Exponential sampling of red and infrared signals
Methods and systems are provided that include sampling a light intensity signal at different frequencies based on the waveform of the signal to produce a more accurate digitized signal. The light intensity signal is an analog signal proportional to the intensity of light received at a detector of a pulse oximetry system. In one embodiment, the signal may be sampled exponentially during pulse width periods, such that the end of the pulse width periods where the signal reaches a maximum amplitude may be sampled more frequently. The signal may also be exponentially sampled or oversampled during periods when the signal is expected to near maximum amplitude. Further, the signal may be sampled less frequently during low amplitude periods of the signal, and during dark periods, such that processing power may be conserved.
1. A method of processing a signal from a pulse oximeter system comprising an emitter configured to transmit light into tissue, a detector configured to receive light from the tissue and output a signal proportional to an intensity of the light from the tissue, and a processor configured to sample the signal, wherein the signal comprises a red pulse width period, an infrared pulse width period, and a dark period, the method comprising:
using the pulse oximeter system, sampling the signal at a rate substantially in proportion to an amplitude of the signal during the red pulse width period and the infrared pulse width period, and sampling at a substantially constant rate during the dark period.
2. The method of claim 1 , wherein sampling the signal substantially in proportion to the amplitude comprises oversampling the signal toward an end of the red or infrared pulse width period.
3. The method of claim 1 , wherein sampling the signal substantially in proportion to the amplitude comprises sampling at or above a Nyquist rate during at least a portion of the red or infrared pulse width period, and wherein the constant rate is below the Nyquist rate.
4. The method of claim 1 , further comprising using a processor to calculate an oxygen saturation value based at least in part on the sampled signal.
5. A pulse oximetry system, comprising:
an emitter configured to transmit light into tissue;
a detector configured to receive the light from the tissue and output a signal proportional to an intensity of the light from the tissue, wherein the signal comprises a red pulse width period, an infrared pulse width period, and a dark period; and
a processor coupled to the detector, wherein the processor is configured to sample the signal at a first rate when the signal is at or below a first amplitude and sample the signal at a second rate when the signal is at or below a second amplitude, wherein the second amplitude is higher than the first amplitude, and wherein the processor is configured to sample the signal at a rate substantially in proportion to an amplitude of the signal during the red pulse width period and the infrared pulse width period and to sample the signal at a substantially constant rate during the dark period.
6. The pulse oximetry system, as set forth in claim 5 , wherein the first rate is below a Nyquist rate of the signal and the second rate is at or above the Nyquist rate of the signal.
7. The pulse oximetry system of claim 5 , wherein the processor is further configured to calculate an oxygen saturation value based at least in part on the sampled signal.
8. The pulse oximetry system of claim 5 , wherein sampling the signal substantially in proportion to the amplitude comprises oversampling the signal toward an end of the red or infrared pulse width period.
9. The pulse oximetry system of claim 5 , wherein sampling the signal substantially in proportion to the amplitude comprises sampling at or above a Nyquist rate during at least a portion of the red or infrared pulse width period, and wherein the constant rate is below the Nyquist rate.