Oximeter ambient light cancellation

• US 20050187448A1
• Filed: 02/25/2004
• Published: 08/25/2005
• Est. Priority Date: 02/25/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Petersen, Ethan, Chew, Bradford B., Shea, William
• Granted Patent: US 7,190,985 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• 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
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: A61B 5/725 using specific filters therefor, e.g. Kalman or adaptive filters A61B5/316 take precedence
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence

Abstract

A pulse oximeter method and apparatus which provides (1) a notch filter at a distance between a modulation frequency and a common multiple of commonly used power line frequencies (50, 60, 100 and 120) and also (2) a demodulation frequency greater than a highest pulse rate of a person and lower than any harmonic of 50, 60, 100 or 120 Hz, to filter ambient light interference, while choosing an optimum demodulation frequency that avoids interference from the notch filter or from harmonics of the line interference. Also, ambient light for any low frequency interference, such as power line interference, is measured both before and after each of the light emitter wavelengths and the average of the ambient light is then subtracted from the detected signal.

Claims

1. A method for operating a pulse oximeter, comprising:

alternately generating light at two wavelengths using a modulation frequency that is a predetermined distance from a common multiple of 50, 60, 100, and 120;

said predetermined distance being greater than a highest pulse rate of a person and lower than a distance of any harmonic of 50, 60, 100, or 120;

directing said light at a tissue site;

detecting a detected light signal scattered off said tissue site;

notch filtering said detected light signal at a frequency at said common multiple.

2. The method of claim 1 further comprising:

digitizing said detected light signal to provide a digitized signal; and

said notch filtering being performed on said digitized signal.

3. The method of claim 1 wherein said common multiple is 1200.

4. The method of claim 1 wherein said predetermined distance is between 5 and 20 hertz.

5. The method of claim 1 wherein said predetermined distance is approximately 11 hertz.

6. The method of claim 1 wherein said notch filtering is performed by providing a zero in a digital filter at said distance.

7. The method of claim 1 wherein

said modulation frequency is used to alternate between a first period of light of said first wavelength being generated, a dark period of no light being generated, and a second period of light of said second wavelength being generated;

estimating a level of ambient light in said detected light signal during said first period by averaging an amount of said detected light signal in said dark periods before and after said first period;

estimating a level of ambient light in said detected light signal during said second period by averaging an amount of said detected light signal in said dark periods before and after said second period.

16. The pulse oximeter of claim 1 further comprising:

said control circuit being configured to alternate between a first period of light of said first wavelength being generated, a dark period of no light being generated, and a second period of light of said second wavelength being generated;

said processing circuit being configured to estimate a level of ambient light in said detected light signal during said first period by averaging an amount of said detected light signal in said dark periods before and after said first period, and estimate a level of ambient light in said detected light signal during said second period by averaging an amount of said detected light signal in said dark periods before and after said second period.

8. A method for measuring a detected light signal in a pulse oximeter comprising:

alternately generating light at two wavelengths using a modulation frequency;

directing said light at a tissue site;

detecting a detected light signal scattered off said tissue site;

using said modulation frequency to alternate between a first period of light of said first wavelength being generated, a dark period of no light being generated, and a second period of light of said second wavelength being generated;

estimating a level of ambient light in said detected light signal during said first period by averaging an amount of said detected light signal in said dark periods before and after said first period; and

estimating a level of ambient light in said detected light signal during said second period by averaging an amount of said detected light signal in said dark periods before and after said second period.

9. The method of claim 8 further comprising:

subtracting an estimated level of ambient light from said detected light signal.

10. A pulse oximeter, comprising:

at least one light emitter;

a control circuit for alternately generating light at two wavelengths from said at least one light emitter using a modulation frequency that is a predetermined distance from a common multiple of 50, 60, 100, and 120;

said predetermined distance being greater than a highest pulse rate of a person and lower than a distance of any harmonic of 50, 60, 100, or 120;

a detector for detecting a detected light signal scattered off a tissue site;

a processing circuit for processing said detected light signal, said processing circuit including a notch filter configured to filter said detected light signal at a frequency at said common multiple.

11. The pulse oximeter of claim 10 further comprising:

an analog-to-digital converter for digitizing said detected light signal to provide a digitized signal; and

said notch filter being a digital filter.

12. The pulse oximeter of claim 10 wherein said common multiple is 1200.

13. The pulse oximeter of claim 10 wherein said predetermined distance is between 5 and 20 hertz.

14. The pulse oximeter of claim 10 wherein said predetermined distance is approximately 11 hertz.

15. The pulse oximeter of claim 10 wherein said notch filter provides a zero in a digital filter at said distance.

17. A pulse oximeter comprising:

at least one light emitter;

a control circuit for alternately generating light at two wavelengths from said at least one light emitter using a modulation frequency;

a detector for detecting a detected light signal scattered off said tissue site;

said control circuit being configured to alternate between a first period of light of said first wavelength being generated, a dark period of no light being generated, and a second period of light of said second wavelength being generated;

a processing circuit configured to estimate a level of ambient light in said detected light signal during said first period by averaging an amount of said detected light signal in said dark periods before and after said first period, and estimate a level of ambient light in said detected light signal during said second period by averaging an amount of said detected light signal in said dark periods before and after said second period.

18. An oximeter apparatus comprising:

a sigma-delta modulator having an input coupled to receive an analog sensor signal;

a hardware amplifier for amplifying a sensor signal to provide an amplified signal as said analog sensor signal;

a multiple bit analog-to-digital converter coupled to said output of said sigma-delta modulator to provide a digital output;

a decimator configured to operate on said digital output;

a digital filter configured to operate on said digital output; and

a control program for preloading said decimator and digital filter with an estimate of a settled output value after modifying the gain of said hardware amplifier.