, 600473, 600476, 600500
• CPC Class Codes: A61B 2560/0247 for compensation or correction of the measured physiological value
• CPC Class Codes: A61B 5/0082 adapted for particular medical purposes
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/02427 Details of sensor
• CPC Class Codes: A61B 5/02438 with portable devices, e.g. worn by the patient
• CPC Class Codes: A61B 5/1455 using optical sensors, e.g. spectral photometrical oximeters
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7214 using signal cancellation, e.g. based on input of two identical physiological sensors spaced apart, or based on two signals derived from the same sensor, for different optical wavelengths

Abstract

Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps to remove a component of a signal corresponding to ambient light in a photoplethysmographic sensor device, including capturing a first detected light signal representing an ambient light at a first time, causing a light emitter to generate a source light signal driven at a first level, capturing a second detected light signal representing the source light signal after interacting with a user'"'"'s tissue plus the first detected light signal, generating a first output signal based on the second detected light signal adjusted by the first detected light signal, causing the light emitter to generate a source light signal driven at a second level, capturing a third detected light signal representing the source light signal driven at the second level after interacting with the user'"'"'s skin plus the first detected light signal, and generating a second output signal based on the third detected light signal adjusted by the first detected light signal.

Claims

1. A method for removing a component of a signal corresponding to ambient light in a photoplethysmographic sensor device, the method comprising:

capturing, using a switch in a first position, a first detected light signal and holding the first detected light signal as voltage across a capacitor, the first detected light signal representing ambient light;

causing a light emitter to generate a source light signal driven at a first level;

capturing a second detected light signal, the second detected light signal representing the source light signal after interacting with a user'"'"'s tissue;

generating, using the switch in a second position, a first output signal based on the second detected light signal adjusted by the first detected light signal held in the capacitor;

changing the light emitter to generate a source light signal driven at a second level that is different than the first level;

capturing a third detected light signal, the third detected light signal representing the source light signal driven at the second level after interacting with the user'"'"'s tissue;

generating, using the switch in the second position, a second output signal based on the third detected light signal adjusted by the first detected light signal held in the capacitor; and

changing the light emitter again to generate the source light signal driven at the first level and capturing a fourth detected light signal representing the source light signal driven at the first level interacting with the user'"'"'s tissue;

generating a third output signal based on the fourth detected light signal adjusted by the first detected light signal held in the capacitor; and

generating a photoplethysmographic signal based on a weighted combination of the first, second, and third output signals,

wherein initially detected ambient light is removed during the generating of the first, second, and third output signals, and dynamic changing ambient light is removed as a result of the weighted combination of the first, second, and third output signals.

2. The method of claim 1, wherein the photoplethysmographic signal is generated based on a first difference between a weighted combination of the first and second output signals and a second difference between the first difference and the third output signal that has been weighted.

3. The method of claim 1, wherein the capturing of the first detected light signal further comprises:

activating a light detector to detect the first detected light signal, wherein the first detected light signal represents at least one of (i) an ambient light condition, and (ii) an ambient light condition and a light signal representing a source light signal driven at non-zero level after interacting with the user'"'"'s tissue.

4. The method of claim 1, wherein the first output signal is generated at an output of an amplifier, the amplifier receiving a first input based on the second detected light signal adjusted by the first detected light signal.

5. The method of claim 4, wherein the amplifier is an adjustable gain amplifier.

6. The method of claim 4, further comprising:

deactivating the light emitter after generation of the first output signal.

7. The method of claim 1, further comprising:

converting the first output signal to a digital signal representing a voltage for processing by a processor.

8. The method of claim 1, wherein the first output signal and the second output signal are generated at different times at an output of an amplifier, the amplifier receiving a first input at a first time based on the second detected light signal adjusted by the first detected light signal to generate the first output signal, and the amplifier receiving a second input at a second time based on the third detected light signal adjusted by the first detected light signal to generate the second output signal.

9. The method of claim 1, wherein the source light signal driven at the second level is a higher level than the first level.

10. The method of claim 8, further comprising:

deactivating the light emitter after generation of the second output signal.

11. The method of claim 1, further comprising:

converting the second output signal to a second digital signal representing a voltage for processing by a processor.

12. The method of claim 1, wherein the first output signal, the second output signal and the third output signal are generated at different times at an output of an amplifier, the amplifier receiving a first input at a first time based on the second detected light signal adjusted by the first detected light signal to generate the first output signal, the amplifier receiving a second input at a second time based on the third detected light signal adjusted by the first detected light signal to generate the second output signal, and the amplifier receiving a third input at a third time based on the fourth detected light signal adjusted by the first detected light signal to generate the third output signal.

13. The method of claim 1, wherein the source light signal driven at the first level is a low level, and the source light signal driven at the second level is a high level, where the high level is at least twice the low level.

14. The method of claim 12, further comprising:

deactivating the light emitter after generation of the third output signal.

15. The method of claim 1, further comprising:

converting the third output signal to a third digital voltage signal for processing by a processor.

16. A method performed by a photoplethysmographic sensor device, the method comprising:

causing a light emitter to generate a source light signal driven at a first level;

measuring a first detected light signal, the first detected light signal representing the source light signal driven at the first level interacting with a user'"'"'s skin, and subtracting ambient light from the first detected light signal to generate a first output signal;

increasing the source light signal driven by the light emitter to a second level;

measuring a second detected light signal, the second light signal representing the source light signal increased to the second level interacting with the user'"'"'s skin, and subtracting ambient light from the second detected light signal to generate a second output signal;

decreasing the source light signal driven by the light emitter back to the first level;

measuring a third detected light signal representing the source light signal driven at the first level interacting with the user'"'"'s skin, and subtracting ambient light from the third detected light signal to generate a third output signal; and

generating a photoplethysmographic sample based on a weighted combination of the first, second, and third output signals, wherein dynamic changing ambient light is removed as a result of the weighted combination of the first, second, and third output signals.

17. A non-transitory computer-readable medium storing executable instructions for removing a component of a signal corresponding to ambient light in a photoplethysmographic sensor device, the instructions, when executed, cause one or more processors to:

capture, using a switch in a first position, a first detected light signal and holding the first detected light signal as voltage across a capacitor, the first detected light signal representing ambient light;

cause a light emitter to generate a source light signal driven at a first level;

capture a second detected light signal, the second detected light signal representing the source light signal interacting with a user'"'"'s tissue;

generate, using the switch in a second position, a first output signal based on the second detected light signal adjusted by the first detected light signal held in the capacitor;

change the light emitter to generate a source light signal driven at a second level that is different than the first level;

capture a third detected light signal, the third detected light signal representing the source light signal driven at the second level interacting with the user'"'"'s tissue;

generate, using the switch in the second position, a second output signal based on the third detected light signal adjusted by the first detected light signal held in the capacitor;

change the light emitter again to generate the source light signal driven at the first level and capture a fourth detected light signal representing the source light signal driven at the first level interacting with the user'"'"'s tissue;

generate a third output signal based on the fourth detected light signal adjusted by the first detected light signal held in the capacitor; and

generate a photoplethysmographic signal based on a weighted combination of the first, second, and third output signals,

wherein initially detected ambient light is removed during the generating of the first, second, and third output signals, and dynamic changing ambient light is removed as a result of the weighted combination of the first, second, and third output signals.

18. The computer readable medium of claim 17, wherein the instructions further cause, when executed, the one or more processors to:

activate a light detector to detect the first detected light signal.

19. The computer readable medium of claim 17, wherein the instructions further cause, when executed, the one or more processors to:

deactivate the light emitter after generation of the first output signal, the second output signal and the third output signal.

20. The computer readable medium of claim 17, wherein the source light signal driven at the first level is a low level, and the source light signal driven at the second level is a high level different from the low level.

21. The computer readable medium of claim 17, wherein the instructions further cause, when executed, the one or more processors to:

convert the first output signal, the second output signal, and the third output signal to digital voltage signals for processing by a processor.

22. The computer readable medium of claim 17, wherein the instructions further cause, when executed, the one or more processors to:

generate a heart rate signal after removing a common-mode offset between the first and second output signals.

23. A photoplethysmographic sensor device, comprising:

one or more processors;

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

cause a light emitter to generate a source light signal driven at a first level;

measure a first detected light signal, the first detected light signal representing the source light signal driven at the first level after interacting with a user'"'"'s skin, and subtract ambient light from the first detected light signal to generate a first output signal;

increase the source light signal driven by the light emitter to a second level;

measure a second detected light signal, the second light signal representing the source light signal increased to the second level after interacting with the user'"'"'s skin, and subtract ambient light from the second detected light signal to generate a second output signal;

decrease the source light signal driven by the light emitter back to the first level;

measure a third detected light signal representing the source light signal driven at the first level interacting with the user'"'"'s skin, and subtract ambient light from the third detected light signal to generate a third output signal; and

generate a photoplethysmographic sample based on a weighted combination of the first, second, and third output signals, wherein dynamic changing ambient light is removed as a result of the weighted combination of the first, second, and third output signals.

24. The photoplethysmographic sensor device of claim 23, further comprising:

at least a first light detector configured to detect the source light signal after interacting with the user'"'"'s skin, the at least first light detector providing a signal representing the detected source light signal to a first circuit stage including a low gain amplifier.

25. The photoplethysmographic sensor device of claim 24, further comprising:

a gain stage circuit coupled to receive an output of the first circuit stage and to apply a gain to the output of the first circuit stage.