WO2005009221A2

WO2005009221A2 - Data acquisition system for pulse oximeters

Info

Publication number: WO2005009221A2
Application number: PCT/US2004/023426
Authority: WO
Inventors: John Denny Bryars
Original assignee: The Titan Corporation
Priority date: 2003-07-21
Filing date: 2004-07-20
Publication date: 2005-02-03
Also published as: WO2005009221A3

Abstract

A data acquisition system for pulse oximetry detects a background light level sensed by a pulse oximeter probe and obtains a pulse stream for each of a plurality of wavelengths with which the pulse oximeter probe illuminates tissue. The pulse streams are corrected for the effect of the background light, amplified, and filtered to obtain pulse waveforms representing heart activity and blood oxygen saturation (Sp02).

Description

DATA ACQUISITION SYSTEM FOR PULSE OXIMETERS

Priority is claimed to US Provisional application 60/488,839, filed July 21, 2003 for "Data Acquisition System for Pulse Oximeters". Background and Summary

This invention represents a significant improvement over earlier pulse oximeter technology in that zooming techniques are used to extract useful signal information at the peaks of electrical pulse signal pedestals being detected by a photo diode. The amplitudes of these pedestals are controlled by a microcontroller that drives a digital to analog converter (DAC). A bi-colored light emitting diode (LED) is switched "ON" for approximately 3 milliseconds during each data acquisition cycle. The pedestal amplitude of data acquisition circuitry is calibrated. Once calibrated, the peak amplitude of the detected pedestals varies with the level of the signal being detected at the measurement site. By establishing the average amplitude of the detected pedestals a reference voltage is set by the DAC and only a differential level is amplified. This technique produces a small differential signal voltage that can be greatly amplified giving the acquisition system a wide dynamic range. This technique can best be described as "ZOOMING" the analog to digital converter (ADC) input so that the full input range of the ADC is centered on the peak of the detected pedestal. By employing the improved data acquisition technology embodied in this new optical sensor data acquisition approach an instrument for monitoring blood oxygen and heart rate can be produced. This system can be used with conventional transmission type sensors as well as a series of new and improved reflectance sensors disclosed in co- pending patent applications filed on even date herewith. Production of an instrument derived from this new and innovative technology will be useful as a medical instrument. Descriptions of the Drawings

Figure 1 is a circuit diagram illustrating the invention in a first embodiment; Figure 2 is a waveform diagram showing pedestal voltage amplitude plotted against a sample and hold output; Figure 3 is a timing diagram illustrating a data acquisition timing sequence; and Figure 4 is a circuit diagram illustrating the invention in a second embodiment.

Description of the Embodiments and Operation of the Invention

In this specification, a "heart pulse signal" is a signal that represents the pulse produced in a living being such as a person by the throbbing of an artery in response to contractions of the being's heart. See the second definition of "pulse" in Webster's New Collegiate Dictionary, 1974 at page 934. In contrast an "optical pulse signal" is an optical signal having a narrow or short spike in magnitude in comparison to a time schedule of interest. An "electrical pulse signal" is an electrical signal having a narrow or short spike in magnitude in comparison to the time schedule of interest. See the definition of "pulse" in the IBM Dictionary of Computing, Eighth Edition, 1987, page 344. To "pulse" an optical or electrical component is to cause the component to produce an optical or electrical pulse signal. A heart pulse signal may be constituted of a sequence or an aggregation of one or more optical or electrical pulse signals. Figure 1 shows a functional drawing of the components of the first embodiment of the data acquisition system. In Figure 1, a data acquisition system 100 includes a signal generation section and a data acquisition section. Both sections are connected to a microcontroller (not shown) which controls operations of the two sections according to a sequence represented by the waveforms illustrated in Figure 3. Both sections are also connected to a pulse oximeter probe that produces light at multiple wavelengths in response to the signal generation section. The multiple wavelengths of light illuminate tissue containing blood vessels. Each wavelength interacts optically with the tissue and then is detected by the probe. Each detected wavelength is processed separately from all other detected wavelengths by the data acquisition section. The data acquisition system 100 produces an output constituted of multiple parallel representations of the pulse activity of a person, one representation for each detected wavelength. Each representation consists of a sequence of pulses. A representation of a single heart pulse of blood through an artery is illustrated in Figure 3. In Figure 1, the interface of the data acquisition system 100 with a microcontroller is indicated by reference numeral 110. The interface provides a plurality of signal lines 112a- 112k that carry signals from a microcontroller to the signal generation section and a plurality of signal lines 114a- 114k that carry signals from the data acquisition section to the microcontroller. Of course those skilled in the art will realize that more signal lines than those shown in Figure 1 will be provided to conduct other data, control, and status signals between the data acquisition system 100 and a microcontroller. The interface of the data acquisition system 100 with a pulse oximeter probe 122 is indicated by reference numeral 120. Elements of the pulse oximeter probe 122 include, without limitation, multiple light-emitting diodes (LEDS), each for emitting light in a narrow spectral band centered on a particular wavelength. For example, the pulse oximeter probe 122 may include a first LED 124 that emits light in a narrow visible band of red light (RED) centered at 660 nm and a second LED 126 that emits light in a narrow infrared (IR) band centered at 940 nanometers (nm). The signal generation section of the data acquisition system 100 may be connected to the anodes of the LEDS 124 and 126 in order to control the operations of those devices. The pulse oximeter probe 122 probe further includes a photo detector 128 (which may comprise a photodiode, for example) and a preamplifier 130. Light emitted by the LEDS 124 and 126 and transmitted through tissue is detected by the photo detector 128. The preamplifier 130 amplifies the signals produced by the photo detector 128 and provides a pulse oximeter probe output to the data acquisition section of the data acquisition system 100. The signal generation section of the data acquisition system 100 includes a digital-to-analogue converter (DAC) 140, switches 142 and 144, and buffer amplifiers 143 and 145. The DAC 140 receives a control signal from a microcontroller (not shown) on a bus including the signal lines 112c, 112d, and 112f. The control signal is a digital word indicating a signal magnitude which the DAC 140 converts to an analogue signal of the indicated magnitude. The DAC 140 has a first connection to an input of the switch 142 and a second connection to an input of the switch 144. Each of these connections conducts an analogue signal having an indicated magnitude to the switch to which it is connected. The switch 142 is connected to signal line 112a and the switch 144 is connected to signal line 112b. Each signal line 112a, 112b conducts a signal to enable the switch to which the signal line is connected. The switch 142 is connected to the input of the buffer amplifier 143, the output of which may be connected to an LED of the pulse oximeter probe 122. For example, the output of the buffer amplifier 143 is shown connected to the anode of the RED LED 124. The switch 144 is connected to the input of the buffer amplifier 145, the output of which may be connected to an LED of the pulse oximeter probe 122. For example, the output of the buffer amplifier 145 is shown connected to the anode of the IR LED 126. The DAC 140 also outputs RED and IR REFERENCE voltages, each of which is connected to a differential analogue-to-digital converter 180. The data acquisition section of the data acquisition system 100 includes a first sample and hold circuit 150, a unity gain difference amplifier 155, an amplifier 160, second and third sample and hold circuits 170 and 174, adjustable gain amplifiers 171 and 175, low pass analogue filters 172 and 176, a digitally-controlled potentiometer 178, and a differential analogue-to-digital converter (ADC) 180. The first sample and hold circuit 150 has an input for connection to the output of a pulse oximeter probe. For example the input of the first sample and hold circuit 150 may be connected to the output of the preamplifier 130. The first sample and hold circuit 150 has a control input connected to signal line 112k. The output of the first sample and hold circuit 150 is connected to the negative input of the unity gain difference amplifier 155. The positive input of the unity gain difference amplifier 155 is also connected to the output of the preamplifier 130. The output of the unity gain difference amplifier 155 is connected to the input of the amplifier 160, the output of which is connected to the inputs of the second and third sample and hold circuits 170 and 174. The second sample and hold circuit 170 has a control input connected to signal line 112j and an output connected to the positive input of the amplifier 171 and to a calibration input (RED CAL) of the ADC 180. The third sample and hold circuit 174 has a control input connected to signal line 112i and an output connected to the positive input of the amplifier 175 and to a calibration input (IR CAL) of the ADC 180. The digitally-controlled potentiometer 178 is connected to the three signal lines 112f-112h on which it receives a control signal indicating a resistance value. In the data acquisition section, the digitally-controlled potentiometer 178 has a first resistor 178a with a fixed resistance, one terminal of which is connected to an infrared reference voltage output (RED REF) of the DAC 140 and the other of which is connected to the output of the amplifier 171. A third terminal of the first resistor 178a is adjustable with respect to the other two terminals and may be adjusted in response to the control signal on 112f-112h to provide a selected value of resistance on the third terminal. The third terminal is connected to the negative input of the amplifier 171, so that the adjustable resistance provided by the first resistor 178a between the output and negative input of the amplifier 171 provides an adjustable gain for the amplifier 171. In the data acquisition section, the digitally-controlled potentiometer 178 has a second resistor 178b with a fixed resistance, one terminal of which is connected to a red reference voltage output (IR REF) of the DAC 140 and the other of which is connected to the output of the amplifier 175. A third terminal of the second resistor 178b is adjustable with respect to the other two terminals and may be adjusted in response to the control signal on 112f-112h to provide a selected value of resistance on the third terminal. The third terminal is connected to the negative input of the amplifier 175, so that the adjustable resistance provided by the second resistor 178b between the output and negative input of the amplifier 175 provides an adjustable gain for the amplifier 175. In the data acquisition section, the output of the amplifier 171 is further coupled through a low-pass analogue filter 172 to the ADC 180 and the amplifier 175 is further coupled to the ADC 180 through a low-pass analogue filter 176. The inputs by which the ADC receives the RED and IR channel outputs are differential inputs. Thus, the RED output of the low-pass filter 172 is received on one terminal of a differential input to the ADC 180, with a RED reference (RED REF) signal produced by the DAC 140 received on the other terminal. Similarly, the IR output of the low-pass filter 176 is received on one terminal of a differential input to the ADC 180, with an IR reference (IR REF) signal produced by the DAC 140 received on the other terminal. The inputs to the ADC 180 that are connected to the outputs of the second and third sample and hold circuits 170 and 174 are single-ended inputs; one of those inputs receives a RED calibration signal (RED CAL) signal from the sample and hold circuit 170, the other receives an IR calibration (IR CAL) signal from the sample and hold circuit 174. In the data acquisition section, there are multiple parallel channels, each dedicated to processing an output of the pulse oximeter probe 122 resulting from illumination of tissue with a respective wavelength of light. For example, a RED channel constituted of the elements 170, 171, and 172 processes an output of the pulse oximeter probe 122 resulting from illumination of tissue with RED light produced by the RED LED 124. The output of the RED channel, produced by the low pass analogue filter 172, is a RED heart pulse signal which is coupled to an input of the ADC 180. An IR channel constituted of the elements 174, 175, 176 processes an output of the pulse oximeter probe 122 resulting from illumination of tissue with IR light produced by the IR LED 126. The output of the IR channel, produced by the low pass analogue filter 176, is an IR heart pulse signal which is coupled to an input of the ADC 180. Operations of the signal and data acquisition sections are controlled by signals from a microcontroller which may be connected to the data acquisition system 100 at the interface 110 to implement control circuitry with feedback. These operations presume the connection of the pulse oximeter probe 122 at interface 120. The sequencing of these operations maybe understood with reference to Figures 1 and 3. The LEDS 124 and 126 are turned on and off by the switches 142 and 144 which operate in response to RED LED ON and IR LED ON electrical pulse signals on signal lines 112a and 112b. The phasing of these signals is shown in Figure 3. Each LED is pulsed on to a level determined by the magnitude of a voltage produced by the DAC 140 in response to signals on the signal lines 112c-112e. The switches 142 and 144 are operated in a pulsed mode in order to cause the LEDS 124 and 126 to emit optical pulse signals For example, when the RED LED ON signal is pulsed to a high state, the switch 142 closes and couples a pulse of an analogue voltage produced by the DAC 140 through the amplifier 143 to the RED LED 124. The RED LED 124 turns on and illuminates tissue. The resulting illumination is detected by the photo detector 128 as a pulse of light. The IR LED 126 is pulsed in the same manner by way of 144 and 145. The pulse rate at which the LEDS 124 and 126 are operated is much higher than the heart pulse rate being measured. The first sample and hold circuit 150 is enabled by a pulsed BACKGROUND signal on signal line 112k. In response to the BACKGROUND signal, the first sample and hold circuit accumulates a charge from the photo detector 128 before the LEDS 124 and 126 are turned on. This charge represents background and/or ambient illumination sensed by the photo detector 128. When the first sample and hold circuit 150 is disabled, the accumulated charge is subtracted by the unity gain difference amplifier 155 from the signal produced by the photo detector 128, which is coupled to the positive input of the unity gain difference amplifier 155. This corrects the response of the photo detector 128 to illumination produced by the LEDS 124 and 126 for background illumination also sensed by the photo detector 128. The phasing of this operation with the pulsing of the LEDS 124 and 126 is shown in Figure 3. The second sample and hold circuit 170 is enabled by a pulsed RED SAMPLE signal on signal line 112j. The response of the photo detector 128 to illumination from the RED LED 124, minus the response to background illumination, is thereby coupled to the second sample and hold circuit 170 by way of the unity gain difference amplifier 155 and the amplifier 160. The second sample and hold circuit 170 is then disabled. If the data acquisition system 100 is being operated in a calibration mode, the charge accumulated by the second sample and hold circuit 170 is sensed by the ADC 180 on the RED CAL signal line. The third sample and hold circuit 174 is operated in the same manner in response to the IR SAMPLE signal on signal line 112i. The phasing of the RED SAMPLE and IR SAMPLE signals with respect to the BACKGROUND, RED LED ON and IR LED ON signals is illustrated in Figure 3. There are two modes of operation that must be conducted in the data acquisition system 100: calibration and data acquisition. During the first (calibration) mode, a microcontroller that interfaces at 110 with the signal generation and data acquisition sections sets the drive on the LED's 124 and 126 by varying the output voltage of the DAC 140 to a predetermined level such as 2 volts peak amplitude. This function is accomplished by setting the IR and RED calibration voltages for the RED and IR LED's 124 and 126 by monitoring the RED and IR channels via the ADC 180. Once set so that the detected output of the RED and IR channels is equal, the RED and IR reference voltages are set to the same magnitude, and the data acquisition system 100 is switched to the data acquisition mode. Upon initiating the data acquisition mode the RED and IR reference voltage levels from the DAC 140 are set to equal the RED and IR calibration voltage levels. During data acquisition the analog output from the adjustable gain amplifiers 171 and 175 is the differential between the RED and IR reference voltages and the outputs of the RED and IR sample and hold circuits 170 and 174. This method literally "zooms" on to the peak of the detected voltage pedestal and extracts useful signal information that is presented as small pedestal amplitude variations. A graphic representation of this process is shown in Figure 2. In Figure 2, presume that the output of the second sample and hold circuit 170 is plotted as time progresses from left to right. The narrow pulses are electrical pulse signals output by the second sample and hold circuit 170. These are referred to as "pedestals" hereinafter. The pedestals of the second sample and hold circuit 170 are compared against the RED reference voltage signal by the differential amplifier 171. The amplifier 171 produces a waveform constituting an envelope on the peaks of the pedestals. Such an envelope is represented by the solid line 190 in Figure 2. The envelope is filtered by the low pass filter 172 and the filtered envelope is provided to the ADC 180. The ADC 180 quantizes the filtered envelope and provides a quantized, filtered envelope signal to a microcontroller via the interface 110. The IR channel 174, 175, 176 similarly provides a filtered envelope signal to the ADC 180. The envelope 190 represents a single heart pulse signal. A sequence of such heart signals represents the heart pulse of a being. With reference to Figures 1-3, representative sequence timing for the data acquisition process may progress as follows: 1. A BACKGROUND subtraction sample of ~ 1 millisecond in duration is taken which closes the FET switch in the first sample and hold circuit 150 and stores a charge on the capacitor 151 equaling the ambient illumination bias voltage on the photo detector 128. This level is maintained throughout the data acquisition process and is subtracted from detected pedestal signals from either the detected RED or IR channels by the the unity gain difference amplifier 155. 2. A sequence of 3-millisecond duration RED LED ON and IR LED ON pulses is initiated. As can be seen, these signals do not overlap and are separated by approximately 1 millisecond. Approximately 1 millisecond after the initiation of the RED ON pulse the RED SAMPLE pulse turns on the second sample and hold circuit 170 for ~ 1 millisecond to sample the detected pedestal and store this voltage on the sample and hold capacitor. Then approximately 1 millisecond after the initiation of the IR ON pulse the IR SAMPLE pulse turns on the third sample and hold circuit 174 for ~ 1 millisecond to sample the detected pedestal and store this voltage on the sample and hold capacitor. These voltages are the instantaneous detected levels of a Plethysmography signal from the measurement site. The output of each of the second and third sample and hold circuits is a varying analog voltage following the Plethysmographic signal from the measurement site as shown in Figure 2. Each of these varying signals is compared differentially to corresponding RED or IR reference voltages and amplified. Varying voltages output from the adjustable gain amplifiers 171, 175 are then smoothed at 172, 176 to remove small voltage transients caused by sample and hold switching and noise introduced from the sensor. The -3dB roll-off of each of the low-pass the filters 172 and 176 is ~12 Hz. Smoothed and filtered signals are then sent to the differential input of the ADC and nominally digitized to 12-bits. 3. The sequential process illustrated in Figure 3 is repeated every 10 milliseconds. This equates to a nominal data sampling rate of 100 samples per second for each channel. Programming the microcontroller can easily change the data sampling rate for this system. Data acquisition continues until a calibration sequence is initiated due to anomalies in the digitized output signal. A calibration sequence can be initiated quite frequently during measurements at body sites where there is a good deal of motion or changes in the coupling of the sensor to tissue. According to a second embodiment of the data acquisition system, the number of calibration sequences required to make accurate measurement is minimized by reducing the effect of body motion artifacts in the detected signal. The auto calibration circuitry 200 as seen in Figure 4 accomplishes this. With reference to the RED channel, the input operational amplifier 210 has the alternating voltage components fed into differential inputs where the effect is to cancel or zero the AC voltage. The DC operating point of the differential amplifier 212 is set by the voltage on the positive terminal. The DC output of the amplifier 212 is the average output of the sample and hold circuit 170 minus the AC component. This signal serves as both the RED reference and the RED calibration signals. The IR reference and calibration signals are similarly generated by 214/218. The time constant for the auto calibration circuit is approximately one second, which means that the system will only be able to respond to relatively slow DC variations in the pedestal amplitude. The purpose of adding the auto calibration feature is to minimize the number of calibrations required during data acquisition. All other components of the data acquisition circuitry operate in the same manner as discussed above with respect to Figure 1 with the exception of the method of generating the RED and IR reference voltages and the RED and IR calibration and reference voltages. Changing the manner in which these voltages are developed eliminates two outputs from the DAC 140. The RED and IR CAL inputs to the ADC 180 are used to monitor the absolute level of the reference voltage and set the pedestal level during a calibration sequence. This process streamlines data acquisition and will provide more accurate SpO2 (blood oxygen saturation) readings. Several advantages over existing pulse oximeters have been realized by these data acquisition techniques. First among these is the "ZOOMING" technique that significantly reduces the number of ADC bits required to achieve high-resolution digital data. By adding the auto-calibration circuitry to the design the number of calibration sequences required over time will be significantly reduced thus achieving a long term stability of the measurements being made. Additionally, the electronic circuitry required to achieve high accuracy SpO2 measurements has been reduced from what similar systems utilize.

Claims

1. A data acquisition system for a pulse oximeter, comprising: means for pulsing at least two light sources in a pulse oximeter probe; means for detecting a background signal of the oximeter probe; first channel means responsive to the background signal for detecting a first signal produced by the pulse oximeter probe at a first light wavelength; at least a second channel means responsive to the background signal for detecting a second signal produced by the pulse oximeter probe at a second light wavelength; and a first converter connected to the first and second channel means for converting the first and second signals to digital form.

2. The system of claim 1 , in which the means for detecting a background signal include: a first sample and hold circuit having a first input for being connected to receive an output from the pulse oximeter probe, a second input to receive a control signal and an output; and a unity gain difference amplifier having a negative input connected to the output of the sample and hold circuit, a positive input for being connected in common with the first input of the sample and hold circuit, and an output coupled to the first and second channel means.

The system of claim 2, in which: the first channel means include: a second sample and hold circuit having a first input coupled to the output of the unity gain difference amplifier, a second input to receive a control signal and an output; a differential amplifier having a positive input connected to the output of the unity gain difference amplifier, a negative input for receiving a first reference signal, and an output; and a low pass filter having an input connected to the output of the differential amplifier and an output connected to the first converter; and the second channel means include: a third sample and hold circuit having a first input coupled to the output of the unity gain difference amplifier, a second input to receive a control signal and an output; a differential amplifier having a positive input connected to the output of the unity gain difference amplifier, a negative input for receiving a second reference signal, and an output; and a low pass filter having an input connected to the output of the differential amplifier and an output connected to the first converter.

4. The system of claim 3, in which the means for pulsing include: a second converter having an input for receiving a digital signal, a first output for providing a first analogue drive signal in response to a digital signal, a second out put for providing a second analogue drive signal in response to a digital signal, a third output coupled to the negative input of the first channel means differential amplifier for providing the first reference signal in response to a digital signal, and a fourth output coupled to the negative input of the second channel means differential amplifier for providing the second reference signal in response to a digital signal; a first switch means connected to the first output of the second converter for producing a pulsed drive signal in response to the first analogue drive signal; and a second switch means connected to the second output of the second converter for producing a pulsed drive signal in response to the second analogue drive signal.

5. The system of claim 4, further including a potentiometer having: a first resistance with a first terminal connected to the output of the first channel means differential amplifier, a second terminal connected to the third output of the second converter, and an adjustable resistance terminal connected to the negative input of the first channel means differential amplifier; and a second resistance with a first terminal connected to the output of the second channel means differential amplifier, a second terminal connected to the fourth output of the second converter, and an adjustable resistance terminal connected to the negative input of the second channel means differential amplifier.

6. The system of claim 5, wherein the first light wavelength is a visible red wavelength and the second light wavelength is an infrared wavelength.

7. A data acquisition system for a pulse oximeter, comprising: means for pulsing at least two light sources in a pulse oximeter probe; means for detecting a background signal of the oximeter probe; first channel means responsive to the background signal and a first reference signal for detecting a first signal produced by the pulse oximeter probe at a first light wavelength; at least a second channel means responsive to the background signal and a second reference signal for detecting a second signal produced by the pulse oximeter probe at a second light wavelength; circuit means connected to the first and second channel means for producing the first and second reference signals; and a first converter connected to the first and second channel means for converting the detected first and second signals to digital form.

8. The system of claim 7, in which the means for detecting a background signal include: a first sample and hold circuit having a first input for being connected to receive an output from the pulse oximeter probe, a second input to receive a control signal and an output; and a unity gain difference amplifier having a negative input connected to the output of the sample and hold circuit, a positive input for being connected in common with the first input of the sample and hold circuit, and an output coupled to the first and second channel means.

9. The system of claim 8, in which: the first channel means include: a second sample and hold circuit having a first input coupled to the output of the unity gain difference amplifier, a second input to receive a control signal and an output; a differential amplifier having a positive input connected to the output of the unity gain difference amplifier, a negative input for receiving the first reference signal, and an output; and a low pass filter having an input connected to the output of the differential amplifier and an output connected to the first converter; and the second channel means include: a third sample and hold circuit having a first input coupled to the output of the unity gain difference amplifier, a second input to receive a control signal and an output; a differential amplifier having a positive input connected to the output of the unity gain difference amplifier, a negative input for receiving the second reference signal, and an output; and a low pass filter having an input connected to the output of the differential amplifier and an output connected to the first converter.

10. The system of claim 9, in which the circuit means include: first differential amplifier means connected to the output of the second sample and hold circuit and coupled to the negative input of the first channel means differential amplifier for producing the first reference signal in response to signal variations on the output of the second sample and hold circuit; and second differential amplifier means connected to the output of the third sample and hold circuit and coupled to the negative input of the second channel means differential amplifier for producing the second reference signal in response to signal variations on the output of the second sample and hold circuit.

11. The system of claim 10, in which the means for pulsing include: a second converter having an input for receiving a digital signal, a first output for providing a first analogue drive signal in response to a digital signal, and a second out put for providing a second analogue drive signal in response to a digital signal; a first switch means connected to the first output of the second converter for producing a pulsed drive signal in response to the first analogue drive signal; and a second switch means connected to the second output of the second converter for producing a pulsed drive signal in response to the second analogue drive signal.

12. The system of claim 11 , further including a potentiometer having: a first resistance with a first terminal connected to the output of the first channel means differential amplifier, a second terminal connected to the first differential amplifier means, and an adjustable resistance terminal connected to the negative input of the first channel means differential amplifier; and a second resistance with a first terminal connected to the output of the second channel means differential amplifier, a second terminal connected to the second differential amplifier means, and an adjustable resistance terminal connected to the negative input of the second channel means differential amplifier.

13. The system of claim 12, wherein the first light wavelength is a visible red wavelength and the second light wavelength is an infrared wavelength.

14 A method for acquiring data from a pulse oximeter probe, comprising: producing a first electrical pulse signal in response to the first signal and pulsed illumination of tissue by the probe at a first light wavelength; correcting the first electrical pulse signal for background light sensed by the probe; producing at least a second electrical pulse signal representing pulsed illumination of tissue by the probe at a second light wavelength; and correcting the second electrical pulse signal for background light sensed by the probe.

15. The method of claim 14, further including: producing a first reference signal; producing a second reference signal; amplifying a difference between a sequence of first electrical pulse signals and the first reference signal to produce a first heart pulse signal; and amplifying a difference between a sequence of second electrical pulse signals and the second reference signal to produce a second heart pulse signal.

16. The method of claim 15, further including: converting a difference between the first heart signal and the first reference signal to digital form; and converting a difference between the second heart signal and the second reference signal to digital form.

17. The method of claim 14, further including iteratively performing the acts to produce a first sequence of first electrical pulse signals and a second sequence of second pulse signals.

18. The method of claim 17, further including: amplifying a difference between the first sequence and a first reference signal to produce a first heart pulse signal; filtering the first heart signal; amplifying a difference between the second sequence and a second reference signal to produce a second heart pulse signal; and filtering the second heart signal.

19. The method of claim 18, further including: converting a difference between the first heart signal and the first reference signal to digital form; and converting a difference between the second heart signal and the second reference signal to digital form.

20. The method of claim 19, wherein the first light wavelength is a visible red light wavelength and the second light wavelength is an infrared light wavelength.