Signal acquisition circuit for detecting a wanted signal in the presence of an unwanted signal

1. A signal acquisition circuit for detecting a wanted signal in a composite signal containing the wanted signal and an unwanted signal, where the highest frequency in the unwanted signal is higher than the highest frequency in the wanted signal, comprising:

a first sensor arranged to sense the composite signal, the composite signal being sensed at a rate that is at least equal to the Nyquist rate for the highest frequency in the wanted signal and less than the Nyquist rate for the highest frequency in the unwanted signal;

a second sensor arranged to sense the unwanted signal, without the wanted signal, at a rate that is at least equal to the Nyquist rate for the highest frequency in the unwanted signal;

an analog-to-digital converter arranged to receive the outputs of the first sensor and the second sensor, and sample the received outputs of the first sensor and the second sensor at a sampling rate that is at least equal to the Nyquist rate for the highest frequency in the unwanted signal;

a demultiplexer arranged to receive the output of the analog to digital converter and output a signal representative of the unwanted signal separately from a signal representative of the composite signal; and

a filter arranged to subtract the output signal representative of the unwanted signal from the signal representative of the composite signal and thereby output a signal representative of the wanted signal. containing the sum of the wanted signal and the unwanted signal sampled at a low rate at least equal to the Nyquist rate for the wanted signal but less than the high rate and a second component containing the unwanted signal sampled at the high rate.

2. The signal acquisition circuit as claimed in claim 1 , further comprising an active element for which energy must be expended to make the wanted signal observable by the first sensor, said active element being energized at the sensing rate of the first sensor.

3. The signal acquisition circuit as claimed in claim 1 , wherein the sensing of the second sensor is regular but intermittent.

4. The signal acquisition circuit as claimed in claim 1 , wherein the sensing of the second sensor is irregular.

5. The signal acquisition circuit as claimed in claim 1 , wherein the sensing of the first and second sensors are in phase with each other.

6. The signal acquisition circuit as claimed in claim 1 , wherein the sensing of the first and second sensors are out of phase with each other.

7. The signal acquisition circuit as claimed in claim 1 , wherein the first and second sensors are provided by a common sensor element.

8. The signal acquisition circuit as claimed in claim 1 , wherein the first and second sensors are provided by a common sensor element, and further comprising an active element for which energy must be expended to make the wanted signal observable by the single sensor, said active element being energized at the sensing rate of the first sensor, and the common sensor element being the second sensor when said active element is not energized.

9. A method of acquiring a wanted signal in a composite signal containing the wanted signal and an unwanted signal, where the highest frequency in the unwanted signal is higher than the highest frequency in the wanted signal, comprising the steps of:

sensing the composite signal at a low rate that is at least equal to the Nyquist rate for the highest frequency in the wanted signal but less than the Nyquist rate for the highest frequency in the unwanted signal;

sensing the unwanted signal, without the wanted signal, at a rate that is at least equal to the Nyquist rate for the highest frequency in the unwanted signal;

sampling said composite and unwanted signals at a sampling rate that is at least equal to the Nyquist rate for the highest frequency in the unwanted signal;

producing a signal representative of the unwanted signal separately from a signal representative of the composite signal; and

subtracting the signal representative of the unwanted signal separately from the signal representative of the composite signal to produce the wanted signal;

wherein said sensing includes expending energy to make said wanted signal observable.

10. The method as claimed in claim 9 , wherein said composite signal and said unwanted signal are sensed in separate sensors.

11. The method as claimed in claim 9 , wherein said composite signal and said unwanted signal are sensed in a common sensor element.

12. A method as claimed in claim 9 , wherein the sampling sensing of the unwanted signal is regular but intermittent.

13. A method as claimed in claim 9 , wherein the sensing of the unwanted signal is irregular.

14. A method as claimed in claim 9 , wherein the sensing of the composite signal is in phase with the sensing of the unwanted signal.

15. A method as claimed in claim 9 , wherein the sensing of the composite signal is out of phase with the sensing of the unwanted signal.

16. A pulse oximetry sensor comprising:

a first and a second light source;

a pulse generator arrangement for sequentially pulsing said first and second light sources at a low rate that is at least equal to the Nyquist rate of a photoplethysmograph signal but less than a high rate at least equal to the Nyquist rate for the highest frequency of the ambient light to be removed;

a photodetector for detecting light pulses from said first and second light sources in the presence of artificial ambient light and the ambient light in the absence of the light pulses from said light sources;

an analog-to-digital converter (ADC) triggered at the high rate for acquiring and converting an output of the photodetector to digital format, wherein the analog-to-digital converter outputs a signal having a first component containing the signals from the pulsed first and second light sources in the presence of ambient light sampled at the low rate and a second component containing the ambient light signal sampled at the high rate;

a demultiplexer for separating said first and second components into:

a first signal representative of pulsed first light source;

a second signal representative of the pulsed second light source; and

a third signal representative of the ambient light; and

a filter downstream of the demultiplexer for subtracting the third signal representative of the ambient light signal from each of the first and second signals light to output a first output signal representative of the pulsed first light source and a second output signal representative of the pulsed second light source.

17. A pulse oximetry sensor as claimed in claim 16 , further comprising an input for receiving a base clock signal, and a divide-by −N unit for dividing the base clock signal by N, where N is an integer, and wherein said pulse generator arrangement comprises a first pulse generator for driving the first light sources receiving the output of the divide-by-N unit, and a second pulse generator for receiving a delayed version of the output of the divide-by-N unit for driving the second light sources, and a delay unit for applying a delayed version of the base clock signal to a trigger input of the ADC.

18. A pulse oximetry sensor as claimed in claim 17 , which is configured such that the delay applied to the output of the divide-by-N unit before application to the second pulse generator is given by the expression n*Ts where Ts is the base clock period and n is within the range 1≦n≦N−1.

19. A pulse oximetry sensor as claimed in claim 18 , which is configured such that the ambient light in the absence of the light pulses from said light sources is converted in the gaps between signals from the first and second light sources.

20. A pulse oximetry sensor as claimed in claim 17 , which is configured such that the trigger for the ADC for the acquiring of the detected light pulses from said first and second light sources in the presence of artificial ambient light is 180 degrees out of phase with the trigger for the acquiring of the ambient light in the absence of the light pulsed.

21. A pulse oximetry sensor as claimed in claim 17 , wherein the first light source and the second light source emit respectively at red and infrared wavelengths, and wherein N=10, and wherein said high rate is approximately equal to said base clock rate, which is ten times the pulse rate of said light sources.

22. A pulse oximetry sensor as claimed in claim 17 , wherein the low rate is at least 10 Hz and less than 100 Hz.

23. A method of acquiring a wanted signal in a pulse oximetry sensor comprising:

sequentially pulsing a first and a second light sources, having different effective wavelengths, at a first rate that is at least equal to the Nyquist rate of a photoplethysmograph signal but less than a second rate at least equal to the Nyquist rate for the highest frequency of an ambient light to be removed;

detecting light pulses from said first and second light sources in the presence of the ambient light and further detecting ambient light in the absence of light pulses from the first and second light sources to produce a composite signal;

acquiring and converting said composite signal to digital format in an analog-to-digital converter triggered at the second rate to produce an converted signal having a first component representative of the detected light pulses from the first light source in the presence of ambient light, a second component representative of the detected light pulses from the second light source in the presence of ambient light and a third component representative of the detected ambient light and second light sources in the absence of light pulses from the first and second light sources;

separating said components of the signal; and

subtracting the third component from each of the first and second components to output a first signal representative of the pulsed signal of the first light source and a second signal representative of the pulsed signal of the second light source.

24. A method as claimed in claim 23 , further comprising receiving a base clock signal, dividing the base clock signal by N to produce a divided signal, where N is an integer, and pulsing the first light sources by the divided signal and the second light sources by a delayed version of the divided signal, and triggering the analog-to-digital converter with a delayed version of the base clock signal.

25. A method as claimed in claim 24 , wherein the second light sources is pulsed by the divided signal delayed by n*Ts where Ts is the base clock period and n is within the range 1≦n≦N−1.

26. A method as claimed in claim 23 , wherein the ambient light signal is acquired in the gaps between signals from the first and second light sources.

27. A method as claimed in claim 23 , wherein the trigger for the acquiring of the analog-to-digital converter for the acquisition of the composite signal having detected light pulses from the first and second light sources is 180 degrees out of phase with the trigger for the acquisition of the ambient light.

28. A method as claimed in claim 23 , wherein the low rate is at least 10 Hz and less than 100 Hz.