Circuits and methods for photoplethysmographic sensors

• US 10,624,542 B2
• Filed: 08/28/2017
• Issued: 04/21/2020
• 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 F
• Assistant Examiner(s): Liu, Chu Chuan
• Application Number:
  US15/687,768
• Publication Number:
  US 20180064345A1
• Time in Patent Office: 967 Days
• 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 comprising:

obtaining a first detected light signal representative of an amount of light detected by a skin-facing light detector of a photoplethysmographic (PPG) sensor device at a first time, wherein any skin-facing light emitter or emitters of the PPG sensor device are off at the first time;

causing one or more light emitters of the PPG sensor device to be driven at a first, non-zero level at a second time to generate a first source light signal;

obtaining a second detected light signal representative of the amount of light detected by the light detector at the second time;

causing the one or more light emitters to be driven at a second level higher than the first, non-zero level and at a third time to generate a second source light signal;

obtaining a third detected light signal representative of the amount of light detected by the light detector at the third time;

generating a first output signal based on the second detected light signal adjusted by the first detected light signal;

generating a second output signal based on the third detected light signal adjusted by the first detected light signal; and

generating a PPG signal based, at least in part, on a difference between the second output signal and at least a portion of the first output signal.

2. The method of claim 1, wherein the PPG signal is generated by subtracting the first output signal from the second output signal.

3. The method of claim 1, wherein:

the first output signal is generated by subtracting the first detected light signal from the second detected light signal, and

the second output signal is generated by subtracting the first detected light signal from the third detected light signal.

4. The method of claim 1, further comprising:

causing the one or more light emitters of the PPG sensor device to be driven at a third level different from the second level and at a fourth time to generate a third source light signal;

obtaining a fourth detected light signal representative of the amount of light detected by the light detector at the fourth time;

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

generating the PPG signal based, at least in part, on the difference between the second output signal and at least a portion of the first output signal and on a difference between the second output signal and at least a portion of the third output signal.

5. The method of claim 4, wherein the first, non-zero level and the third level are the same.

6. The method of claim 5, wherein the PPG signal is generated by subtracting, from the second output signal, the first output signal multiplied by 0.5 and the third output signal multiplied by 0.5.

7. An apparatus comprising:

a housing configured to be worn on a person'"'"'s wrist;

a PPG sensor including one or more light emitters and a light detector,

one or more processors, and

a memory, wherein;

the PPG sensor is located in the housing such that the one or more light emitters and the light detector face towards the skin of the person when the person wears the apparatus, and

the memory stores computer-executable instructions for controlling the one or more processors to;

obtain a first detected light signal representative of an amount of light detected by the light detector at a first time, wherein the one or more light emitters are off at the first time;

cause the one or more light emitters to be driven at a first, non-zero level at a second time to generate a first source light signal;

obtain a second detected light signal representative of the amount of light detected by the light detector at the second time;

cause the one or more light emitters to be driven at a second level higher than the first, non-zero level and at a third time to generate a second source light signal;

obtain a third detected light signal representative of the amount of light detected by the light detector at the third time;

generate a first output signal based on the second detected light signal adjusted by the first detected light signal;

generate a second output signal based on the third detected light signal adjusted by the first detected light signal; and

generate a PPG signal based, at least in part, on a difference between the second output signal and at least a portion of the first output signal.

8. The apparatus of claim 7, wherein the memory further stores computer-executable instructions for controlling the one or more processors to generate the PPG signal by subtracting the first output signal from the second output signal.

9. The apparatus of claim 7, wherein the memory further stores computer-executable instructions for controlling the one or more processors to:

generate the first output signal by subtracting the first detected light signal from the second detected light signal, and

generate the second output signal by subtracting the first detected light signal from the third detected light signal.

10. The apparatus of claim 7, wherein the memory further stores computer-executable instructions for controlling the one or more processors to:

cause the one or more light emitters of the PPG sensor to be driven at a third level different from the second level and at a fourth time to generate a third source light signal;

obtain a fourth detected light signal representative of the amount of light detected by the light detector at the fourth time;

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

generate the PPG signal based, at least in part, on the difference between the second output signal and at least a portion of the first output signal and on a difference between the second output signal and at least a portion of the third output signal.

11. The apparatus of claim 10, wherein the first, non-zero level and the third level are the same.

12. The apparatus of claim 11, wherein the memory further stores computer-executable instructions for controlling the one or more processors to generate the PPG signal by subtracting, from the second output signal, half of the first output signal and half of the third output signal.

13. A non-transitory, computer-readable medium storing computer-executable instructions for controlling one or more processors of a wearable photoplethysmographic (PPG) sensor, wherein the wearable PPG sensor includes including one or more light emitters and a light detector that face towards a person'"'"'s skin when the wearable PPG sensor is worn by the person, to:

obtain a first detected light signal representative of an amount of light detected by the light detector at a first time, wherein the one or more light emitters are off at the first time;

cause the one or more light emitters to be driven at a first, non-zero level at a second time to generate a first source light signal;

obtain a second detected light signal representative of the amount of light detected by the light detector at the second time;

cause the one or more light emitters to be driven at a second level higher than the first, non-zero level and at a third time to generate a second source light signal;

obtain a third detected light signal representative of the amount of light detected by the light detector at the third time;

generate a first output signal based on the second detected light signal adjusted by the first detected light signal;

generate a second output signal based on the third detected light signal adjusted by the first detected light signal; and

generate a PPG signal based, at least in part, on a difference between the second output signal and at least a portion of the first output signal.

14. The non-transitory, computer-readable medium of claim 13, further storing computer-executable instructions for controlling the one or more processors to generate the PPG signal by subtracting the first output signal from the second output signal.

15. The non-transitory, computer-readable medium of claim 13, further storing computer-executable instructions for controlling the one or more processors to:

generate the first output signal by subtracting the first detected light signal from the second detected light signal, and

generate the second output signal by subtracting the first detected light signal from the third detected light signal.

16. The non-transitory, computer-readable medium of claim 13, further storing computer-executable instructions for controlling the one or more processors to:

cause the one or more light emitters of the PPG device to be driven at a third level different from the second level and at a fourth time to generate a third source light signal;

obtain a fourth detected light signal representative of the amount of light detected by the light detector at the fourth time;

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

generate the PPG signal based, at least in part, on the difference between the second output signal and at least a portion of the first output signal and on a difference between the second output signal and at least a portion of the third output signal.

17. The non-transitory, computer-readable medium of claim 16, wherein the first, non-zero level and the third level are the same.

18. The non-transitory, computer-readable medium of claim 17, further storing computer-executable instructions for controlling the one or more processors to generate the PPG signal by subtracting, from the second output signal, half of the first output signal and half of the third output signal.