United States Patent [19J
`Fein et al.
`
`I 1111111111111111 11111 1111111111 111111111111111 IIIII lllll 111111111111111111
`US006044283A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,044,283
`*Mar.28,2000
`
`[54] MEDICAL SENSOR WITH MODULATED
`ENCODING SCHEME
`
`[75]
`
`Inventors: Michael E. Fein, Mountain View;
`David C. Jenkins, Loomis; Michael J.
`Bernstein, San Ramon; K. L.
`Venkatachalam, Palo Alto; Adnan I.
`Merchant, Fremont; Charles H.
`Bowden, San Ramon, all of Calif.
`
`[73] Assignee: Nellcor Puritan Bennett Incorporated,
`Pleasanton, Calif.
`
`[ *] Notice:
`
`This patent is subject to a terminal dis(cid:173)
`claimer.
`
`[21] Appl. No.: 09/073,361
`
`[22] Filed:
`
`May 6, 1998
`
`Related U.S. Application Data
`
`[60] Continuation of application No. 08/451,630, May 26, 1995,
`Pat. No. 5,779,630, which is a division of application No.
`08/168,449, Dec. 17, 1993, Pat. No. 5,645,059.
`
`Int. Cl.7 ........................................................ A61B 5/00
`[51]
`[52] U.S. Cl. ........................... 600/310; 600/322; 600/323
`[58] Field of Search ............................ 600/310, 322-324,
`600/475-480, 300; 356/41
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`7/1997 Fein ........................................ 600/323
`5,645,059
`7/1998 Fein ........................................ 600/323
`5,779,630
`Primary Examiner-Robert L. Nasser
`Attorney, Agent, or Firm-Townsend and Townsend and
`Crew LLP
`
`[57]
`
`ABSTRACT
`
`The present invention provides an encoding mechanism for
`a medical sensor which uses a modulated signal to provide
`the coded data to a remote analyzer. The modulated signal
`could be, for instance, a pulse width modulated signal or a
`frequency modulated signal. This signal is amplitude inde(cid:173)
`pendent and thus provides a significant amount of noise
`immunity.
`
`24 Claims, 5 Drawing Sheets
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`
`Petitioner Apple Inc. – Ex. 1048, p. 1
`
`

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`
`Petitioner Apple Inc. – Ex. 1048, p. 2
`
`

`

`U.S. Patent
`
`Mar.28,2000
`
`Sheet 2 of 5
`
`6,044,283
`
`SENSOR -
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`
`Petitioner Apple Inc. – Ex. 1048, p. 3
`
`

`

`U.S. Patent
`
`Mar.28,2000
`
`Sheet 3 of 5
`
`6,044,283
`
`FIG 5.
`
`1140
`DRIVER (119) ~
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`FIG. 8B.
`
`Petitioner Apple Inc. – Ex. 1048, p. 4
`
`

`

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`
`Petitioner Apple Inc. – Ex. 1048, p. 5
`
`

`

`U.S. Patent
`US. Patent
`
`Mar.28,2000
`Mar. 28,2000
`
`Sheet 5 0f5
`Sheet 5 of 5
`
`6,044,283
`6,044,283
`
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`
`Petitioner Apple Inc. — Ex. 1048, p. 6
`
`Petitioner Apple Inc. – Ex. 1048, p. 6
`
`

`

`6,044,283
`
`1
`MEDICAL SENSOR WITH MODULATED
`ENCODING SCHEME
`
`This application is a continuation of U.S. Ser. No.
`08/451,630, filed May 26, 1995, now U.S. Pat. No. 5,779,
`630, which is a divisional of U.S. Ser. No. 08/168,449, filed
`Dec. 17, 1993, now U.S. Pat. No. 5,645,059.
`
`2
`memory used can be fusible links connected to the inputs of
`a counter, shift register or other device. The coded informa(cid:173)
`tion is provided in the duty cycle of the pulse width
`modulated signal. The averaged duty cycle can be made to
`5 match a fixed voltage level for resistors used in existing
`oximeter probes. In this way, an existing oximeter, which
`would read an averaged pulse width modulated signal,
`would think that it is reading a resistor and get the same
`value. Thus, the probe can be made compatible with oxime(cid:173)
`ters in the field.
`The present invention additionally allows multiple types
`of information to be coded. In addition to the duty cycle in
`one embodiment, for instance, the pulses could be placed in
`different time slots within a period to indicate information.
`15 Thus, one set of information is indicated by the location of
`the pulse or pulses, and another set of information by the
`fraction of time over the period a pulse or pulses are present.
`A further understanding of the nature and advantages of
`the invention may be realized by reference to the remaining
`portion of the specification and the drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a prior art resistor encoding
`pulse oximeter system;
`FIG. 2 is a block diagram of a sensor according to the
`present invention;
`FIG. 3 is a timing diagram illustrating pulse width modu(cid:173)
`lation according to the present invention;
`FIG. 4 is a timing diagram illustrating time slot encoding
`according to the present invention;
`FIG. 5 is a diagram illustrating frequency modulation
`according to the present invention;
`FIG. 6 is a detailed circuit diagram of one embodiment of
`a sensor according to FIG. 2;
`FIG. 7 is a diagram of an open collector output stage for
`the circuit of FIG. 6;
`FIGS. SA and SB are diagrams of two alternative
`resistance-reading circuits; and
`FIG. 9 is an example of a set of curves relating an
`intermediate value, R, computed by a pulse oximeter, to the
`oxygen saturation value, S, which the pulse oximeter will
`report.
`
`45
`
`BACKGROUND OF THE INVENTION
`The present invention relates to medical sensors which 10
`include coded calibration information relating to character(cid:173)
`istics of the sensor.
`An example of such an encoding mechanism is shown in
`U.S. Pat. No. 4,700,708. This relates to an optical oximeter
`probe which uses a pair of light emitting diodes (LEDs) to
`shine light through a finger, with a detector picking up light
`which has not been absorbed by oxygen in the blood. The
`operation depends upon knowing the wavelength of the
`LEDs. Since the wavelength of LEDs actually manufactured
`can vary, a resistor is placed in the sensor with the value of 20
`the resistor corresponding to the actual wavelength of at
`least one of the LEDs. When the instrument is turned on, it
`first applies a current to the coding resistor and measures the
`voltage to determine the value of the resistor and thus the
`value of the wavelength of the LED in the probe. A disad- 25
`vantage of this system is that it is dependent upon an analog
`amplitude level which can be affected by wiring impedance,
`noise, etc. Another disadvantage is that considerations of
`cost and error budget limit the number of distinguishable
`resistance values that may be employed, so that the amount 30
`of information conveyable by this means is limited.
`Another method of storing coded information regarding
`the characteristics of the LEDs is shown in U.S. Pat. No.
`4,942,877. This patent discloses using an EPROM memory 35
`to store digital information, which can be provided in
`parallel or serially from the sensor probe to the remote
`oximeter. This system either requires more wires to read the
`data in parallel, or requires the reading of a multiple, serial
`bit pattern rather than a single amplitude level as in the 40
`system using the resistor. In addition, an oximeter designed
`to read a resistor would be incompatible with a probe having
`such an EPROM memory structure.
`Other examples of coding sensor characteristics exist in
`other areas. In U.S. Pat. No. 4,446,715, assigned to Camino
`Laboratories, Inc., a number of resistors are used to provide
`coded information regarding the characteristics of a pressure
`transducer. U.S. Pat. No. 3,790,910 discloses another pres(cid:173)
`sure transducer with a ROM storing characteristics of the
`individual transducer. U.S. Pat. No. 4,303,984 shows 50
`another sensor with digital characterization information
`stored in a PROM, which is read serially using a shift
`register.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`Operation of Prior Art Resistor Calibration
`FIG. 1 illustrates an oximeter probe 1 and an oximeter 60
`according to the prior art. Such an oximeter is described in
`more detail in U.S. Pat. No. 4,700,708, which is incorpo(cid:173)
`rated herein by reference. Oximeter 60 contains a micro(cid:173)
`processor 61, and a read only memory 62 and random access
`memory 63. A table of resistances and corresponding coef-
`55 ficients may be easily programmed into ROM 62 at the time
`oximeter 60 is fabricated.
`When each probe 1 is manufactured, a value for resistor
`40 is chosen to correspond to the wavelength of one or both
`of LEDs 10 and 20. When the probe is then connected to
`oximeter 60, the value of resistor 40 can be read by oximeter
`60. Typically, this resistance corresponds to the wavelength
`of at least one of the LEDs, and allows the oximeter to look
`up stored coefficients for a calibration curve corresponding
`to that wavelength or those wavelengths (FIG. 9 shows
`65 examples of these curves). Current I from current source 69
`is passed through resistor 40. The resulting voltage is
`detected at the input of multiplexer 66 and is passed through
`
`SUMMARY OF THE INVENTION
`
`The present invention provides an encoding mechanism
`for a medical sensor which uses a modulated signal to
`provide the coded data to a remote analyzer. The modulated
`signal could be, for instance, a pulse width modulated signal
`or a frequency modulated signal. This signal is amplitude 60
`independent and thus provides a significant amount of noise
`immunity.
`In a preferred embodiment, the coding is provided in a
`memory, which is connected to a shift register or counter
`which produces a pulse width modulated output as it is
`cycled by a clock. For a pulse oximeter using LEDs, the
`code would indicate the wavelength of at least one LED. The
`
`Petitioner Apple Inc. – Ex. 1048, p. 7
`
`

`

`6,044,283
`
`5
`
`3
`multiplexer 66 through comparator 65, to microprocessor
`61. The analog value presented to the input of comparator 65
`is compared to a series of different analog values from
`digital to analog converter 70. When there is a match at the
`output of comparator 65, microprocessor 61 knows which
`digital input to DAC 70 produced this match, and thus the
`digitally encoded value of the analog voltage produced by
`resistor 40.
`Microprocessor 61 may be programmed to calculate the
`resistance of resistor 40 and thereafter to look up the
`coefficients of the calibration curves for the wavelengths of
`LEDs 10, 20 from a table in ROM 62. Microprocessor 61 is
`also programmed to itself use the coefficients in its oxygen
`saturation calculations. By this means, it is not required to
`recalibrate by hand oximeter 60 for each new probe 1 nor,
`alternatively, to require that LEDs 10, 20 be of precisely
`standardized wavelengths.
`The specific function and design of the circuitry sche(cid:173)
`matically illustrated in FIG. 1 is seen as obvious when taken
`in combination with the general description of its function.
`The function of microprocessors and read only memories are
`well known and understood and it is well within the capa(cid:173)
`bility of a person with ordinary skill in the art to design and
`program microprocessor 61 to calculate the resistance of
`resistor 40 and thereby obtain the coefficients corresponding
`to the wavelengths of LEDs 10, 20 from a simple lookup 25
`table in a ROM 62.
`Probe 1 may be used with any number of oximeters, the
`method of operation of which is well understood and beyond
`the scope of the teaching of the present invention. An
`example is found in U.S. Pat. No. 5,078,136, which is
`incorporated herein by reference. Basically, for each heart
`beat, fresh arterial blood is pumped into the arterioles and
`capillaries of, for example, a finger, thereby causing a
`periodic increase and decrease in light intensity observed by
`detector 30. The oxygen saturation of hemoglobin in the
`pulsatile blood may be determined by the oximeter 60. For
`any known wavelength, there are a number of known
`calibration coefficients referred to above. Given these coef(cid:173)
`ficients and measuring the intensity of diffused light
`received by detector 30, the oxygen saturation can be
`computed and displayed.
`Microprocessor 61, through LED control circuitry 67,
`operates LEDs 10, 20. Light from LEDs 10, 20 results in
`current in detector 30 which passes through amplification
`and filtration circuitry 68 to multiplexer 66. Comparator 65
`and a digital to analog converter 70 are operative as an
`analog to digital converter means to present a digital signal
`to the microprocessor 61, thereby allowing microprocessor
`61 to determine oxygen saturation and/or pulse rate. Results
`are shown on display 64.
`Present Invention Modulation Encoding
`FIG. 2 shows a sensor 80 with an innovative Memory
`Readout Circuit 82 according to one embodiment of the
`present invention. Alternately, the sensor and Memory Read(cid:173)
`out Circuit could be separate. The Memory Readout Circuit
`could be in an ASIC built into the sensor head, in the sensor
`connector, in a patient module, or in an oximeter instrument
`box. The sensor includes two LEDs 84 and 86 connected by
`lines 85 and 87, respectively, to a remote oximeter. A
`photodetector 88 detects light which is not absorbed by the 60
`patient's tissue and provides a signal on a line 89 to the
`remote oximeter. Also shown is a ground wire 90 and a
`calibration line 92 ( corresponding to the line used to connect
`to the calibration resistor of the prior art). In this embodi(cid:173)
`ment of the present invention, a memory 94 is provided, 65
`coupled to a triggerable pulse pattern generator 96 (shift
`register or counter). Also included is a clock 98.
`
`4
`In operation, when the circuit of FIG. 2 is provided with
`a voltage VDD from a voltage generator 100, the clock starts
`cycling the shift register or counter 96 which has an initial
`input provided by memory 94. This initial input determines
`the count and thus the duty cycle provided on serial output
`line 92. The duty cycle will indicate the value of at least one
`of the LEDs. Thus, on assembly, the memory is programmed
`to correspond to the value of the LEDs used. This duty cycle
`signal could be continuously generated, or only generated
`10 for a short time in response to an interrogating signal from
`the oximeter. The interrogating signal could be an enable
`signal applied to the clock or counter.
`An illustration of the output signal is shown in FIG. 3,
`wherein a pulse width modulated signal with a period T is
`15 shown. The signal is high for a portion of time P. Thus, the
`duty cycle of the signal is P/T. The duty cycle can be made
`to correspond to the wavelength of at least one of the LEDs
`used. For instance, duty cycles ranging from 1 % to 100%
`can correspond to related increments in the wavelength of
`20 the LED.
`The voltage generator 100 of FIG. 2 receives its power
`from a current provided from the oximeter on line 92.
`Alternatively, the pulsing of LEDs by the oximeter could
`provide the power to send the pulse modulated signal back
`to the oximeter monitor on line 92. Alternately, a battery
`could be used or a separate Memory Readout Circuit module
`could have its own power supply connection. By putting the
`power supply, shift register and clock elements in a separate
`Memory Readout Circuit module, the sensor can be made
`30 disposable while still being relatively inexpensive.
`Alternately, an ASIC design could be generated which
`would make the entire Memory Readout Circuit 82 very
`inexpensive. In this case it could be economical to include
`circuit 82 as part of every sensor assembly, even when
`35 sensors are designed to be disposable.
`In addition to the pulse width modulation, information
`could be encoded, in an alternate embodiment, by the
`location of the pulses within a period. FIG. 4 illustrates an
`example of a period having ten time slots, with pulses
`40 occurring in time slots 2, 3 and 8. The location of the pulses
`can indicate certain information which is different from the
`information conveyed by the duty cycle of the signal. The
`example in FIG. 4 has a duty cycle of 30%. In one example,
`the duty cycle could indicate the wavelength of an LED,
`45 while the location of the pulses in the time slots might
`indicate the brand or type of sensor being used, such as a
`finger sensor, a nose bridge sensor, etc. In another example,
`the second type of information could be the wavelength of
`a second LED. With the single resistor of the prior art, the
`50 resistor had to convey either the combined wavelength
`values of the two LEDs, or the wavelength value of one
`LED. The additional information provided by this invention
`could improve the accuracy of the calibration coefficients
`determined by the oximeter, because more information
`55 about the sensor would be known to the oximeter.
`Yet another example of the potential uses of the additional
`information-carrying capacity of the digital code would arise
`if a manufacturer wished to introduce special sensors incor(cid:173)
`porating three or more LEDs of different wavelengths, in
`addition to the more common sensors having LEDs of only
`two wavelengths. The number of LEDs present could be
`indicated by the digital code, so that the instrument could
`select an appropriate strategy for operating the LEDs and
`interpreting the light pulses seen by the detector.
`FIG. 5 shows an alternate method for transmitting modu(cid:173)
`lated information. A frequency generator could be used with
`the frequency being related to the wavelength of the LED or
`
`Petitioner Apple Inc. – Ex. 1048, p. 8
`
`

`

`6,044,283
`
`5
`other characteristic of a transducer in the sensor. FIG. 5
`illustrates a first portion 50 with a wavelength with a high
`frequency, indicating one coded value, and a second portion
`52 with a low frequency indicating a different coded value.
`Such a signal could be generated with a current-to-frequency
`converter, for instance. The current provided to the input of
`the converter could be controlled by a memory, or resistor
`which is used to vary the value of a current source.
`FIG. 6 is a circuit diagram of one embodiment of a portion
`of the sensor circuitry of FIG. 2. The block diagram shows
`an application specific integrated circuit (ASIC) design. The
`design uses four counters, counters 110, 112, 114 and 116.
`Input lines A to H can come from an external memory to the
`pads connected to the parameter input (PI) lines, or, a fusible
`link or other coding mechanism can be connected to each of
`the input lines. Counters 110 and 114 are connected in series
`to provide a first count. The time it takes for counters 110
`and 114 to progress from their initially set count to the
`capacity of the counters (255) will define the high portion of
`the pulse on output 118. The starting count of counters 110
`and 114 is defined by the inputs A-H, which are program(cid:173)
`mable in accordance with the wavelength of the LED placed
`in the individual sensor.
`Counters 112 and 116 are connected in series, with the
`inputs grounded. The amount of time between when the first
`two counters (110, 114) reach their full count and when the 25
`second two counters (112, 116) reach their full count will
`define the low portion of the output pulse on line 118.
`When first set of counters 110 and 114 reach their maxi(cid:173)
`mum count, the carry out signal of counter 114 passes
`through a delay circuit 120 to the K input of flip-flop 122.
`The delay is provided to ensure that the clock arrives at
`flip-flop 122 first. This causes the Q output to go low, which
`propagates through NAND gate 124 to output line 118. This
`defines the end of the high portion of the pulse width output.
`The low output is also provided as a feedback through delay
`line 126 to the parallel enable input (P) of counters 110 and
`114 to disable further counting of these counters until the
`next period.
`Counters 112 and 116 will have started their count at the
`same time as the other counters, but will count to a full count
`from 0. When the full count is reached, the carry out output
`of counter 116 is provided through a delay line 128 to the J
`input of flip-flop 122. This causes the Q output to go high,
`defining the end of the low period of the pulse width output
`on line 118. This output is also provided as a feedback
`through inverter 130 to the load input (L) of all the counters.
`This will reload the initial counts into all the counters to start
`the cycle over again. When the first set of counters reach the
`top of their count, this will define the end of the high portion
`of the duty cycle again.
`The counters receive their clock from an oscillator circuit
`132. A precise clock is not needed since the high and low
`portions of the ultimate output signal will be equally affected
`by the clock frequency, and thus variations in the clock's
`frequency will not affect the duty cycle, which encodes the 55
`information. This allows the circuit to be made relatively
`inexpensively.
`Similar immunity to variations in clock frequency can be
`achieved when digital information is to be communicated, as
`in the waveform of FIG. 4. The oximeter could be pro- 60
`grammed to calculate the actual clock rate by detecting the
`repeating pattern. Alternately, the clock frequency can be
`detected by requiring that every permitted code string begin
`with a regular series of ON/OFF pulses, so that the instru(cid:173)
`ment can recognize the clock frequency.
`The other pins and circuits are basically for test purposes.
`The CD_N, EN and ZA signals on the left of the diagram
`
`6
`are test inputs, as are the J, Kand CKI signals on the right.
`The PROCMON circuit is a process monitor cell used in
`testing the ASIC.
`As can be seen, output line 118 is driven by NAND gate
`5 124 to provide a pulse width modulated output. If this is
`hooked up to an existing oximeter input, such as input line
`41 of the oximeter of FIG. 1, this will meet a circuit 69
`which provides a current to this line. One way to make the
`circuit backward compatible is to use an open collector
`10 output for an output driver 119 such as shown in FIG. 7. As
`shown in FIG. 7, the driver 119 output would connect to the
`base of a transistor 140, with a collector of transistor 140
`being provided to output 118 which would connect to input
`41 of FIG. 1. The current provided from the oximeter would
`15 thus have nowhere to go; indicating a very high resistance,
`when transistor 140 is turned off due to line 118 being low.
`When line 118 is high, the current will go to ground,
`indicating a short. The effective voltage or average voltage
`seen by the oximeter monitor will be the duty cycle times the
`20 maximum voltage provided on line 41 by the circuitry of the
`oximeter monitor 60 of FIG. 1.
`Alternately, a separate switch Sl as shown in FIG. 6 could
`be used. The switch alternately connects between two resis(cid:173)
`tance values, RA and Rs. Switch Sl could be implemented,
`for example, as a pair of field effect transistors (FET's). One
`FET could be responsive to a high level signal from driver
`119, while the other FET would be responsive to a low level
`signal. These could also be implemented as part of the
`outputs of driver 119 itself. Such a circuit has an advantage
`30 where an infinite or zero impedance is undesirable for the
`reading circuit in the oximeter itself, as discussed below.
`FIGS. SA and SB show block diagrams for two alternative
`circuits which may be used in the oximeter monitor to read
`the value of calibration-indicating resistor 40, which is
`35 commonly known as Real. In FIG. SA appears the same type
`of circuit that is also shown in FIG. 1. Current source 69
`sends a known current I through resistor 40, and the result(cid:173)
`ing voltage is passed to a multiplexer, which in turn will
`deliver the voltage to an analog-to-digital converter, which
`40 will produce a digital value proportional to the voltage. The
`digital value can then be interpreted by microprocessor 61 to
`determine the value of resistor 40. A disadvantage of this
`method is that part of the error budget (margin for error) for
`reading of resistor 40 will be consumed by errors in the
`45 value of current I and errors in reading the resulting voltage.
`FIG. SB shows a method which eliminates these contri-
`butions to the error budget. A reference voltage Vref is
`applied to the series combination of a standard impedance
`Zstd and Real resistor 40 which is to be measured. The
`50 resulting voltage across Real, which is labelled Veal in FIG.
`SB, is fed to a multiplexer, and from there to an analog to
`digital converter. By one of several known means, micro(cid:173)
`processor 61 then gains information as to the ratio of Veal
`to Vref. One such means is to apply Vref to the reference
`input of the analog to digital converter, so that Vref becomes
`the reference voltage used by the analog to digital converter.
`The output of the ADC will then automatically represent the
`ratio of Veal to Vref. Alternatively, an analog to digital
`converter may separately measure the values of Veal and
`Vref, using some third voltage as a reference, and the two
`resulting readings may be delivered to microprocessor 61.
`Microprocessor 61 can then determine the ratio by digital
`division.
`In the description of FIG. 2, it was explained that a
`65 function of Memory Readout Circuit 82 is to cause alterna(cid:173)
`tion of the voltage level on Real readout line 41, between
`high and low states, in accordance with the pattern stored in
`
`Petitioner Apple Inc. – Ex. 1048, p. 9
`
`

`

`6,044,283
`
`8
`of instruments. To accomplish this end, it would produce
`different values of fA depending on the value of Rstd. Circuit
`82 could include a state machine or processor programmed
`to determine the value of Rstd by any method equivalent to
`the following sequence of steps:
`1. Observe the open circuit value of applied voltage,
`which would be Vref;
`2. observe the current I which flows when its output line
`is connected through a finite or zero resistance RK to ground;
`and
`
`- RK
`
`Vret
`3. compute Rstd = -
`
`1-
`
`As in the case of the current-source readout of FIG. SA,
`it would be possible to establish the value ( or values) of fA
`at time of manufacture, to correct for statistical variability in
`the values of RA and Rs.
`The voltage supply VDD can be provided to the circuit of
`FIG. 6 from the current provided by the oximeter on its
`output line 41 of FIG. 1. This is shown in FIG. 6 where line
`41 connects to output 118 through a resistor 150. A diode
`152 and capacitor 154 are added to provide a supply voltage
`VDD from the current generated by the oximeter to read the
`value of the "resistor" the oximeter expects to see. Resistor
`150 is provided to limit the current flow.
`The memory for encoding of a pulse width ( or frequency
`for frequency modulation) can be provided in a number of
`different ways. Programmable links connected to data lines
`can be programmed at the time each LED is placed into a
`sensor during manufacturing. Alternately, a memory chip
`can be programmed to correspond to the LED value.
`Alternately, a set of, say, 20 pre-programmed memory chips
`can be available, with the assembler picking the one having
`35 the code which most closely represents the measured wave(cid:173)
`length of the LED.
`One advantage of sensors made according to this inven(cid:173)
`tion is that they can be designed to be back-compatible with
`early-generation instruments, while offering new capabili-
`40 ties when used in combination with new instruments. For
`example, oximeters made according to U.S. Pat. 4,700,708
`are designed to recognize an identifying resistor, such as
`resistor 40 in FIG. 1, whose value instructs the oximeter to
`select a particular calibration curve from among a set of
`45 possible curves whose defining coefficients are stored in
`ROM.
`For example, FIG. 9 shows a set of curves relating an
`intermediate computed value, called R, to the final value, S,
`of oxygen saturation, which is to be delivered by an oxime(cid:173)
`ter. The third of a set of five possible curves has been
`selected, in accordance with the value of an identifying
`resistor, and has been used to compute from particular value
`R0 a saturation value S0 • Five curves are shown for ease of
`illustration, 21 curves is more desirable. Also shown in FIG.
`55 9 is dashed line 6. This is a calibration curve for a hypo(cid:173)
`thetical newly-designed sensor. While old sensors may use
`21 curves, curve 6 could be, for instance, one of a hundred
`more accurate curves for newer sensors. Curve 6 does not
`exactly match any of standard curves 1 through 5. It would
`60 be possible to choose a digital pattern for this sensor such
`that its duty cycle would simulate resistor number 3 (so that
`existing oximeters would assign best-match curve 3 to the
`sensor). Meanwhile, many different digital codes would be
`available, all of which would have the same percentage ON
`65 time, which could be used to communicate to properly(cid:173)
`designed newer instruments a choice among a much larger
`variety of standard curves, one of which might be substan-
`
`15
`
`7
`memory 94. FIG. 6 schematically shows an output circuit
`that may be incorporated in Memory Readout Circuit 82, in
`order to accomplish this alternation. Switching element Sl,
`(which may, for example, be implemented using two FET's)
`switches line 41 alternately between two alternate resistance 5
`values, RA and Rs· Either of these resistance values may be
`selected to be zero (i.e. a short circuit to ground) or an open
`circuit (i.e. a quasi-infinite resistance).
`When the output circuit of FIG. 6 is to be used with the
`readout circuit of FIG. SA, resistors RA and Rs are restricted 10
`to being finite or zero (i.e. not an open circuit), since the
`voltage created by current source 69 across a quasi-infinite
`resistance would be undefined. When the readout circuit is
`to be of the form shown in FIG. SB, this restriction to finite
`values is not necessary.
`Now consider how the duty cycle of the output of
`Memory Readout Circuit 82 may be selected, so as to
`simulate the value of a particular value of Real. Define fA as
`the fraction of time during one full readout cycle that the
`switch is connected to resistor RA, and fs as the fraction of 20
`time that the switch is connected by resistor Rs. By design,
`fA+fs=l. First take the case of the current-source readout of
`FIG. SA When current I from source 69 is applied to the
`circuit of FIG. 6, the mean voltage that appears on line 41
`will be I(RA +fsRs)- So, for example, if Rs=0, a selected 25
`resistance Rs can be simulated by choosing fA=R5/RA. To
`provide a more specific example, if the resistance to be
`simulated is 6.04 KQ, RA=lO.0 KQ, and Rs=0, then the
`appropriate value of fA will be 0.604.
`In order to eliminate the effect of normal statistical 30
`variations in values of RA during manufacturing, it may be
`desirable to simulate a particular resistance not by selecting
`a particular value of fA, but rather by selecting fA at the time
`of manufacturing so that the product fA RA has the desired
`value. Thus two differ