US005954644A
`,15
`United States Patent
`5,954,644
`(i1} Patent Number:
`Dettling et al.
`[45] Date of Patent:
`Sep. 21, 1999
`
`
`[54] METHOD FOR AMBIENTLIGHT
`SUBTRACTION INA
`PHOTOPLETHYSMOGRAPHIC
`7A
`MAT
`1
`7
`MEASUREMENT INSTRUMENT
`Inventors: Allen Dettling, Broomfield, Colo.; Alan
`Martin, San Jose, Calif.; Kurt
`Aronow, Lafayette, Colo,
`
`[75]
`
`[73] Assignee: Ohmeda Ine., Louisville, Colo.
`
`[21] Appl. No.: 08/823,526
`
`cessesssesssersseeessesseersersnees 600/492
`5,152,296 10/1992 SimMOMs
`5,385,144
`1/1995 Yamanishi et al.
`.
`128/633
`
`5,503,148
`4/1996 Pologe el al.
`...........
`600/323
`5,555,882
`9/1996 Richardson etal.
`600/336
`5,632,272
`5/1997) Diab et al.
`seccccscsccessecsecseeseeses 600/323
`OTHER PUBLICATIONS
`
`“A New Family of Sensors for Pulse Oximetry” by Kastle,
`Noller, Falk, Bukta, Mayer
`and Miller Feb.
`1997
`Hewlett-Packard Journal Article 7.
`Sedra et al., “Microelectronic Circuits, 3rd Edition,” Saun-
`ders College Publishing, pp. 68 & 69, 1991,
`
`[56]
`
`Primary Examiner—Michael Peffley
`Mar.24, 1997
`Filed:
`[22]
`[SE] Unt, C12 cnssnnnsnnnnnnenenennetenne ASIB Soe, Asian Examner—Sryin & Yerell
`(52 US. Cl.
`_... 600/322; 600/336
`Attorney, Agent, or Firm—Holme Roberts & Owen, LLP
`
`Field of Search occ 600/310, 322-328,
`[58]
`[57]
`ABSTRACT
`600/336, 340, 473, 476; 356/39-41
`;
`:
`An improved photoplethysmographic measurement system
`is disclosed in whicha portion of a time division multiplexed
`References Cited
`(TDM) signal represents an ambient light level, and other
`ao
`oS oe 5
`TDM signal portions represent detected levels of two or
`U.S. PATENT DOCUMENTS
`more centered wavelengths of transmitted light. The ambi-
`7/1991
`Isaacson et als scccssssssssmeeeeeeom 600/336
`Re, 33,643
`ent and detected light portions of the signal are simulta-
`1/1972 Sedivyetal. .
`sae 356/41
`3,632,211
`neously applied to the inputs of an instrumentation
`4/1974 Tehang.........
`356/41
`3,802,776
`amplifier(s) so as to produce a continuous outpul vollage
`7/1974 Lubbers et al.
`. 356/41
`3,825,342
`that is proportional to a difference in voltage between the
`1/1977 Bober...........
`see 3237/1
`4,001,667
`ambient and detected light portions of a TDM signal. Such
`9/1981 Hamaguri....
`-- 600/323
`4,250,544.
`an approach provides for ambient
`light
`level subtraction
`Obs toes Seta etal.
`. aes
`“cad
`ae
`so
`a
`{198
`TEED seesesesseeees
`, 781,16
`ie
`336
`
`4,863,265 * 600/322~—-With reduced noise and componenttry.9/1989 Flower etal.
`
`5,144,951
`9/1992 Uematsu et al.
`«+
`128/633
`..
`5,149,503
`9/1992 Kohno et al. wo...
`_ 422/82.05
`
`26 Claims, 3 Drawing Sheets
`
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`PRE-AMP LIGHT SOURCE
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`AMPLIFICATION SECTION
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`PROCESSING/CONTROL
`SECTION
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`50
`AMPLIFICATION FILTERING
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`DRIVER
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`0001
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`Apple Inc.
`APL1048
`U.S. Patent No. 8,652,040
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,652,040
`
`0001
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`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 1 of 3
`
`5,954,644
`
`FIG.1
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`FIG.2
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`0002
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`

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`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 2 of 3
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`5,954,644
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`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 3 of 3
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`5,954,644
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`5,954,644
`
`1
`METHOD FOR AMBIENT LIGHT
`SUBTRACTION IN A
`PHOTOPLETHYSMOGRAPHIC
`MEASUREMENT INSTRUMENT
`
`FIELD OF THE INVENTION
`
`This invention relates to systemsthat utilize time division
`multiplexed (TDM) signals and, more particularly, to an
`improved photoplethysmographic measurement instrument
`in which an ambient light component is subtracted from a
`TDM signal. The inventionis particularly apt for implemen-
`lation using instrumentation amplifiers.
`BACKGROUND OF THE INVENTION
`
`In the field of analog data transmission, oneefficient data
`transmission technique is to utilize a TDM signal in which
`information corresponding with a plurality of sources is
`transmitted over a single data line. Data corresponding with
`each source is transmitted over the line in dedicated intervals
`which are generally regular in duration and sequenced. That
`is, al One particular point in time, data present on the line
`corresponds with only one of the sources. If the dedicated
`interval rate is sufficiently rapid, an apparency of continuous
`data transmission corresponding with each source is realized
`at the receiving end of the data line. In this regard, the TDM
`signal is de-multiplexed al the receiving end so as to separate
`the data into parallel channels, one corresponding with each
`source. De-multiplexing is generally performed in a syn-
`chronous switching operation.
`In some systems, after de-multiplexing, a first series of
`signal conditioning steps is performed which operate on the
`parallel channel source data. Thereafter, a second series of
`steps is performed which, once again, require the signal to
`be in a TDM form. Re-multiplexing ofthe parallel channels
`is necessary to regain the TDM signal format required for
`the second series of steps. After the second series of signal
`conditioning steps, the signal is de-multiplexed a second
`lime into parallel channels for completion of analog signal
`processing. The performance of each multiplexing/de-
`multiplexing iteration introduces switching noise into the
`resultant signal(s). As can be appreciated, such noise pre-
`sents system design considerations and limitations.
`Other limitations are also introduced by the performance
`of multiple de-multiplexing/multiplexing iterations,
`Specifically, each time either of these operations is
`performed, the overall parts count in the system is increased,
`Such an increase may significantly limit the reliability ofthe
`system and increase manufacturing costs. Moreover,
`the
`associated increase in signal line length resulting from the
`additional parts, along with their
`interconnections, may
`serve to couple still further noise into the system from the
`ambient environment,
`thereby reducing system perfor-
`mance.
`
`The noted design considerations/limitations are of par-
`ticular importance in medical
`instruments that determine
`pulse rate and blood oxygen saturation level via measure-
`ment of certain blood analytes such as, for example, the
`concentration (as a percentage of total hemoglobin) of
`oxyhemoglobin (O,Hb), deoxyhemoglobin (RHb), carboxy-
`hemoglobin (COHb) and methemoglobin (MetHb) of a
`patient. Such photoplethysmographic measurement instru-
`ments are configured to emit light of at least two different,
`predetermined wavelengths through a selected portion of a
`patient’s anatomy (e.g., a finger tip). The analytes to be
`identified within the patient’s blood must each have unique
`light absorbance characteristics for at
`least
`two of the
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`emitted wavelengths. By measuring changesin intensity of
`the transmitted (the light exiting an absorber is referred to as
`transmitted) light from the patient's finger (or other suitable
`area of anatomy) at these wavelengths, each analyte may be
`determined. Thereafter, characteristics such as blood oxygen
`saturation may be determinedbasedon these analytes. Other
`characteristics such as pulse rate may be determined based
`on certain components of the transmitted light signal which
`passes through the patient’s anatomy. Specifically, the trans-
`mitted light includes a large DC component and a smaller
`AC or pulsatile component. By using the pulsatile
`component,
`the patient’s pulse rate may be determined,
`since fluctuations in the pulsatile component are a function
`ofarterioles pulsating with the patient’s heart rate.
`In one photoplethysmographic measurement system
`known as a pulse oximeter, at least two wavelengthsoflight
`may be emitted during dedicated, alternating intervals. The
`transmitted light from the selected body portion is detected
`by a light-sensitive element (e.g., a photodiode). The light-
`sensitive element then outputs a TDM signal that includes
`portions corresponding with each wavelength of the trans-
`mitted light. As will be appreciated, the photodiode ts also
`sensitive to light which is present in the ambient environ-
`ment. Consequently, the TDM output signal can include a
`corresponding ambient light component. Such component
`must be removed from the TDM signal for proper process-
`ing. For this purpose, at least one interval within a TDM
`signal
`is typically dedicated to measuring a component
`corresponding with only the detected ambient light.
`For example in one known pulse oximeter, cach emitted
`light
`level
`is immediately preceded by an ambient
`light
`interval which may also be referred to as a “dark ume”
`interval. The system first de-multiplexes the TDM signal
`into parallel channels. Signal processing then proceeds
`wherein a first series of steps performs preliminary filtering.
`Immediately following the first series of steps, the parallel
`channels are re-multiplexed. Next, a second series of steps
`is performed in which the re-multiplexed signal facilitates
`subtraction of the dark time signal from the signal corre-
`sponding with each emitted light interval in a manner known
`in the art. Such subtraction process relies on a dark time
`interval immediately preceding each and every emitted light
`interval in a TDM format. Following the second series of
`steps, in which ambient light subtraction is accomplished,
`the TDMsignalis de-multiplexed a secondtimeinto parallel
`channels prior to the completion of signal processing. Such
`multiple de-multiplexing/multiplexing raises the very noise
`introduction and cost concerns noted above.
`
`SUMMARY OF THE INVENTION
`
`Accordingly, primary objectives of the present invention
`are to provide an improved photoplethysmographic mea-
`surement system wherein ambient light subtraction from a
`TDMsignal is achieved with reduced noise and/or reduced
`componentry.
`In order to achieve such objectives, a system is provided
`having at least one TDM signal that includes at least a first
`identifiable portion that correspondswith detectedlight from
`at least one predetermined, light source plus any ambient
`light present in the system, and a second identifiable portion
`that corresponds with only the detected ambientlight present
`in the system, In one aspect of the invention, the system
`further includes amplification means havingfirst and second
`inputs and an output. The amplification means is configured
`to produce an amplified output on its output proportionalto
`a difference between signals present onits first and second
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`5,954,644
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`3
`inputs. Means are provided for substantially simultaneously
`applying the firstTDM signal portion to the first input and
`for applying the second TDM signal portion to the second
`input, at substantially the same time, such that the amplified
`output produced by the amplification means is proportional
`to the difference between the first and second signal
`portions, thereby achieving contemporaneous subtraction of
`the ambient light component and desired signal amplifica-
`tion. The contemporaneous amplification and ambient light
`level removal may be advantageously performed using an
`instrumentation amplifier.
`In another aspect of the invention, the system ts config-
`ured for emitting light through a region of interest al two or
`more different primary wavelengths in an environment
`which includes an ambient light level. The system separately
`detects the ambient light level and transmitted light level for
`each primary wavelength that has passed through the region
`of interest, such that the levels of detected ambient light and
`transmitted light form corresponding portions of a TDM
`signal. The level of transmitted light detected at each pri-
`mary wavelength includes the ambient light level. To pre-
`pare for removing the ambient light level from such trans-
`mitted light
`levels, demultiplexing means is provided for
`demultiplexing the TDM signal to provide (1) a first portion
`signal correspondingto the transmitted light levelat the first
`primary wavelength, (i) a second signal portion correspond-
`ing to the transmitted light
`level at
`the second primary
`wavelength and (iii) an ambient light signal portion corre-
`sponding to the detected ambientlight level. A first subtrac-
`tion means then produces a first ambient compensated
`output corresponding to the first primary wavelength by
`removing the ambient light level from the first signal por-
`tion. Similarly, while second subtraction means, separate
`from thefirst subtraction means, produces a second ambient
`compensated output corresponding to the second primary
`wavelength by removing the ambient light level from the
`second signal portion. Such separate first and second sub-
`traction means may, for example, comprise first and second
`instrumentation amplifiers, A processor means is employed
`to determine the value of the characteristic(s) of interest
`within the region of interest based on the first and second
`ambient compensated outputs.
`In a primary embodiment of the invention, a photopl-
`ethysmographic measurement system includes means for
`emitting light through a portion of a patient’s anatomy al two
`or more different, predetermined and centered wavelengths
`(¢.g. by intermittent emission). The transmitted portions of
`the emitted light for each centered wavelength and the
`ambient light
`level are detected so as to form respective
`ambient and detected light signal portions within the TDM
`signal. First and second amplification means are provided,
`each of which includesa first input, a second input and an
`output for producing an amplified output. The output pro-
`duced by each amplification means is proportional
`to a
`difference between signals present on its first and second
`inputs multiplied by a predetermined and variable gain. The
`system is configured to apply the ambient light signal to the
`first input of each amplification means while, at substantially
`the same time, applying the transmitted detected light sig-
`nals to the second input of the first and second amplification
`means. Consequently, the first amplification means produces
`a first output that is proportional to the difference between
`the detected signal corresponding to a first predetermined,
`centered wavelength and the ambient light level; and the
`second amplification means produces a second output thatis
`proportional to the difference between the detected signal
`corresponding to a second predetermined, centered wave-
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`length and the ambientlight of interest. The first and second
`amplification means may advantageously comprise separate
`first and second instrumentation amplifiers, each having a
`substantively linear response over a frequency range that
`accommodates the detected light. The first and second
`outputs from the amplification means are then processed to
`determine/output certain characteristics including, but not
`limited to, a patient’s pulse rate and blood oxygen saturation
`level and/or specific blood analyte information such as, for
`example,
`the concentration (as a percentage oftotal
`hemoglobin) of oxyhemoglobin (O,Hb), deoxyhemoglobin
`(RHb), carboxyhemoglobin (COHb) or methemoglobin
`(MetHb) or to otherwise provide an indication when one of
`such measures exceeds a predetermined level of interest.
`The concentrations of a plurality of the noted analytes of
`Interest may be determined by using at
`least a common
`plurality of emitted wavelengths, provided that the analytes
`exhibit unique absorbance behavior at
`the emitted light
`wavelengths. By measuring changes in intensity of the
`transmitted light, for example, from a finger at the emitted
`wavelengths and based on the corresponding outputs ofthe
`amplification means, the aforementioned analytes are among
`those which may be determined in processing. Thereafter,
`characteristics such as blood oxygen saturation may be
`determined based on these analytes. Other characteristics
`such as pulse rate may be determined based on certain
`components of the transmitted light signal which passes
`through the patient’s anatomy. Specifically, the transmitted
`light includes a large DC component and a smaller AC or
`pulsatile component. By using the pulsatile component, the
`patient’s pulse rate may be determined, since fluctuations in
`the pulsatile component are a function ofarterioles pulsating
`with the patient’s heart rate.
`As will be appreciated, the present invention allows a
`system to be defined that employs only a single demulti-
`plexing step within the overall system. In such a system, a
`TDM, such as deseribed above, is demultiplexed into (i) a
`first signal corresponding to transmitted light at
`the first
`primary wavelength, (ii) a second output corresponding to
`the transmitted light at the second primary wavelength and
`(iii) an ambient light signal corresponding to the detected
`ambient
`light
`level. Thereafter, first and second ambient
`compensated outputs can be produced by removing the
`ambient
`light
`level
`from the first and second signals,
`respectively, using the ambient light signal. The first and
`second ambient compensated outputs may then be separately
`conditioned and combinatively processed to determine one
`or more ofthe noted characteristics of interest.
`
`Switching noise can be reduced in the present invention
`since a TDM signal need only be demultiplexed a single
`time into individual channels.
`
`Additionally, parts count and complexity can be reduced.
`Finally, as noted, the invention is particularly apt for imple-
`mentations using instrumentation amplifiers, thereby further
`yielding improved system performance andreliability.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present inven-
`tion and further advantages thereof, reference is now made
`to the following Detailed Description, taken in conjunction
`with the Drawings, in which:
`FIG, 1 is a diagrammatic illustration of a photoplethys-
`mographic measurement system implementing the present
`invention.
`
`FIG. 2 is a waveform illustrating a TDM signal produced
`at the preamp output by the system of the present invention.
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`5,954,644
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`5
`FIG. 3 is a block diagram illustrating the components
`which make up selected portions of the system of FIG. 1.
`FIG. 4 is a schematic representation of an amplification
`section used in the system of FIGS. 1 and 3.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 is a diagrammatic illustration of a photoplethys-
`mographic measurement system embodiment, generally
`indicated by reference numeral 10, constructed in accor-
`dance with the present invention. As will be described, the
`embodiment utilizes a time division multiplexed (TDM)
`signal in conjunction with an instrumentation-type amplifier.
`The system is configured to apply specific portions of the
`TDM signal to the inputs of the instrumentation amplifiers
`so as to produce continuous output voltages that are pro-
`portional to differences in voltage between different portions
`of the TDM signal.
`System 10 includes a sensor probe 12 and a signal
`conditioning/processing assembly 30 mounted in housing
`14. Probe 12 is configured for emitting light 16 centered
`about a first wavelength and light 18 centered about a second
`wavelength. Light of the first and second wavelengths is.
`alternately emitted at regular intervals from first and second
`light sources 20 and 22,
`respectively, which may,
`for
`example, comprise light emitting diodes or laser diodes, One
`known combination offirst and second wavelengths com-
`prises light centered about 660 nm and 940 nm, respectively.
`It is to be understood, of course, that many other combina-
`tions can be employed. Furthermore, it should be appreci-
`ated that the present invention can be employed in systems
`utilizing light of more than two centered, or primary, wave-
`lengths of light.
`Continuing to refer to FIG. 1, a portion of the emitted light
`is transmitted through a portion of a patient’s anatomy, such
`as a finger 24, and is detected by a light-sensitive device. In
`the described embodiment, a photodiode 26 is utilized.
`Other areas of the patient’s anatomy may also be used
`provided that the transmitted light suitably passes through
`such areas. In this regard, the output indications provided by
`system 10 pertain to arterial blood flow data. More
`particularly, based upon the absorption oflight at the emitted
`wavelengths certain characteristics may be determined
`including, but not limitedto, a patient’s pulse rate and blood
`oxygen saturation level, including the concentration (as a
`percentage of total hemoglobin) of oxyhemoglobin (O.,Hb),
`deoxyhemoglobin (RHb), carboxyhemoglobin (COHb) or
`methemoglobin (MetIb).
`Sensor probe 12 is electrically connected to the signal
`conditioning/processing assembly 30 via multi-conductor
`cable 32. A first set 34 of conductors within cable 32 carries
`drive signals to light sources 20 and 22, while a second set
`36 of conductors is used to bias photodiode 26 and to carry
`a TDMsignal to the signal conditioning/processing assem-
`bly 30.
`Referring to FIG. 2 in conjunction with FIG. 1, the TDM
`signal 38 includes a series of pulse groups 40 output by the
`photodiode 26 in response to the detection of light passed
`through finger 24. Each pulse group includes, in this case, a
`negative going “light
`1” (hereinafter “LT1") portion, or
`interval, and a negative going “light 2”(hereinafter “LT2”)
`portion, or interval, corresponding to the detectedlevels of
`light at each of the two transmitted wavelengths. Ambient
`light is also detected by photodiode 26 together with the
`detected light corresponding with the light at
`the first
`wavelength 16 and light of the second wavelength 18. This
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`ambient light is manifested within the output signal of the
`photodiode 26 as an offset voltage. That is, the LT1 and LT2
`portions each include an offset which results from ambient
`light that is incident upon the photodiode 26 during the time
`that the LT1 and LT? signal portions are generated. In order
`to facilitate removal of the offset,TDM signal 38 includes a
`“dark 1”(hereinafter *“DK1”") portion, or interval, immedi-
`ately preceding LT1, and a “dark 2”(hereinafter “DK2”)
`interval 48 immediately preceding LT2. The voltage level
`during each of the DK1 and DK2 intervals represents the
`ambient
`light
`level
`incident upon photodiode 26 in the
`absence of transmitted light at the first wavelength 16 or
`transmitted light at the second wavelength 18.
`By way of example and in total darkness, a signal
`corresponding to V,,,,, is output by the photodiode which
`may be offset shightly from the zero voltage level V_,,.,. The
`zero
`difference in voltage between V_,,,,
`and DK1, and between
`V..,. and DK2, illustrated as V,,,,,. Tepresents the overall
`ambient
`light offset present
`in TDM signal 38. Such an
`ambient
`light
`level may result
`from any light source
`including, for example, room lighting or sunlight. As will be
`appreciated, subtraction of the DK1 and DK2 voltages from
`the LT and LT2 portions, respectively, will result in elimi-
`nation of both the ambient light data and the photodiode dark
`current from the signal of interest, i.e., data corresponding
`with the transmitted light 16 and 18 which has passed
`through finger 24. It should also be appreciated that ambient
`light may not produce a D.C.offset as illustrated in FIG. 2.
`For example, certain common types of lighting induce a
`time-varying A.C. offset. One such type of lighting is 120
`Volt A.C., 60 Hz. powered fluorescent lighting which may
`produce ambient
`light pulses at a frequency of 120 Hz.
`Therefore, circuitry for providing ambient light level sub-
`traction should also be effective in the removal of such
`time-varying offset signals, as will be described in further
`detail below.
`
`is to be understood that TDM signal 38 may be
`It
`configured in a number of different ways in accordance with
`the present
`invention. In the described embodiment, each
`pulse group 40 includes a dark time interval preceding each
`light time interval. It is noted that some prior art systems
`require this signal configuration to perform ambient
`light
`subiraction. The present invention also contemplates ambi-
`ent light subtraction utilizing one dark time interval per each
`pulse group 40, as will be described further below,
`Referring to FIGS.
`1 through 3, signal conditioning/
`processing assembly 30 will now be described. A
`processing/control section 50 is included which provides
`drive signals to a light source driver section 52 through a
`control signal line 53. The light source driver section may be
`configured, for example, to drive LEDs,laser diodes or other
`such suitable light sources which may become available.
`The light source driver section 52 provides drive signal
`waveforms to probe 12 so as to excite sources 20 and 22 to
`emil light 16 (of the first wavelength) and light 18 (ofthe
`second wavelength). In turn, photodiode 26 detects the light
`passing through the selected body portion to output the TDM
`signal 38 of FIG. 2.
`As shown in FIG. 3, TDM signal 38 is coupled to a
`preamp section 54 via conductors 36 from probe 12. Preamp
`54 converts the relatively small magnitude current of TDM
`signal 38 to a voltage level more useful for processing. A
`silicon switch 36 is connected to preamp 54 through resistor
`58 (c.g., a 2.1 KW resistor). In the present example, silicon
`switch 56 comprises a single pole, quadruple throw switch
`which is controlled by processing/control section 50 by
`means of control lines 59. Synchronous control of switch 56
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`84 of amplifier 70. Variable resistors 74 and 80 are adjusted
`is coordinated by processing/control section 50 with drive
`by processing/control section 50 using control line sets 72a
`signals provided to light source driver section 52 such that
`and 72b, respectively. Amplifier 70a includes an output 86
`TDM signal 38 is de-multiplexed. Specifically, silicon
`and is configured to output a voltage according to the
`switch 36 outputs four data channels A, B, C and D wherein
`difference in voltage level between its inverting and non-
`channel A comprises the LT signal portion, channel B
`inverting inputs multiplied by the gain of the amplifier, as
`comprises the DK1 signal portion, channel C comprises the
`determinedby the setting of the variable resistor. Amplifier
`LT2 signal portion and channel D comprises the DK2 signal
`70b includes an output 88 and is configured in the manner
`portion.
`of amplifier 70¢. Amplifiers 70a and 706 each include a
`Following de-multiplexing, the signal on each channel,
`reference input 90 and 92, respectively. The reference inputs
`A-D, charges one of four holding capacitors 60a—d(e.g., 1.0
`may be grounded but, alternatively, they may be provided
`uF capacitors). These holding capacitors are configured with
`with offset vollages Vop-, and Voey, On control lines 72c
`resistor 58 to form part of a sample and hold circuit (as well
`and 72d, respectively, which are added to each amplifier’s
`as a low-pass filter) in which an average value of each
`output voltage. Thus, cach amplifier outputs a voltage
`channel is stored for that cycle,
`Vawea OF VAMP,, respectively,
`in accordance with the
`Thereafter, signals on each of channels A-D are filtered
`equations:
`by one offour first order low-pass filters 62a—d. Each filter
`Vaso"VorratGAIN, x(LT1-DK1)f; or
`includesaresistor 64 (e.g., 60.4 K© resistors) and a capaci-
`tor 66 (e.g., 0.1 wF capacitors).
`In accordance with the
`VanesVorritGAIN,x(LT2-DRK2)]
`present
`invention,
`the sample and hold/low pass circuit
`comprised of resistor 58, capacitors 60 and silicon switch 56
`cooperates with low-passfilters 62 so as to simultaneously
`and continuously apply signals LT1', DK1', LT2' and DK2'
`to an amplification section 68. It should be appreciated that
`the values ofresistor 58 and resistors 62,
`in the low pass
`filter, are selected along with the values of capacitors 60 and
`capacitors 62, in the low pass filter, to filter out common,
`time-varying offset voltages such as those produced by
`fluorescent lighting to effectively remove the time-varying
`ambient light signal component. The values of these various
`passive components may be modified as required by the
`TDM signal being processed and, in fact, the components
`may be of different values from one channel to the next for
`a particular application. For example, the circuitry of FIG. 3
`may readily be modified for a TDM signal which includes a
`single dark time interval per pulse group (not shown). In
`such a case for a two-wavelength system, channel A may
`process the LT1 signal, channel B may process the LT2
`signal and channel C may process a DK signal, with channel
`D not being required.
`Continuing to refer to FIG. 3, amplification section 68
`includes first and second instrumentation amplifiers 70a and
`70b. As employed herein, the term “instrumentation ampli-
`fier” refers to an amplifier which has an output, V,,,,=[(V,-
`V_) G+V,.,] where V, and V_ are the inverting and non-
`inverting inputs, respectively. V,., is a reference voltage
`which may be set to ground. Gis the gain. Typically, the
`common mode rejection ratio, CMRR, is very high, for
`example, greater than 100 dB. The instrumentation amplifier
`may be a single integrated circuil or made up ofa group of
`integrated circuits and/or discrete transistors.
`A plurality of control lines 72 connect processing/control
`section 50 with amplification section 68. Each amplifier 70a,
`70b includes an inverting input, indicated by a minus sign,
`and a non-inverting input, indicated by a plus sign. LT1' and
`DK are applied to the inverting and non-inverting inputs of
`amplifier 70a, respectively, while L' 2' and DK2' are applied
`to the inverting and non-inverting inputs of amplifier 70d,
`respectively. Alternatively, where a single dark time interval
`is presented within each pulse group, the dark time channel
`is applied to both of the non-inverting inputs of amplifiers
`70a and 70b.
`
`wherein Voe-, and Voper,are provided from the processing/
`control section and wherein GAIN, and GAIN,, are,
`likewise, determined by the processing/control section and
`implemented via settings of variable resistors 74 and 80.
`Vames 86 and V.,¢p;, 88 are provided to an amplification/
`filtering section 91, then to processing/control section 50.
`Following processing, data is provided by processing/
`control section 50 to a display 92 including a display screen
`94 of a suitable configuration including, for example, LCD
`and CRT types. Information and related warnings are pro-
`vided in conjunction with or as an alternative to visual
`display. For example, in the event that the determined value
`of a monitored characteristic falls above and/or below
`predetermined threshold values an audio alarm may soundto
`alert attending medical personnel.
`One advantage in the configurationofthe circuitry of FIG,
`3 relates to control of the gain and offset voltage settings of
`amplifiers 70 by the processing/controlsection. Specifically,
`to achieve a relatively high signal to noise ratio, the gain and
`offset settings provided by processing/control section 50
`should cooperatively center each amplifier’s output within
`the input
`range of the processing/control measurement
`system, al
`the same time, maximizing the swing of the
`output voltage therebetween without clipping either the top
`or bottom of the waveform. Processing/control section 50
`may accomplish this task by monitoring the Vays... and
`Viner signals received from amplification/filtering section
`91, or Viaagp, aNd Vijaypp. Teceived directly from amplifica-
`tion section 68 using, for example, automatic gain control
`techniques which have previously been implemented in the
`art using, for example, control algorithms.
`FIG. 4 illustrates an amplification section 68, In that all of
`the instrumentation amplifiers which form part of system 10
`are arranged in a directly analogous manner to amplifier 70a,
`only instrumentation amplifier 70@ and its associated cir-
`cuitry including channels A and Bfor producing V4,yp, will
`be described in detail. De-multiplexed signals LT] and DK1
`of channels A and Bare first applied to capacitors 60a and
`60b, respectively, which form the sample and hold circuit in
`conjunction with resistor 58 (see FIG. 3). Next, the signals
`are fillered by low-passfilters 62¢ and 626 to present signals
`LT and DK1' to the inverting and non-inverting inputs of
`amplifier 70a , as previously described, Power supply volt-
`ages are applied to amplifier 70a with each of V, and V_
`being filtered by decoupling capacitors 100 (e.g., 0.1 aF
`capacitors).
`to FIG. 4, variable resistor 74
`Continuing to refer
`(indicated within a dashed line) is connected to amplifier
`
`45
`
`$0
`
`55
`
`6t
`
`The gain, G, of each amplifier 70a, 70b is adjusted by
`varying the resistance between a pair of terminals.
`Specifically,a first variable re