(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization |
`International Bureau
`
`(43) International Publication Date
`1 November 2007 (01.11.2007)
`
`
`
`ND OMAAA
`
`(10) International Publication Number
`WO 2007/122375 A2
`
`(51)
`
`International Patent Classification:
`AGIB 5/1455 (2006.01)
`AGIB 5/08 (2006.01)
`AGIB 5/024 (2006.01)
`
`(74) Agent: CHARIG, Raymond; Eric Potter Clarkson LLP,
`Park View House, 58 The Ropewalk, Nottingham NG1
`SDD (GB).
`
`(21)
`
`International Application Number:
`PCT/GB2007/001355
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(81) Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`AT, AU, AZ, BA, BB, BG, BH, BR, BW, BY, BZ, CA, CH,
`(22)
`International Filing Date:—11 April 2007 (11.04.2007)
`CN, CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, EG, ES,
`FI, GB, GD, GE, GH, GM, GT, HN, HR, HU, ID, IL, IN,
`IS, JP, KE, KG, KM, KN, KP, KR, KZ, LA, LC, LK, LR,
`LS, LT, LU, LY, MA, MD, MG, MK, MN, MW, MX, MY,
`MZ, NA, NG, NI, NO, NZ, OM, PG, PH, PL, PT, RO, RS,
`RU, SC, SD, SE, SG, SK, SL, SM,SV, SY, TJ, TM, TN,
`.
`a
`C0) Petontly Dates
`
`
`0607270.6 LL April 2006:(11.04.2006)|GE TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.
`(71) Applicant (for all designated States except US): THE
`UNIVERSITY OF NOTTINGHAM[GB/GB]; Univer-
`sity Park, Nottingham NG7 2RD(GB).
`
`(54) Title: PHOTOPLETHYSMOGRAPHY
`
`100
`
`101
`
`102
`
`103
`
`405
`
`Circuit FF
`
`| Mi1(t)
`
`Photo
`Detector
`
`106 Driver
`
`107
`
`D1(t)
`
`(57) Abstract: A photoplethysmograph device includes a light sourcefor illuminating a target object. A modulator drives the light
`source suchthat the output intensity varies as a function of a modulation signal at a modulation frequency. A detector receiveslight
`from the target object and generates an electrical output as a function of the intensity of received light. A demodulator with a local
`oscillator receives the detector output and produces a demodulated output representative of the modulation signal. The demodulator
`is insensitive to any phase difference between the modulation signal and the oscillator of the demodulator. From the demodulated
`output, a signal indicative of blood volumeas a function of time and / or blood composition is generated. A number of demodulators
`may be provided to derive signals from multiple light sources of different wavelengths, or from an array of detectors. The plethys-
`mograph may operate in a transmission modeora reflectance mode. When inareflectance mode, the device may use the green part
`=
`of the optical spectrum and may use polarising filters.
`
`0001
`
`Apple Inc.
`APL1049
`U.S. Patent No. 8,923,941
`FITBIT, Ex. 1049
`
`(72)
`(75)
`
`Inventors; and
`Inventors/Applicants (for US only): CROWE, John
`[GB/GB]; School of Electrical and Electronic Engineer-
`ing, The University of Nottingham, University Park,
`Nottingham NG7 2RD (GB). GRUBB, Mark [GB/GB];
`School of Electrical and Electronic Engineering, The
`University of Nottingham, University Park, Nottingham Published:
`NG7 2RD (GB). HAYES-GILL, Barrie [GB/GB]; School —_—without international search report and ta be republished
`of Electrical and Electronic Engineering, The University
`upon receipt of that report
`of Nottingham, University Park, Nottingham NG7 2RD
`(GB). MILES, Nicolas [GB/GB]; School ofElectrical and=For two-letter codes and other abbreviations, refer to the "Guid-
`Electronic Engineering, The University of Nottingham,
`ance Notes on Codes and Abbreviations” appearing at the begin-
`University Park, Nottingham NG7 2RD (GB).
`ning ofeach regular issue ofthe PCT Gazette.
`
`(84) Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARIPO (BW, GH,
`GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM,
`ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ,'TM),
`European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES,FI,
`FR, GB, GR, HU, IE, [S, IT, LT, LU, LV, MC, MT, NL, PL,
`PT, RO, SE, SI, SK, TR), OAPI (BE, BJ, CF, CG, CI, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`2007/122375A2IMTMINITATTAA
`
`Apple Inc.
`APL1049
`U.S. Patent No. 8,923,941
`
`0001
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001355
`
`PHOTOPLETHYSMOGRAPHY
`
`The present invention relates photoplethysmography and in particular to a method and
`
`apparatus for measuring pulse rate, breathing rate and blood constituents in the human
`
`or animal body.
`
`The word plethysmography is a combination of the Greek words Plethysmos,
`
`meaning increase, and graph, meaning write. A plethysmograph is an instrument,
`method or apparatus used to measure the variations in blood volumein the body.
`Photoplethysmography (hereinafter also referred to as 'PPG') refers to the use oflight
`to measure these changes in volume, and therefore a photoplethysmograph is an
`
`10
`
`instrument, method or apparatus that uses light to perform these measurements.
`
`Although the human or animal body is generally assumed to be opaque tolight, most
`
`15
`
`soft
`
`tissue will
`
`transmit and reflect both visible and near-infrared radiation.
`
`Therefore, if light is projected onto an area of skin and the emergent light is detected
`
`after its interaction with the skin, blood and other tissue, time varying changes of light
`
`intensity having a relation with blood volume, known as the plethysmogram, can be
`
`observed. This time varying light intensity signal will depend on a number offactors
`
`20
`
`including the optical properties of the tissues and blood at the measurement site, and
`
`the wavelength of the light source. The signal results because blood absorbs light and
`
`the amount of light absorbed, and hence the intensity of remaining light detected,
`
`varies in relation with the volume of blood illuminated.
`
`Variation in the
`
`plethysmogram is caused by the variation in blood volume flowingin the tissue.
`
`25
`
`30
`
`This technique was introduced in 1937 by Hertzman. He wasthe first to use the term
`
`photoplethysmography and suggested that the resultant plethysmogram represented
`
`volumetric changes of blood in the skin's vessels.
`
`The plethysmogram is usually described with respect to its AC and DC components.
`
`The absorption of light by non-pulsatile blood, bone and tissue is assumed to be
`constant and gives rise to the DC component. The DC componentrepresents the
`volume of non-pulsatile blood below the sensor, plus light reflected and scattered off
`
`1
`
`0002
`
`FITBIT, Ex. 1049
`
`0002
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`the skin, bone and other tissues. The AC component is caused by the time varying
`
`absorption oflight caused by temporal changes in blood volumebelow the sensor.
`
`Changes in the blood volume can be caused by cardiovascular regulation, blood
`
`pressure regulation, thermoregulation and respiration. Thus the plethysmogram can
`
`be analysed to determine information on such parameters as pulse rate, breathing rate,
`
`blood pressure, perfusion, cardiac stroke volume and respiratory tidal volume. These
`
`can be observed as periodic and non-periodic changes in the amplitude of AC and DC
`components in the plethysmogram. This has been described in more detail in Kamal
`
`10
`
`et al:
`
`‘Skin Photoplethysmography — a review’, Computer Methods and Programs in
`
`Biomedicine, 28 (1989) 257-269). The plethysmogram can also be analysed to
`
`determine blood constituents.
`
`One such technique is pulse oximetry, which
`
`determines the relative amount of oxygen in the blood. Other blood constituents can
`
`also be measured by using photoplethysmography.
`
`I5
`
`There are two modes of photoplethysmography, the transmission mode and the
`
`reflection mode. In transmission mode the light source is on one side ofthe tissue and
`
`the photodetector is placed on the other side, opposite the light source. The use of
`
`transmission modeis limited to areas where thetissue is thin enough to allow light to
`
`20
`
`propagate, for example the fingers, toes and earlobes of a human subject.
`
`In reflection mode the light source and photodetector are place side-by-side. Light
`
`entering the tissue is reflected and a proportionofthis is detected at the photodetector.
`
`This source-detector configuration is more versatile and allows measurements to be
`
`Zo
`
`performed on almost any area of tissue. However, the use of reflectance mode is
`
`much harder to design than transmission because the signal levelis significantly lower
`
`at the most effective wavelengths. Thus, considerable attention must be given to
`maximising signal-to-noise ratio. As a result, the most common PPG sensors use
`
`transmission mode and hence are restricted to positions where light can pass through
`
`30
`
`tissue.
`
`As a photodetectoris used to measure light from the source, the photoplethysmograph
`
`can also respond to interfering signals from other sources of light, for example
`
`2
`
`0003
`
`FITBIT, Ex. 1049
`
`0003
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`fluorescent lighting and computer monitors. The sensor must also respond to changes
`
`in the light propagating through tissue, i.e. the plethysmogram. These physiological
`
`changes contain frequency components between DC and 25 Hz. However,it is
`
`desirable for the sensor not to respond to ambient light noise. Accordingly, the
`
`photoplethysmograph should reject ambient
`plethysmogram in the bandwidth ofinterest.
`
`light noise while detecting the
`
`A second source of interference is other electrical apparatus. Other electrical devices
`
`can generate radio frequency signals that a photoplethysmograph can detect.
`
`It is
`
`10
`
`desirable to minimise the sensitivity of the system to interfering sources ofthis nature.
`
`A third source of
`
`interference
`
`is
`
`the
`
`electrical noise generated by the
`
`photoplethysmograph itself. Such noise can be generated by electronic components,
`
`and can include thermal noise, flicker noise, shot noise, as well as noise spikes, for
`
`15
`
`example, harmonics generated by missing codes in an analogue-to-digital converter.
`
`It is also desirable to minimise the sensitivity of the system to interference from these
`sources,
`
`A known technique for reducing the noise generated by these three sources of
`interference is to drive the sensor's light source with a carrier modulated at a
`
`20
`
`light, electrical radio
`in the ambient
`frequency that is not present, or dominant,
`frequency signals, or photoplethysmograph system noise. This can be done by
`modulating the sensor’s light source with a square wave, by pulsing it on and off The
`
`detected signals are then band pass filtered to attenuate interference outside the
`
`25
`
`frequency range of
`
`interest.
`
`Subsequent demodulation will
`
`recover
`
`the
`
`plethysmogram.
`
`In general, any periodic signal such as a sine wave may be used to
`
`modulate the light source.
`
`Though modulated light photoplethysmography exists in the prior art, there are still
`
`30
`
`critical limitations in how it has been applied, especially in terms of suitable signal
`
`conditioning circuits for attenuating or removing noise, and demodulation. For
`
`example, EP0335357, EP0314324, WO0144780 and WO9846125 disclose modulated
`
`light photoplethysmography. However,
`
`they use a demodulation method and
`
`3
`
`0004
`
`FITBIT, Ex. 1049
`
`0004
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`apparatus that requires
`
`the modulating and demodulating carrier phase to be
`
`synchronised. Error in the synchronisation timing will add noise to the demodulated
`
`signal (timing jitter or phase noise). The prior art also fails to make full use of band
`
`pass filter characteristics to remove ambient interfering light, by still relying on a
`
`separate channel to measure ambient light, and later subtracting it from the signal,
`
`which adds
`
`further complexity and is arguably less efficient at attenuating
`
`interference. These limitations reduce immunity to broadband and narrowband noise
`
`from sources such as fluorescent lighting, computer monitors, sunlight, incandescent
`
`light, electrical RF interference, thermal noise, flicker noise, and shot noise.
`
`10
`
`A further limitation in the prior art is the choice of wavelength for reflectance mode
`
`sensors. Both reflection mode and transmission mode sensors use light sources in the
`
`red and/or infrared part of the spectrum, wavelengths between 600nm and 1000nm
`
`being typical. However, red / infrared reflectance sensors do not function well
`
`15
`
`because light at red and infrared wavelengths is poorly absorbed by blood. This
`
`results in low modulation of the reflected signal and therefore a small AC component.
`
`Therefore red / infrared reflectance probes give poor results when compared to
`
`transmittance probes. It has been shown in Weija Cui ef al: “In Vivo Reflectance of
`
`Blood and Tissue as a Function of Light Wavelength”, TEEE Transactions on
`
`20
`
`Biomedical Engineering, Volume 37, No 6, June 1996), that a larger plethysmogram
`
`AC component amplitude can be recorded if a reflectance mode sensor uses light of
`
`wavelengths between 500nm and 600nm (green light).
`
`A continuous non-modulated green light photoplethysmograph was described in WO
`
`25
`
`9822018A1. However, the objective of this invention was reflectance pulse oximetry,
`
`and the patent does not explain the steps necessary to produce a reliable
`
`photoplethysmograph suitable for measuring the plethysmogram AC and DC
`
`component. Such a green light sensor would be necessary to reliably detect the AC
`
`component, for example heart rate, but moreover the breathing signal, which is
`
`30
`
`extremely small and was not detected by this system.
`
`In Benten etal: “Integrated synchronous receiver channelfor optical instrumentation
`
`applications” Proceedings of SPIE - The International Society for Optical
`
`4
`
`0005
`
`FITBIT, Ex. 1049
`
`0005
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`Engineering, Volume
`
`3100,
`
`75-88,
`
`1997),
`
`a modulated
`
`light
`
`reflectance
`
`photoplethysmograph is described that uses a switching multiplier to systematically
`change the gain of the signal path between +1 and -1. This is the equivalent of
`mixing the modulated signal with a square wave to recover the plethysmogram.
`
`However, similar to the other prior art described previously, this method needs the
`
`modulating carrier and demodulating local oscillator signals to be in-phase.
`
`It is an object ofthe present invention to provide an improved plethysmograph.
`
`According to one aspect,
`
`the present
`
`invention provides a photoplethysmograph
`
`device comprising:
`
`a light source for illuminating a target object;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`15
`
`a detector for receiving light from the target object and generating an electrical
`
`output as a function of the intensity of received light;
`
`a demodulator for receiving the detector output, having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`sidebands thereof, in which the demodulator is insensitive to any phase difference
`
`20
`
`between the modulation signal and the oscillator of the demodulator; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`blood volumeas a function of time and / or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`25
`
`plethysmogram, comprising thesteps of:
`
`illuminating a target object with a light source;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector and generating an
`
`30
`
`electrical output as a function ofthe intensity of received light;
`
`receiving the detector output in a demodulator having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`0006
`
`FITBIT, Ex. 1049
`
`0006
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`sidebands thereof, in which the demodulator is insensitive to any phase difference
`
`between the modulation signal and the oscillator of the demodulator; and
`generating, from the demodulated output, a signal indicative of blood volume
`
`as a function of time and / or blood composition.
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device comprising:
`
`one or morelight sources each for illuminating a portion of a target object;
`
`one or more modulators for driving the light sources such that the output
`
`10
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`one or more detectors for receiving light from the snc object and generating
`one or more electrical outputs as a function of the intensity of received light;
`
`a plurality of demodulators each for receiving one or more of the electrical
`
`15
`
`outputs and producing a demodulated output representative of the modulation signal
`
`of one of the modulated light sources and any sidebandsthereof, to thereby produce a
`
`plurality of demodulated outputs correspondingto the plurality of light sources and/or
`
`plurality of detectors; and
`
`means for generating, from the demodulated outputs, plethysmogram signals
`
`20
`
`indicative of blood volume as a function of time and / or blood composition for each
`
`of the demodulator outputs.
`
`According to another aspect, the present invention provides a method of generating a
`
`plethysmogram, comprising the steps of:
`
`25
`
`illuminating a portion of a target object with one or morelight sources;
`
`driving the light sources with one or more modulators such that the output
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`
`receiving light from the target object with one or more detectors and
`
`30
`
`generating one or moreelectrical outputs as a function of the intensity of received
`
`light;
`
`receiving one or more of the electrical outputs with a plurality of
`demodulators, each producing a demodulated output representative of the modulation
`
`6
`
`0007
`
`FITBIT, Ex. 1049
`
`0007
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`signal of one of the modulated light sources and any sidebands thereof, to thereby
`
`produce a plurality of demodulated outputs corresponding to the plurality of light
`
`sources; and
`
`generating, from the demodulated outputs, plethysmogram signals indicative
`of blood volume as a function of time and / or blood composition for each of the
`
`demodulator outputs of the pixel array.
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`
`10
`
`a light source for illuminating a target object via a first polarising filter;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object via a second polarising
`
`filter having a different polarisation state than the first polarising filter, the detector
`adapted to generate an electrical output as a function ofthe intensity ofreceived light;
`
`15
`
`a demodulator for receiving the detector output and producing a demodulated
`
`outputrepresentative of the modulation signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`blood volumeas a function of time and / or blood composition.
`
`20
`
`According to another aspect, the present invention provides a method of generating a
`
`photoplethysmogram, comprising the stepsof:
`
`illuminating a target object with a light sourcevia a first polarising filter;
`
`driving the light source with a modulator such that the output intensity varies
`
`25
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector via a second polarising
`
`filter having a different polarisation state than the first polarising filter, the detector
`
`generating an electrical output as a function ofthe intensity of received light;
`
`30
`
`receiving the detector output with a demodulator and producing a demodulated
`output representative of the modulation signal and any sidebands thereof; and
`generating, from the demodulated output, a signal indicative of blood volume
`
`as a function oftime and / or blood composition.
`
`0008
`
`FITBIT, Ex. 1049
`
`0008
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`a light source for illuminating a target object with optical
`
`radiation of
`
`wavelength less than 600 nm;
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object and adapted to generate an
`
`electrical output as a function of the intensity of received light, the light source and
`detector being disposed laterally adjacent to one another on a substrate such that the
`
`10
`
`active surfaces thereof can be directed towards substantially the same point on a
`
`surface ofthe target body;
`
`a demodulator for receiving the detector output and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`15
`
`blood volumeas a function of time and / or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`photoplethysmogram, comprising thestepsof:
`
`illuminating a target object with optical radiation of wavelength less than 600
`
`20
`nm fromalight source;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector to generate an electrical
`
`output as a function of the intensity of received light, the light source and detector
`
`25
`
`being disposed laterally adjacent to one another on a substrate such that the active
`
`surfaces thereof can be directed towards substantially the same point on a surface of
`
`the target body;
`receiving the detector output with a demodulator and producing a demodulated
`output representative ofthe modulation signal and any sidebands thereof; and
`generating, from the demodulated output, a signal indicative of blood volume
`
`30
`
`as a function oftime and / or blood composition.
`
`0009
`
`FITBIT, Ex. 1049
`
`0009
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`The invention provides a modulated light photoplethysmograph device.
`
`In selected
`
`embodiments, it combines the features of modulated light, band pass filtering, and IQ
`
`demodulation to give a plethysmogram of perfuse tissue. When used in reflectance
`mode, light in the blue and/or green portion ofthe optical spectrum is used which
`
`gives a larger pulsatile signal and improvedsigna! to noiseratio.
`
`Selected embodiments of the invention provide improved reliability through the
`
`reduction of noise when the photoplethysmograph device is used in transmission
`
`mode.
`
`In addition, the choice of light in the blue / green portion of the optical
`
`10
`
`spectrum (i.e. wavelengths of between 400 nm and 600nm) gives improved reliability
`through the reduction of noise and the increase in AC component signal amplitude,
`
`when the photoplethysmograph device is used in reflection mode.
`
`Selected embodiments can be applied to different photoplethysmography techniques
`
`15
`
`including
`
`single wavelength
`
`photoplethysmography, multiple wavelength
`
`photoplethysmography,
`
`pixel
`
`array
`
`photoplethysmography,
`
`and
`
`non-contact
`
`photoplethysmography.
`
`Embodiments of the present invention will now be described by way of example and
`
`20
`
`with reference to the accompanying drawings in which:
`
`Figure
`
`1
`
`is
`
`a
`
`functional block diagram of
`
`a
`
`single wavelength
`
`photoplethysmograph device;
`
`Figure 2 is a functional block diagram of a demodulator suitable for use in the
`
`photoplethysmograph device offigure 1;
`
`25
`
`Figure 3
`
`is
`
`a
`
`functional block diagram of a multiple wavelength
`
`photoplethysmograph device;
`
`Figure 4 is a schematic plan view of a pixel array photoplethysmograph
`
`device;
`
`Figure 5a is a schematic side view of a non-contact photoplethysmograph
`
`30
`
`device with polarising filters;
`
`Figure 5b is a plan view of a polarisingfilter for use with the reflectance mode
`
`photoplethysmograph device of figure 7;
`
`0010
`
`FITBIT, Ex. 1049
`
`0010
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`single wavelength
`
`block diagram of a
`
`functional
`a
`is
`6
`Figure
`photoplethysmograph device;
`Figure 7 is a schematic plan view, side view and end view of a reflectance
`mode photoplethysmograph device;
`Figure 8 is a circuit diagram of a transimpedance amplifier suitable for use in
`the photoplethysmograph devices described herein;
`Figure 9 is a circuit diagram of a bandpassfilter circuit suitable for use in the
`photoplethysmograph devices described herein;
`Figure 10 is a circuit diagram ofa light source driver circuit suitable for use in
`the photoplethysmograph devices described herein;
`Figure 11 is a process flow diagram illustrating a demodulation algorithm
`suitable for use in the photoplethysmograph devices described herein;
`Figure 12 is a functional block diagram of a light source brightness control
`loop suitable for use in the photoplethysmograph devices described herein;
`Figure 13a is a photoplethysmogram showing a combined AC and DCoutput
`of a photoplethysmograph device;
`Figure 13b is a photoplethysmogram showing the magnified AC component
`from figure 13a;
`Figure 14a is a photoplethysmogram showing combined pulsatile and
`breathing signal;
`Figure 14b is a photoplethysmogram showing the breathing signal of figure
`
`14a only;
`Figure 15a is a photoplethysmogram showinga breathing signal only;
`Figure 15b is a corresponding breathing signal as measured by an oral
`thermistor;
`Figure 16 is a photoplethysmogram recorded using a green light source of
`
`wavelength 510 nm;
`Figure 17 is a photoplethysmogram recorded using a red light source of
`
`wavelength 644 nm;
`Figure 18 is a functional block diagram ofan alternative demodulator suitable
`for use in the photoplethysmograph device offigure 1;
`Figure 19 is a functional block diagram ofan alternative demodulator. suitable
`for use in the photoplethysmographdevice of figure 1;
`
`10
`
`0011
`
`FITBIT, Ex. 1049
`
`10
`
`20
`
`25
`
`30
`
`0011
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`Figure 20 is a functional block diagram of an alternative demodulator suitable
`for use in the photoplethysmograph device offigure 1.
`
`Single wavelength photoplethysmograph device
`With reference to figure 1, a photoplethysmograph device 100 comprises a driver
`circuit 101 which is coupled to energise a light source 102 with modulated drive
`signal such that the output intensity of the light source varies as a function of a
`modulation signal having a specific modulation frequency (fn) and modulation
`amplitude (M1(t)). The waveform driving the light source is therefore a modulating
`carrier characterised byits frequency and amplitude.
`
`The light source 102 is configured to illuminate a target object 103 such as an area of
`tissue of the human or animal body. The light source 102 preferably comprises one or
`morelight emitting devices each of a given wavelength or range ofwavelengths.
`
`A photodetector 104 is configured to receive light from the target object 103 after its
`interaction therewith. Depending on the relative positioning of the light source 102,
`the target object 103 and the photodetector 104, this received light may be one or
`more oflight that has been transmitted through the target object, light that has been
`reflected from the surface ofthe target object, and light that has been scattered by and
`/ or reflected from structures or fluids within the target object. The photodetector will
`generate an electrical current that is a function of, e.g. proportional to, the amount of
`light incident to its active area.
`
`from the
`the electrical current
`A detector 105 may be provided to convert
`photodetector 104 to a voltage that is proportional to the current. The detector 105
`may incorporate an amplifier (not shown). Thegain ofthat amplifier can be rolled off
`at a frequencygreater than the modulation frequency. The detector 105 and amplifier
`can, with careful design, minimise the noise at the input to a band passfilter 106
`coupled thereto. Ina general sense, the photodetector 104 and detector 105 functions
`may be provided by any detector capable of receiving light from the target object and
`generating an electrical outputthat is a function of the intensity ofthe receivedlight.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`11
`
`0012
`
`FITBIT, Ex. 1049
`
`0012
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`The band passfilter 106 may be provided for attenuating signals outside a bandwidth
`of interest. Thefilter bandwidth is preferably centred on the modulation frequency fm
`and is sufficiently wide to pass the modulating carrier and sidebands caused by
`plethysmogram amplitude modulation, but narrow enough to attenuate frequency
`components of interference and noise. To reduce noise, the bandwidth of the band
`pass filter 106 should be as narrow aspossible.
`It need only be wide enough to pass
`the upper and lower sidebands ofthe plethysmogram, typically but not limited to 50
`Hz. The band pass filter 106 may incorporate an amplifier (not shown) to provide
`additional gain. The bandpassfilter 106 and amplifier are preferably designed to
`minimise noiseat the input of the following stage, namely a demodulator 107. It will
`be appreciated that the provision of a bandpassfilter 106 is not always necessary but,
`if employed,an increase in signal-to-noise ratio (SNR) mayresult.
`
`15
`
`20
`
`25
`
`30
`
`A preferred arrangement of demodulator 107 is shown in more detail in figure 2, The
`demodulator 107 is adapted to demodulate the output of the band passfilter 106 and
`hence recover a plethysmogram from the detected light received from the target
`object. The preferred demodulator 107 uses a methodthat is insensitive to the phase
`difference between the modulation carrier and a demodulation carrier. In other words,
`the demodulatoris insensitive to any phase difference between the modulation signal
`and an oscillator in the demodulator, as will be explained later.
`Thus,
`it
`is
`unnecessary to maintain a predetermined phaserelationship between the modulation
`
`and demodulation process.
`
`The demodulator 107 may comprise a multiplexer 210 for splitting the modulated
`signal M1(t) into two channels. A first channel processes a first modulated input
`signal M1(t)a and a second channel processes a second modulated input signal
`Ml(t)b. The first modulated input signal M1(t)a is provided as input to a first
`multiplier 201 together with an output of a first demodulator local oscillator (LO)
`signal 204, D1(t).
`‘The frequency of the local oscillator signal 204 is preferably
`substantially equal to the frequency of the modulation signal and therefore equal to
`the modulating carrier
`frequency of input signal MI(t).
`The result of the
`multiplication of M1(t)a with the first LO signal 204is an I (“in phase’) signal. In the
`second channel, the second modulated input signal is multiplied, using a multiplier
`
`12
`
`0013
`
`FITBIT, Ex. 1049
`
`0013
`
`FITBIT, Ex. 1049
`
`

`

`WO 2007/122375
`
`PCT/GB2007/001335
`
`206, with a second demodulatorlocal oscillator (LO) signal that also has a frequency
`preferably substantially equal to the frequency of the modulation signal. However,
`the second demodulator LO signal is phase shifted by phase shifter 205 with respect
`to the first demodulator LO signal.
`The phase difference between the first
`demodulator LO and second demodulator LO is preferably 90 degrees. The result of
`the multiplication of M1(t)b with the second demodulator LO signal
`is the Q
`(‘quadrature phase’) signal.
`It will be understood that the local oscillator, although
`shown as a producing a sine wave output, could produce other waveforms of the
`required frequency.
`
`10
`
`The separate I and Q signals are preferably separately low pass filtered in filter
`elements 202 and 207 respectively to remove unwanted harmonics and products of the
`multiplication process. Optionally,
`the resulting signals may be decimated in
`decimators 203 and 208 respectively to reduce the sample rate. The results of this are
`
`15
`
`the I’ and Q’signals.
`
`20
`
`25
`
`30
`
`The I’ and Q’ signals can be demultiplexed back into one signal at mixer 209 to
`provide the demodulated plethysmogram S1(t). The demultixplexing process can
`include an algorithm or circuit that determines the square root of the sum of the
`squares ofthe I’ and Q’signals.
`
`The demodulator arrangement of fi