Abstract:
A method and apparatus for providing a substantially real-time representation of an analog representation of a physiological signal is provided. The waveform signal from the sensor is converted into digital form. A delta-sigma modulator is used as a simple Digital-to-analog Converter (DAC). The output can then be provided through a simple hardware filter to give an analog output signal in nearly real-time, which can be used for other instruments, synchronization, display, etc.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to oximeters, and in particular to analog waveform displays in pulse oximeters. 
     Pulse oximetry is typically used to measure various blood chemistry characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient&#39;s tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed at various frequencies is then used to calculate the amount of blood constituent being measured. 
     The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation. 
     Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor. 
     Pulse oximeters, after processing the sensor data and calculating oxygen saturation, present that information to a display. In some pulse oximeters, it is also desirable to display the analog waveform itself. For example, U.S. Pat. No. 6,188,470 shows a signal for a display reflecting the waveform. U.S. Pat. No. 6,385,471 also discusses a waveform display, and sets forth that the data is first digitized, prefiltered, and then reconstructed for the display. 
     Nellcor Puritan Bennett, the assignee of the present invention, provides analog outputs in a number of its products. The analog outputs are used for such purposes as synchronizing to other instruments (e.g., EKG, multi-parameter monitor) as well as for a display, The analog waveforms are sometimes provided from the hardware pre-processing circuitry, to insure the analog signal is close in time to the actual patient waveform. 
     A problem with providing an analog waveform to a display after processing is that the processing takes some time, and thus the signal provided is delayed and not real-time. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for providing a substantially real-time representation of an analog representation of a physiological signal. The waveform signal from the sensor is converted into digital form. A delta-sigma modulator is used as a simple Digital-to-analog Converter (DAC). The output can then be provided through a simple hardware filter to give an analog output signal in nearly real-time, which can be used for other instruments, synchronization, display, etc. 
     The invention allows a waveform to be converted into digital form, and supplied to the software, while still allowing fast conversion back into hardware after initial processing in software. In particular, for a pulse oximeter that does demodulation in software, the digital IR signal can be obtained after this software demodulation, but before the much slower software filtering process used as part of the process to calculate oxygen saturation. 
     In one embodiment, in a first path the digital signal is processed, but a second path applies this digitized waveform to the delta-sigma modulator. The second path picks off the signal immediately after it is converted into digital form and demodulated. For a pulse oximeter, an IR signal is chosen for the analog output because it typically has less noise. 
     In one embodiment, the delta-sigma modulator is a software modulator which operates on the digitized version of the waveform. The delta-sigma modulator provides a single bit, serial output. This output is provided to a hardware RC filter, and then to the display. 
     For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an oximeter incorporating the present invention. 
         FIG. 2  is a block diagram of a prior art circuit for generating an analog output signal. 
         FIG. 3  is a block diagram of an embodiment of a circuit for generating an analog output signal according to the present invention. 
         FIG. 4  is a block diagram of the software delta-sigma modulator according to an embodiment of the present invention. 
         FIG. 5  is a circuit diagram of an embodiment of the hardware RC filter according to an embodiment of the present invention. 
         FIG. 6  is a front view of a monitor showing an analog display according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overall System 
       FIG. 1  illustrates an embodiment of an oximetry system incorporating the present invention. A sensor  10  includes red and infrared LEDs and a photodetector. These are connected by a cable  12  to a board  14 . LED drive current is provided by an LED drive interface  16 . The received photocurrent from the sensor is provided to an I-V interface  18 . The IR and red voltages are then provided to a sigma-delta interface  20  incorporating the present invention. The output of sigma-delta interface  20  is provided to a microcontroller  22 . Microcontroller  22  includes flash memory for a program, and RAM memory for data. The oximeter also includes a microprocessor chip  24  connected to a flash memory  26 . Finally, a clock  28  is used and an interface  30  to a digital calibration in the sensor  10  is provided. A separate host  32  receives the processed information, as well as receiving an analog signal on a line  34  for providing an analog display. 
     Prior Art Demodulation in Hardware 
       FIG. 2  shows an example of a prior art circuit for generating an analog output signal. A signal from a patient sensor is processed in hardware through a current-to-voltage converter (I-V)  36 , and a filter  37 . The red and IR signals are then demodulated in a demodulator  38 . A red signal is provided through a first channel of a filter  39  and an amplifier  40  to an ADC  41 . Similarly, the IR signal is provided through a second channel of filter  42 , amplifier  43  and ADC  44 . The analog output is obtained from the IR signal at the input of ADC  44 . 
     Demodulation in Software in the Present Invention 
       FIG. 3  shows an embodiment of the present invention where demodulation isn&#39;t done in hardware, rather in software, so an analog IR signal simply is not available in hardware. A signal from a patient sensor is processed in hardware through a current-to-voltage converter  45  and a filter  46 , then is supplied to an ADC  47 . In software, a demodulator separates the red and IR signals. The red signal is then provided to a software filter  49  and further processing not shown. The IR signal is similarly provided through a software filter  50  and further processing not shown. Since the software filtering can cause a significant time delay, the IR signal before the filter  50  is converted back into analog form. A sigma-delta modulator  51  is used as a simple Digital-to-analog Converter (DAC). By using a sigma-delta modulator, the conversion process is simple and can be done quickly. The resulting analog signal then only needs to be filtered in a simple RC filter  52 . 
     Sigma-Delta Modulator and Filter for Simple DAC 
       FIG. 4  is a block diagram of a delta-sigma modulator  51  of  FIG. 3  according to an embodiment of the invention. This modulator is preferably implemented in software running on microcontroller  22  of  FIG. 1 . An input on line  52  is the digitized sensor signal. In a preferred embodiment this signal is the infrared (IR) signal as opposed to the red signal. The infrared is chosen because it is typically a cleaner signal than the red signal.  FIG. 4  is a graphical representation of the difference equations implemented to create the second order noise shaping for the quintile signal. In one implementation, pseudocode that implements the difference equations is: 
                                                 X is the input on line 52       Y is the output on line 54       A, B, and C are intermediate variables that store data from one iteration to       the next loop every 206 μS                A = X − C + (2 * B)           C = B           if A &gt; ½ then                 Y = 1           else                 Y = 0           end if           B = A − Y            end loop                    
This code is executed in a loop that executes every 206 μS, so the output (Y) is a 4845 bits/sec bit stream with an average value that is equal to the input (X).
 
     The output on line  54  is preferably a 4845 Hz bitstream. This is provided to the input  60  of a hardware filter as shown in  FIG. 5 . This filter includes resistors  62  and  64  and capacitors  66  and  68 . This filter acts on the digital output signal to convert it into analog form to produce an output on line  70  that can be provided to a display. The filter is a passive, second order RC filter, without a buffer on the output. Any buffering could be done by the host system before displaying, if required. 
       FIG. 6  shows an example of an analog display  72  on a pulse oximeter monitor  74 . The signal for this display is provided from line  70  of  FIG. 5 . 
     As will be understood by those of skill in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, the delta-sigma modulator could be of a different order than a second order. Some filtering could be done in software prior to the hardware filter, and a different configuration of the hardware filter could be used. Accordingly, the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.