Abstract:
A pulse oximeter with drive lines for driving red and IR LEDs, and a drive circuit for driving those drive lines. A processor controls the drive circuit using a red zero output line and an IR zero output line directly connected between the processor and the drive circuit. This allows a control signal to directly control the turning off of either the red or IR drive transistors to prevent forward current flow through the red and IR LEDs by overriding the ongoing programmable logic state machine control of the drive transistors. The effects of crosstalk and capacitive coupling are reduced as a result.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to oximeters, and in particular to LED drive circuits in pulse oximeters.  
         [0002]     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 wavelengths is then used to calculate the amount of blood constituent being measured.  
         [0003]     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.  
         [0004]     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.  
         [0005]     The light sources, typically light emitting diodes (LEDs), need to be driven with current to activate them. In order to reduce the effects of leakage and capacitively coupled transients, it is desirable to be able to drive one of the LEDs, without any current going through the other one. Typically, this can be done by controlling the duty cycle with the processor in the pulse oximeter. However, using the duty cycle controls to eliminate current through one of the LEDs has been discovered to still involve an amount of leakage and capacitively coupled transients that is undesirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a pulse oximeter with drive lines for driving red and IR LEDs, and a drive circuit for driving those drive lines. A processor controls the drive circuit using a red zero output line and an IR zero output line directly connected between the processor and the drive circuit. This allows a control signal to directly control the turning off of either the red or IR drive transistors which direct forward current flow through the red and IR LEDs.  
         [0007]     In one embodiment, the red and IR zero output lines are connected to a programmed logic circuit. The programmed logic circuit, which is controlled by the processor, provides the various timing signals for the transistors of the drive circuit. In one embodiment, the drive circuit includes an H-bridge circuit with red and IR FET drive transistors.  
         [0008]     For a further understanding of the nature and advantages of the present invention, reference should be made to the following description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a block diagram of an oximeter incorporating the present invention.  
         [0010]      FIG. 2  is a circuit diagram of a LED drive circuit according to an embodiment of the present invention.  
         [0011]      FIG. 3  is a block diagram of one embodiment of the logic for generating the timing and control signals for the circuit of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Oximeter Front End  
         [0012]      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  which includes a 10-bit A/D converter. Controller  22  includes flash memory for a program, and EEPROM memory for data. The processor also includes a controller 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.  
         [0000]     LED Drive Circuit  
         [0013]      FIG. 2  is a circuit diagram of the LED drive circuit according to an embodiment of the invention, which forms a portion of LED drive interface  16  of  FIG. 1 . A voltage regulator  36  provides a voltage separate from the voltage supply for the overall oximeter circuitry. The output is provided as a 4.5 volt signal on line  38 , with the level being set by the feedback resistor divider of resistors R 89  and R 90 . The voltage on line  38  is provided to a FET transistor Q 11  to an inductor L 6 . The current through inductor L 6  is provided by a switch  40  to one of capacitors C 65  and C 66 , which store charge for the red and IR LEDs, respectively. A red/IR control signal on line  42  selects the switch position under control of the oximeter processor. A control signal LED PWM gate on line  44  controls the switching of transistor switch Q 11 .  
         [0014]     Once the capacitors are charged up, the control signal on line  44  turns off switch Q 11  and current is provided from either capacitor C 65  or C 66 , through switch  40  and inductor L 6  to either the red anode line  46  or the IR anode line  48  by way of transistors Q 5  and Q 6 , respectively. A signal “red gate” turns on transistor Q 5 , while its inverse, “/red gate” turns off transistor Q 7 . This provides current through the red anode line  46  to the back to back LEDs  50 , with the current returning through the IR anode to transistor Q 8  and through resistor R 10  to ground. Transistor Q 8  is turned on by the signal “/IR gate” while the inverse of this signal, “IR gate” turns off transistor Q 6 . The signals are reversed when the IR anode is to be driven, with the “IR gate” and “red gate” signals, and their inverses, changing state, so that current is provided through transistor Q 6  to IR anode  48  and returns through red anode  46  and through transistor Q 7  to resistor R 10  and ground. The “LED current sense” signal is read for calibration purposes not relevant to the present invention.  
         [0015]     When the current from the capacitor C 65  or C 66  is provided through inductor L 6  to the LEDs, and that current is switched off at the desired time, transistor Q 11  is turned on so that the remaining current during the transition can be dumped into capacitor C 64 . This addresses the fact that the FET transistor switching is not instantaneous. Subsequently, C 64  will dump its current through Q 11  and inductor L 6  into the capacitors when they are recharged.  
         [0016]     Resistor R 38  and capacitor C 67  are connected in parallel to inductor L 6  to protect against signal spikes, and provide a smooth transition. Connected to inductor L 6  is a sampling circuit with a switch  52  controlled by an LED sample hold signal on line  54  to sample the signals and provide them through an amplifier  56  to a “LED current” signal on line  58  which is read by the processor. An integrating capacitor C 68  is provided in parallel to amplifier  56 . A switch  60  responds to a “clear LED sample” signal to operate the switch to short out the capacitor between samples.  
         [0017]     The sample and hold circuit measures the voltage at node T 18 , between capacitor C 69  and inductor L 6 , to determine the current. Capacitor C 69  is 1/1000 of the value of capacitors C 65  and C 66 . Thus, a proportional current is provided through C 69 , which is injected through switch  52  to integrating capacitor C 68  to provide a voltage which can be measured at the output of amplifier  56  on line  58 . The voltage measured by the processor on line  58  is used as a feedback, with the processor varying the width of the pulse delivered to transistor Q 11  to selectively vary the amount of energy that&#39;s delivered to the capacitors  65  and  66 , and then is eventually discharged to the LEDs  50 . A PI (Proportional Integral) loop inside the processor then controls the PWM signal at Q 11 . This allows precise control of the LED intensity, allowing it to be maximized, if desired, without exceeding the desired limits (to avoid burning the patient, etc.).  
         [0018]     The lower left of the diagram shows a “4.5 v LED disable” signal which is used by the microprocessor to turn off the voltage regulator  36  in certain instances. For example, diagnostics looking for shorts in a new sensor plugged in will turn off the voltage regulator if there is a problem with the LED line.  
         [0000]     Zero Calibration Control  
         [0019]      FIG. 3  illustrates processor  22 , from  FIG. 1 , connected to programmed logic  62 , which is in the LED drive interface  16  in  FIG. 1 . Programmed logic  62  provides the different control signals used by the circuit of  FIG. 2  in response to basic timing signals from the processor of a clock, a sync pulse, and a pulse width signal.  
         [0020]     As can be seen, processor  22  also provides a red zero signal on a line  64  and an IR zero signal on a line  66 . These two signals go to programmed logic circuit  62 . Programmed logic  62 , in response to assertion of the red zero signal, will provide appropriate control signals on the red gate, /red gate, IR gate and /IR gate control outputs to control the drive transistors in  FIG. 2 . In particular, assertion of the red zero signal will cause the red gate signal to turn off transistor Q 5  and transistor Q 8 . The programmable logic for switching between the LEDs still functions, but is overridden by this zero signal. Thus, the red gate is held at its value regardless of efforts by the programmable logic state machine to cycle it on and off. Similarly, assertion of the IR zero signal on line  66  will cause program logic circuit  62  to turn off transistor Q 6  with the IR gate signal, and turn off transistor Q 7  with the /red gate signal.  
         [0021]     These control signals thus assure that current only flows through the red LED or the IR LED, without any leakage due to switching between them while the appropriate red zero or IR zero signal is asserted. This significantly reduces any switching leakage due to use of the duty cycle controls and any capacitively coupled switching transients.  
         [0022]     As will be understood by those of skill in the art, the present invention can be embodied in other specific forms without departing from the essential characteristics thereof. For example, a different drive transistor structure could be used, such as for LEDs that are not configured back-to-back, but rather have separate connections which are separately driven. 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.