Method, system and apparatus for measuring multiple signals in a body
A pulse oximetry measurement system uses a pseudo-random noise generator to stimulate one or more light emitting diodes (LEDs). The light amplitudes from these LEDs, after passing through a part of a body, are detected by a phototransistor or photodiode and digitized with an analog-to-digital converter (ADC). The digitized ADC light amplitude values are re-correlated with the outgoing pseudo-random noise stimulus. Spread spectrum techniques are known for their noise mitigation properties, and ability to pass multiple signals through the same medium without interference. Thus, these measurements can be performed substantially simultaneously with minimal interference from each other. The pulse oximetry measurement system correlates the measured light intensities using pseudo-random noise generation and phase division multiplexing, and computes the measured and correlated peak-to-peak detected light amplitudes to obtain a ratio between these light amplitudes for determining oxygen saturation in the blood, and may also be used for heart rate monitoring.
1. A system for measuring multiple signals in a body, said system comprising:
at least one first light source generating a first color light;
at least one second light source generating a second color light;
at least one light sensor adapted for detecting light amplitudes, wherein the at least one first and second light sources and the at least one light sensor are adapted for a portion of a body to be located therebetween;
a pseudo-random noise generator adapted for generating a pseudo-random control sequence for turning on and off the at least one first and second light sources at pseudo-random times, wherein the control sequence for the first and second light source use a same code, wherein the code fed to the second light source is phase shifted with respect to the code fed to the first light source;
an analog-to-digital converter (ADC) for converting sampled light amplitudes from the at least one light sensor into digital representations thereof; and
a correlation circuit coupled to a digital output of the ADC and the pseudo-random noise generator, wherein the correlation circuit associates the digital representations with corresponding ones of the at least one first and second light sources,
wherein the pseudo-random noise generator comprises a linear feedback shift register and the pseudo-random control sequence comprises a maximum length (ML) sequence, wherein each of the linear feedback shift register and the ADC receive a clock signal, and wherein the ADC is triggered on a positive going edge of the clock signal and the pseudo-random noise generator is triggered on a negative going edge of the clock signal.
2. The system according to claim 1 , further comprising a digital filter for filtering the correlated digital representations.
3. The system according to claim 2 , further comprising a heartbeat detection circuit coupled to an output of the digital filter.
4. The system according to claim 2 , further comprising a blood oxygen saturation (SpO 2 ) determination circuit coupled to an output of the digital filter.
5. The system according to claim 1 , wherein the ML sequence is phase shifted for each one of a plurality of other light sources.
6. The system according to claim 1 , wherein the linear feedback shift register comprises a plurality of shift registers that are either added to or subtracted from based upon a corresponding output of the pseudo-random noise generator.
7. The system according to claim 1 , further comprising:
at least one first digital-to-analog converter (DAC) having an analog output coupled to the at least one first light source; and
at least one second digital-to-analog converter (DAC) having an analog output coupled to the at least one second light source;
wherein the at least one first and second DACs control light intensities of the first and second light sources.
8. The system according to claim 1 , wherein the first color light is at a red wavelength and the second color light is at an infrared wavelength.
9. The system according to claim 1 , wherein the first color light is at a green wavelength and the second color light is at a yellow-green wavelength.
10. The system according to claim 1 , wherein digital representations of ambient light samples are subtracted from the digital representations of the sampled light amplitudes from the at least one first and second light sources.
11. The system according to claim 1 , wherein interfaces for the at least one first and second light sources and the at least one light sensor, the pseudo-random noise generator, ADC, and correlation circuit are provided by a microcontroller.
12. The system according to claim 11 , further comprising a communications interface coupled to the microcontroller and providing oxygen saturation and heartbeat information.
13. The system according to claim 1 , wherein the at least one first and second light sources comprise light emitting diodes (LEDs) and the at least one light sensor comprises at least one photo-diode or photo-transistor.
14. A method for measuring multiple signals in a body, said method comprising the steps of:
generating a first color light with at least one first light source;
generating a second color light with at least one second light source;
detecting light amplitudes with at least one light sensor, wherein the at least one first and second light sources and the at least one light sensor are adapted for a portion of a body to be located therebetween;
turning on and off the at least one first and second light sources at pseudo-random times generated by a pseudo-random noise generator generating a pseudo-random control sequence, wherein the control sequence for the first and second light source use a same code, wherein the code fed to the second light source is phase shifted with respect to the code fed to the first light source;
converting sampled light amplitudes from the at least one light sensor into digital representations thereof with an analog-to-digital converter (ADC); and
correlating the digital representations of the sampled light amplitudes with corresponding ones of the at least one first and second light sources using the pseudo-random times from the pseudo-random noise generator,
wherein the pseudo-random noise generator comprises a linear feedback shift register and the pseudo-random control sequence comprises a maximum length (ML) sequence, wherein each of the linear feedback shift register and the ADC receive a clock signal, and wherein the ADC is triggered on a positive going edge of the clock signal and the pseudo-random noise generator is triggered on a negative going edge of the clock signal.
15. The method according to claim 14 , further comprising the step of filtering the correlated digital representations with a digital filter.
16. The method according to claim 14 , further comprising the step of determining oxygen saturation (SpO 2 ) of blood from the digital representations of the sampled light amplitudes.
17. A microcontroller configured for measuring multiple signals in a body, comprising:
at least one first driver for turning on and off at least one first light source generating a first color light;
at least one second driver for turning on and off at least one second light source generating a second color light;
at least one analog input for receiving an output from at least one light sensor adapted for detecting light amplitudes, wherein the at least one first and second light sources and the at least one light sensor are adapted for a portion of a body to be located therebetween;
a pseudo-random noise generator adapted for generating a pseudo-random control sequence for turning on and off the at least one first and second light sources at pseudo-random times, wherein the control sequence for the first and second light source use a same code, wherein the code output by the second driver for the second light source is phase shifted with respect to the code output by the first driver for the first light source;
an analog-to-digital converter (ADC) for converting sampled light amplitudes received from the at least one light sensor into digital representations thereof; and
a correlation circuit coupled to a digital output of the ADC and the pseudo-random noise generator, wherein the correlation circuit associates the digital representations with corresponding ones of the at least one first and second light sources,
wherein the pseudo-random noise generator comprises a linear feedback shift register and the pseudo-random control sequence comprises a maximum length (ML) sequence, wherein each of the linear feedback shift register and the ADC receive a clock signal, and wherein the ADC is triggered on a positive going edge of the clock signal and the pseudo-random noise generator is triggered on a negative going edge of the clock signal.
18. The microcontroller according to claim 17 , further comprising:
at least one first digital-to-analog converter (DAC) coupled to at least one first analog output adapted for coupling to the at least one first light source; and
at least one second digital-to-analog converter (DAC) coupled to at least one second analog output adapted for coupling to the at least second light source;
wherein the at least one first and second DACs control intensities of the first and second color lights.
19. The microcontroller according toto claim 17 , wherein the linear feedback shift register has a plurality of outputs each representing a different shift stage and wherein the at least one first driver is coupled with a different output of the linear feedback shift register than the second driver.