LOW FREQUENCY NOISE IMPROVEMENT IN PLETHYSMOGRAPHY MEASUREMENT SYSTEMS

• US 20150257663A1
• Filed: 09/29/2014
• Published: 09/17/2015
• Est. Priority Date: 03/17/2014
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: DELIWALA, Shrenik
• Granted Patent: US 11,229,373 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: A61B 2560/0209 adapted for power saving
• CPC Class Codes: A61B 5/02433 for infrared radiation
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal

Abstract

A plethysmography (“PPG”) measurement system may include at least one source of PPG radiation and at least one auxiliary sensor for detection of PPG radiation. The radiation source emits a portion of the PPG radiation toward a subject and another portion along an optical path for direct communication between the PPG radiation source and the auxiliary sensor. The auxiliary sensor may develop a profile against which measurements from primary PPG sensors, which receive light returning from the subject, may be compared. From this comparison, new PPG signals may be generated that exhibit lower noise than the PPG signals output by PPG sensors. These noise mitigation techniques may be used advantageously by a PPG system to generate more accurate measurements and also to reduce power consumption by the radiation sources.

Claims

1. A system comprising:

a radiation source;

a first sensor for generating a first signal in response to a portion of radiation from the source returned from the subject; and

an auxiliary sensor for detecting a second portion of radiation from the source through a direct path from the source to the auxiliary sensor.

2. The system of claim 1, comprising a noise analyzer to compare the signals from the first sensor and the auxiliary sensor to reduce noise in the first signal.

3. The system of claim 1, wherein the first sensor is positioned to detect radiation reflected by the subject subsequent to exposure by the radiation source.

4. The system of claim 1, wherein the first sensor is positioned detect radiation transmitted through the subject subsequent to exposure by the radiation source.

5. The system of claim 1, further comprising a driver circuit comprising:

a power supply;

a charge storage element, and

a multi-phase switch system interconnecting the radiation source to the driver circuit, wherein in a first phase of operation, the switch system connects the charge storage element to the power supply and, in a second phase of operation, the switch system connects the charge storage element to the radiation source.

6. A method comprising:

illuminating a subject with a radiation source;

measuring, by a first sensor, a portion of the radiation returned from the subject;

generating a first signal based on the measurement by the first sensor;

measuring a portion of the radiation by an auxiliary sensor via an optical path between the radiation source and the auxiliary sensor that does not traverse the subject;

generating a second signal based the measurement by the auxiliary sensor; and

modifying the first signal based on the second signal.

7. The method of claim 6, wherein the modified signal is formed as:

8. The method of claim 6, wherein the modified signal is formed as:

L=L_PPG−

k*L_noise

where L_PPG represents the first signal, L_noise represents a noise estimate derived from the second signal, and k represents a scaling factor to reduce noise from L_PPG.

9. The method of claim 6, further comprising storing the modified signal in a memory.

10. The method of claim 6, further comprising transmitting data representing the modified signal wirelessly to a monitoring device.

11. The method of claim 6, further comprising filtering the first signal and second signal prior to the modifying.

12. The method of claim 6, wherein the generating steps respectively comprise digitizing measurements of the first and second sensors.

13. The method of claim 6, wherein the first sensor measures the radiation reflected by the subject subsequent to exposure by the radiation source.

14. The method of claim 6, wherein the first sensor measures the radiation transmitted through the subject subsequent to exposure by the radiation source.

15. A sensor system, comprising:

a source of photoplethysmographic (PPG) radiation,

a sensor for detection of PPG radiation, and

a housing that accommodates the source and the sensor that defines an aperture for emission of a portion of the PPG radiation toward a subject and an optical path for direct communication of another portion of the PPG radiation from the source to the sensor.

16. The system of claim 15, further comprising a second sensor for detection of PPG radiation returned from the subject.

17. The system of claim 16, wherein the housing accommodates the second sensor and defines a second aperture therefor.

18. The system of claim 16, further comprising a second housing to accommodate the second sensor.

19. The system of claim 15, further comprising a noise cancellation system having an input for a PPG signal from a second sensor and an output from the first sensor, the noise cancellation system to reduce a noise component of the PPG radiation based on an output from the first sensor.

20. The system of claim 15, wherein the sensor and the source are situated on a common substrate.

21. The system of claim 15, wherein the sensor and the source are situated on separate substrates.

22. The system of claim 16, wherein an effective surface area of the sensor is smaller than the corresponding effective surface area of the second sensor.

23. The system of claim 22, wherein the ratio of effective area of the sensor to the effective area of the second sensor is approximately between 1/20^th and 1/80^th.

24. The system of claim 15, wherein the housing further defines a shield to inhibit emission of the PPG radiation reflected from the subject to the sensor.

25. The system of claim 16, wherein the housing includes a partition to inhibit emission from the PPG radiation directed along the optical path for direct communication to the second sensor.

26. A system, comprising:

a photoplethysmographic (PPG) measurement system including;

a source of PPG radiation;

a pair of sensors for detecting PPG radiation, a first sensor to detect PPG radiation that traverses a test subject and a second sensor to detect PPG radiation from the source along an optical path that does not traverse the subject; and

a processor operative to;

manage operation of the system;

receive signals based output signals from the pair of sensors; and

apply noise management logic to the signals, wherein the noise management logic includes algorithms based on the outputs from the pair of sensors.

27. The system of claim 26, wherein the first sensor is positioned to detect PPG radiation reflected by the test subject.

28. The system of claim 26, wherein the first sensor is positioned detect PPG radiation transmitted through the test subject.

29. The system of claim 26, wherein the noise management logic reduces noise in the output of the first sensor based on the output from the second sensor.

30. The system of claim 29, wherein the noise management logic reduces noise in the output of the first sensor based on:

L=L_PPG−

k*L_noise

where L_PPG represents the output of the first second, L_noise represents a noise estimate derived from the second sensor, and k represents a scaling factor to reduce noise from L_PPG.

31. The system of claim 29, wherein the noise management logic reduces noise in the output of the first sensor based on:

32. The system of claim 26, wherein the pair of sensors communicate with the processor via at least one wireless communication link.

33. The system of claim 26, further comprising a monitoring device provided in communication with the processor via a wireless communication link.

34. The system of claim 26, wherein the system is a pulse oximetry measurement system.

35. The system of claim 26, wherein the system is a heart rate monitoring system.