Abstract:
Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps for processing a photoplethysmographic (“PPG”) signal in a monitoring device that monitors a property of blood flow. In some embodiments, the processing includes obtaining a first digital signal representing a detected light signal having a non-pulsatile (e.g., DC) component and a pulsatile component (e.g., AC). An offset control signal is generated from an estimation of the non-pulsatile component and a second digital signal is generated after subtracting the offset control signal from the detected light signal and applying a gain to the subtracted signal. A reconstructed signal is generated that is calculated from the gain and one or more of (i) the first digital signal, and (ii) the second digital signal and the offset control signal.

Description:
BACKGROUND 
     Field 
       [0001]    The embodiments described below relate to the measurement of biometric data. Some embodiments relate to photoplethysmographic sensors. 
       Description 
       [0002]    Recent consumer interest in personal health has led to a variety of personal health monitoring devices being offered on the market. Such devices, until recently, tended to be complicated to use and were typically designed for use with one activity, e.g., bicycle trip computers. 
         [0003]    Recently, personal health monitoring devices (also referred to herein as “biometric tracking” or “biometric monitoring” devices) have expanded to track multiple metrics of the wearer. For example, Fitbit, Inc. produces a number of biometric tracking devices that can have a number of features and elements, such as displays, batteries, sensors, wireless communications capabilities, power sources, and interface buttons, as well as mechanisms for attaching these devices to a pocket or other portion of clothing or to a body part of the wearer, packaged within a small volume which are configured to track, among other things, steps, distance, calorie expenditure, active minutes, elevation (e.g., as measured in floors or stairs), speed, pace, and the like. 
         [0004]    To track these metrics, these devices may collect, process and display a large variety of data using a variety of sensors. One type of sensor used in some biometric tracking devices is a heart rate sensor. These heart rate sensors typically operate by emitting light into the skin of the user and then measuring the light reflected or diffused back after the emitted light interacts with the user&#39;s skin. 
       SUMMARY 
       [0005]    Some embodiments relate to a device, method, and/or computer-readable medium storing processor-executable process steps for processing a photoplethysmographic (“PPG”) signal in a monitoring device that monitors a property of blood flow. In some embodiments, the processing includes obtaining a first digital signal representing a detected light signal having a non-pulsatile (e.g., DC) component and a pulsatile component (e.g., AC). An offset control signal is generated from an estimation of the non-pulsatile component and a second digital signal is generated after subtracting the offset control signal from the detected light signal and applying a gain to the subtracted signal. A reconstructed signal is generated that is calculated from the gain and one or more of (i) the first digital signal, and (ii) the second digital signal and the offset control signal. 
         [0006]    In some embodiments, generation of the offset control signal includes selecting a value for the offset control signal such that the second digital signal, which is generated after the offset control signal is subtracted from the first digital signal and adjusted by the gain, does not exceed a range threshold. The offset control signal may be generated by selecting an initial value, determining that the first digital signal subtracted by the offset control signal and then adjusted by the gain does not exceed a range threshold, selecting a subsequent value for the offset control signal, and determining that the first digital signal adjusted by the subsequent value of the offset control signal and gain does not exceed the range threshold. 
         [0007]    In some embodiments, the monitoring device may include multiple light emitters to generate a source light signal. A control unit associated with the monitoring device may switch between the light emitters to generate the source light signal. The multiple light emitters may include emitters that emit the source light in different wavelength regions (e.g., such as red, green and infrared regions). 
         [0008]    In some embodiments, a monitoring device may be provide which supports two modes of operation to measure different properties of a blood flow. For example, in some embodiments, the device may allow measurement of both heart rate and blood oxygenation levels. In some embodiments, a method for processing a photoplethysmographic (PPG) signal in a monitoring device includes receiving a signal selecting a mode of operation of the monitoring device. A switching component is operated to select a first light source in a first mode of operation and a second light source in a second mode of operation. A selected one of the first and the second light sources are operated to generate a source light signal. A first digital representation of the detected light signal is obtained, and a second digital signal is generated based at least in part on the selected mode of operation. The second digital signal is provided to a processor for use in measuring a property of blood flow. In some embodiments, the first mode of operation is a mode to measure a heart rate and a second mode of operation is a mode to measure a blood oxygenation level. In some embodiments, when a first mode of operation is selected, an offset control component is disabled, and when a second mode of operation is selected, the offset control component is enabled to allow subtraction of a DC component of the detected light signal. 
         [0009]    Embodiments as described herein provide a number of advantages. For example, embodiments provide monitoring devices with reduced cost which can efficiently extract and resolve the pulsatile component of a PPG signal allowing improved processing of PPG signals. In some cases, embodiments provide such improved processing without use of high resolution analog-to-digital (“ADC”) converters or oversampling techniques which can be expensive and consume additional power. Further, embodiments allow faster processing and signal acquisition time. 
         [0010]    A more complete understanding of some embodiments can be obtained by referring to the following detailed description and to the drawings appended hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The construction and usage of embodiments will become readily apparent from consideration of the following specification as illustrated in the accompanying drawings, in which like reference numerals designate like parts, and wherein: 
           [0012]      FIG. 1  is a top perspective view of a device according to some embodiments; 
           [0013]      FIG. 2  is a bottom perspective view of a device according to some embodiments; 
           [0014]      FIG. 3  is a block diagram of a device according to some embodiments; 
           [0015]      FIG. 4  is a block diagram of a sensor pursuant to some embodiments; 
           [0016]      FIG. 5  is a block diagram of components of a sensor pursuant to some embodiments; 
           [0017]      FIGS. 6A-6C  are diagrams illustrating a photoplethysmographic (“PPG”) signal as processed by the device of  FIGS. 4 and 5  pursuant to some embodiments; 
           [0018]      FIG. 7  is a diagram illustrating a PPG signal as processed by the device of  FIGS. 4 and 5  pursuant to some embodiments; and 
           [0019]      FIG. 8  is a diagram illustrating a reconstructed PPG signal pursuant to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The following description is provided to enable any person in the art to make and use the described embodiments. Various modifications, however, will remain readily apparent to those in the art. A specific example will now be described with reference to the appended figures in order to provide an introduction to various features. Embodiments are not limited to the features or description of this example. 
         [0021]    As discussed above, monitoring devices utilizing the methods and/or circuitry discussed herein can be used to collect and monitor biometric information. An example of such a monitoring device  100  is shown in  FIG. 1 . According to the illustrated embodiment, the device  100  is wearable on a user&#39;s wrist. The device  100  includes a display  140 , which may comprise any suitable type of display screen, and which may display graphical indicators based on biometric and other data detected, collected, monitored or otherwise generated by the device  100 . The device  100  may include one or more buttons  180  which may be manipulated by a user to provide input to the device  100 . The display  140  may also incorporate one or more input devices (such as a touch screen). A band  190  may be wrapped around the wrist and is securable using one or more securing elements  195  (e.g., hook and loop, clasp, shape memory elements, magnets). The shape and configuration of the device  100  is one example configuration within which embodiments of the present invention may be deployed. The monitoring devices and PPG sensor system and methods set forth herein may be used with desirable results in devices having a wide variety of shapes and configurations, and the shape and configuration illustrated in  FIG. 1  and  FIG. 2  are for illustrative purposes. 
         [0022]      FIG. 2  is a bottom view of the device  100 , showing a sensor component  210  and a power interface  220 . The sensor component  210  may include sensors which benefit from close proximity and/or contact with a user&#39;s skin tissue. Such sensors may include PPG sensors (e.g., heart rate, pulse oximeter, and the like), moisture, temperature, and/or capacitive touch sensors (e.g., to detect when the device is being worn). The power interface  220  may interface with a docking station or other power source to receive electrical charge for charging of batteries located within the device  100 . Although a single sensor component  210  is shown for simplicity, multiple sensor components may be provided. Further, while the sensor component  210  is illustrated in  FIG. 2  as protruding somewhat from the device  100 , other embodiments may place sensors in proximity to the user without use of a distinct protrusion. 
         [0023]    Features of a particular category or type of sensor—a “PPG” or “photoplethysmographic” sensor—will be described in further detail herein. PPG sensors, such as a heart rate monitor or pulse oximeter, use a light-based technology to sense pulsating blood flow as controlled by the heart&#39;s pumping action. PPG sensors may be used to measure a user&#39;s heart rate, blood oxygenation, and other biometric parameters. In the device  100  shown in  FIG. 1  and  FIG. 2 , sensor component  210  may shield or be associated with one or more light sources (e.g., such as photodiodes, light emitting diodes or “LEDs”) and light detector(s) and corresponding control circuitry (e.g., as described further below). In some cases, light pipes may be used to optically connect the light source(s) or the detector with the surface of the user&#39;s skin tissue. Beneath the skin, the light from the light sources scatters off of blood in the body, some of which may be scattered or reflected back into a photodetector located behind the sensor component  210 . In some embodiments, as will be described further herein, the sensor component  210  may be shaped and formed to improve the operation of the sensor. For example, in some embodiments, the sensor component  210  may utilize light-transmissive structures to improve the performance of a PPG sensor. For example, a light-transmissive structure may include a mask consisting of an opaque material that limits the aperture of one, some or all of the light source(s) and/or detector(s). In this way, the light-transmissive structures may selectively define or control a preferential volume of the user&#39;s body that light is emitted into and/or detected from. 
         [0024]      FIG. 3  is a block diagram of a monitoring system  300  according to some embodiments. System  300  may be operated to control the collection and usage of biometric data pursuant to some embodiments. In some cases, the monitoring system  300  may implement internal features of the device  100  shown in  FIGS. 1 and 2 . 
         [0025]    As  FIG. 3  shows, the system  300  includes one or more processing units  310  (e.g., hardware elements, such as processor cores and/or processing threads, discrete or integrated logic, and/or one or more state machines, and/or field programmable gate arrays (or combinations thereof)). In some cases, one or more processing units  310  are configured to execute processor-executable program code to cause the device  300  to operate as described herein, and memory  320  for storing the program code and any other suitable data. The memory  320  may comprise one or more fixed disks, solid-state random access memory, and/or removable media (e.g., a thumb drive) mounted in a corresponding interface (e.g., a USB port). 
         [0026]    A display interface  330  provides communication with the display  340 , which may comprise any system for visual presentation of information that is or becomes known. The display  340  may comprise a touch screen for receiving user input into the system  300  according to some embodiments. The display  140  shown in  FIG. 1  is an example of the display  340 . 
         [0027]    One or more processing units  310  may execute processor-executable program code stored in memory  320  to cause the system  300  to process sensor data control the operation of sensors and related components, and to perform operations as discussed herein. In some embodiments, the processing unit  310  may be configured as a processor core, memory and programmable input and output peripherals and may be referred to herein as a microcontroller (or “MCU”). According to some embodiments, the system  300  comprises an integrated device such as, but not limited to, a wearable unit (e.g., around a user&#39;s wrist such as the device  100  shown in  FIGS. 1 and 2 , around the user&#39;s neck, attached to an earlobe or inserted in an ear (e.g., an earbud), on a ring, or the like) or an otherwise portable unit (e.g., a smartphone, a dedicated music player, a fob). In some embodiments, elements of the system  300  may be embodied in separate devices, such as a server device (e.g., a desktop computer) including elements  310 ,  320  and  330 , and a terminal device (e.g., a watch) including the display  340 . The system  300  may perform functions other than those attributed thereto herein, and may include any elements that are necessary for the operation thereof 
         [0028]    Some embodiments of the system  300  include a portable monitoring device having a physical size and shape adapted to couple to the body of a user, which allows the user to perform normal or typical user activities (including, for example, exercise of all kinds and type) without hindering the user from performing such activities. An example of such a device is the device  100  of  FIG. 1 . The portable monitoring device may include a mechanism (for example, a clip, strap and/or tie) that facilitates coupling or affixing the device to the user during such normal or typical user activities. 
         [0029]    The system  300  further includes one or more sensor interfaces  350  for exchanging data with one or more sensors  360 . The sensors  360  may comprise any sensors for acquiring data including biometric monitoring data. Examples of sensors  360  include, but are not limited to, an accelerometer, a light sensor, a compass, a switch, a pedometer, a blood oxygen sensor, a gyroscope, a magnetometer, a Global Positioning System device, a proximity sensor, a pressure sensor (e.g., an altimeter), and a heart rate sensor. One or more of the sensors  360  may share common hardware and/or software components. 
         [0030]    As shown in  FIG. 3 , a user  370  is pictured to indicate that, according to some embodiments, the user  370  influences the data acquired by one or more of the one or more sensors  360 . For example, the one or more sensors  360  may generate data based on physical activity of the user  370 . Moreover, one or more of sensors  360  may generate data via direct contact with the user, for example during heart rate, skin temperature, and/or blood oxygen monitoring. 
         [0031]    Reference is now made to  FIG. 4 , where an example block diagram of a sensor  400  is shown which may be used as one of multiple sensors  360  of the device  300  in  FIG. 3 . As will be described further below, embodiments of the sensor  400  may be operated or configured to function as both an optical heart rate sensor and a blood oxygenation sensor. As shown in  FIG. 4 , the sensor  400  includes one or more light sources  410  which emit light toward a user&#39;s skin tissue, and where the reflection and/or diffusion of such light from the skin/internal tissues of the user is sensed by one or more light detectors  420 , the signal from which is subsequently digitized by one or more analog to digital converters (“ADC”)  432 ,  434  (referred to as “ADC  1 ” and “ADC  2 ”, respectively). 
         [0032]    In general, a PPG signal is an optically and non-invasively obtained signal used to sense volumic changes of blood vessels. A light source  410  emits at the tissue of interest (e.g., the skin shown in  FIG. 4 ) and a light detector  420  is placed to measure the transmitted or reflected light. Depending on the wavelengths of the light source  410 , information about a person such as heart rate and blood oxygenation can be determined. 
         [0033]    The light sources  410  may be, for example, one or more photodiodes which are configured to generate a light in a wavelength. For example, a photodiode may be provided as a light source  410  and configured to generate light in the red, infrared, or green regions (660 nm, 940 nm, and 528 nm, respectively). As will be described further herein, the sensor  400  may be configured with one or more light sources  410  operating in the infrared and red wavelengths for use in obtaining blood oxygen measurements (e.g., operating as an SpO2 sensor). One or more additional light sources  410  may be provided which operate in the green wavelength region for use as a heart rate sensor. 
         [0034]    Embodiments of the present invention allow the sensor  400  to be operated in different modes of operation depending on whether the infrared/red wavelengths or the green wavelengths are used. For example, the sensor may be operated in a mode to discern information about a user&#39;s heart rate (when the green wavelength light sources  410  are operated) and in a mode to discern information about a user&#39;s blood oxygenation (when the infrared and red wavelength light sources  410  are operated). In general, both the infrared and red PPG signals&#39; pulsatile components are much weaker in comparison to that of the green wavelength signals and are more susceptible to uncertainty due to noise. In the infrared and red wavelengths, there is a large offset component of the PPG signal (referred to herein as the “non-pulsatile” or “DC” component) and the pulsatile component (also referred to herein as the “AC” component). For example, the AC component of the detected PPG signal can be as small as 0.05% of the signal, leaving up to 99.95% of the dynamic range unusable. Put another way, in a sensor using 12-bit resolution ADCs, the best effective resolution of the AC component of the PPG signal may be less than 2-bits. The use of very high-resolution ADCs to enable higher effective resolution of the AC component is undesirable in many sensor systems, particularly where power, cost and size are important. 
         [0035]    In order to obtain accurate signal information for a processor (such as the processor  310  of  FIG. 3 ) or microcontroller (shown as MCU  402 ) to perform processing of the detected signals, the light information detected by the light detectors  420  is processed using a front-end signal conditioning block  430  which directly interfaces with the light detectors  420  and which outputs an analog PPG signal. This PPG signal is provided to an ADC  432  which is configured with sufficient resolution to measure the non-pulsatile (DC) component of the PPG signal (but maybe not enough resolution for the pulsatile or AC component). This digital signal  433  (which may be referred to elsewhere herein as the “first digital signal”) is provided to the MCU  402  for processing. Upon receipt of the first digital Signal  433 , the MCU  402  performs processing to generate an offset signal (as will be described further herein). The offset signal is provided from the MCU  402  to an offset removal stage  440 . The offset removal stage  440  receives the offset signal from the MCU  402  and removes the offset value from the signal  530 . The MCU  402  calculates what value the offset signal should be and provides it to the offset removal stage  440  to remove the value of the offset signal from the signal from ADC  432 . In general, the offset removal stage  440  provides a modified PPG signal representing primarily the pulsatile (or AC) component of the PPG signal to a second ADC  434  for digitization and for provision to the MCU  402  for use in processing to determine a detected blood oxygenation level. In some embodiments, the MCU  402  includes memory which stores control and application data as well as signal data. In some embodiments, the MCU  402  stores data such as the digital signal  433 , offset signal data and the signal received from ADC  434  in a memory as time series data. For example, the signal data may be stored such that the MCU  402  may reconstruct a PPG signal as described further below in conjunction with  FIGS. 7 and 8 . 
         [0036]    As discussed above, pursuant to some embodiments, multiple light sources  410  may be provided (e.g., including light sources for different wavelengths). The operation and selection of different light sources (as well as the intensity of the selected light source  410 ) may be controlled using one or more light source controls  412 . The light source controls  412  may be operated by control signals received from the MCU  402 . For example, in some embodiments, a light source  410  may be controlled to enable the light source  410  (and, in some embodiments, to set a desired intensity) based on a mode of operation selected by the MCU  402 . As one illustrative example, a mode of operation may be a mode in which an optical heart rate is to be detected. In such a mode, one or more light sources  410  capable of generating light in a green wavelength region may be operated. As another illustrative example, a mode of operation may be a mode in which a blood oxygenation measurement is to be taken. In such a mode, one or more light sources  410  capable of generating light in an infrared wavelength region may be operated as well as one or more light sources  410  capable of generating light in a red wavelength region. In some embodiments, the MCU  402  may be operated to switch between different light sources  410  and/or different light detectors  420  based on a desired mode of operation. 
         [0037]    The light source control module  412  may be controlled by the MCU  402 . The MCU  402  receives digital signals from one or both of the ACDs  432 ,  434  and may use that information to generate control signals including the offset control signal as well as control signals to operate the light source control  412  and light sources  410 . The MCU  402  may adaptively control the operation of the light source  412  based on operating information received from the first and/or second ACDs ( 432 ,  434 ), allowing a wide variety of operating controls. Further, in some embodiments, the MCU  402  may control whether the second ACD  434  is operated based on the mode of operation (e.g., in some embodiments, the second ACD  434  is not operated when the sensor is in a mode of operation to obtain an optical heart rate measurement). 
         [0038]    Features of some embodiments will now be described in further detail by reference to  FIG. 5 , which is a block diagram depicting components of the sensor device of  FIG. 4  in further detail. More particularly,  FIG. 5  depicts the logical blocks and functions that allow the generation of signals for provision to the MCU  520  for further processing of blood oxygenation analysis and heart rate analysis. As shown in  FIG. 5 , a sensor device may be provided with a number of components, including one or more photodiodes  502 ,  504  and one or more signal conditioning components  506 ,  508  which correspond to the photodiodes. While two sets of photodiodes  502 ,  504  and signal conditioning components  506 ,  508  are shown, sensors may be provided with more than two sets as will be described further herein. The sensor further includes or is in communication with an MCU  520  that performs processing to control components of the sensor and to perform processing of signals generated by the sensor. For example, the MCU  520  may generate control signals to operate a multiplexor  510  which allows control over which photodiode  502 ,  504  to use as the signal source. For example, the MCU  520  may issue control signals to select photodiode  502  and signal conditioning block  506  as the source signal. 
         [0039]    As a more specific illustrative example, the MCU  520  may generate the control signal based on a mode of operation selected by a user of the monitoring device (e.g., the user may select to operate the monitoring device to capture an optical heart rate measurement, or the user may select to operate the monitoring device to capture a blood oxygen measurement). For example, the photodiode  502  may be a photodiode that operates in a green wavelength region, and the photodiode  502  (and associated signal conditioning component  506 ) may be enabled by a control signal from the MCU  520  which selects that signal source through the multiplexor  510 . In the following discussion, an embodiment will be described in which the photodiode  502  is a green wavelength photodiode, and the photodiode  504  is a set of photodiodes that operate in the infrared and red wavelengths and are used for SpO2 measurements. 
         [0040]    Once the MCU  520  has selected a PPG signal source (e.g., photodiode  502  or photodiode  504 ), the multiplexor  510  may pass the PPG signal from the selected signal source to an ambient removal component  512 . The PPG signal is shown on  FIG. 5  as signal S 1 (t). The signal S 1 (t) is also provided as an input to the MCU  520  and is used for further processing as described below. The PPG signal S 1 (t) is processed by an ADC (such as the stage one ADC  432  of  FIG. 4 ). In general, the input to the MCU  520  is a digital representation of the PPG signal S 1 (t) that represents the PPG signal (including both the AC and the DC components of the signal) at a point in time. A sample illustration of the PPG signal S 1 (t) is shown in  FIG. 6A , where the magnitude of the PPG signal is shown over time. As shown, the signal has a relatively small amount of information (shown as region  602 ) with a large offset (shown as region  604 ). 
         [0041]    In some embodiments, the PPG signal S 1 (t) may further be processed using an ambient removal component  512  which operates to remove an ambient component of the signal (although this processing is optional and will not be described in detail herein). 
         [0042]    The MCU  520 , having received the digital representation of the PPG signal S 1 (t), performs processing to generate an offset signal. The offset signal may be selected or determined by the MCU  520  based on a predetermined value selected such that the signal S 1 (t) multiplied by a gain less the offset value is within the dynamic range of the ADCs of the sensor. The offset value may be dynamically updated as S 1 (t) is received by the MCU  520 . In some embodiments, the offset value may be selected such that the adjusted signal is above a predetermined or calculated minimum, but also below a maximum (such that the signal S 1 (t) is within the dynamic range of the ADCs of the sensor). Referring to  FIG. 6B , a sample illustration of the signal S 1 (t) less the offset signal is shown. The offset signal is provided as an input to a DC offset signal buffer  516  for provision to a DC subtraction component  514 . The DC offset signal buffer  516  may include a digital to analog converter (“DAC”) to generate the appropriate offset voltage level based on the offset signal from the MCU  520 . The DC subtraction component  514  generates a second PPG signal S 2 (t) which is generally equivalent to the first signal (S 1 (t)) times a gain less the DC offset. Put another way, the output of the DC subtraction component  514  is generally equivalent to the AC component of the PPG signal with some DC left. A representation of the signal S 2 (t) is shown in  FIG. 6C . In general, the offset (illustrated in  FIG. 6B ) is selected to allow the gain (applied in  FIG. 6C ) to be applied while staying within the dynamic range of the system. The result is a signal S 2 (t) that has a larger AC component. The signal S 2 (t) is provided to a second ADC (such as the ADC  434  of  FIG. 4 ) to generate a digital representation of the signal S 2 (t) for provision to the MCU  520 . 
         [0043]    Reference is now made to  FIG. 7 , where a representation of the PPG signals as processed by the MCU  520  over time are shown. In the top portion of  FIG. 7 , the signal S 1 (t) over time is shown. As shown, the offset determined by the MCU  520  may vary over time, but is selected such that the signal S 2 (t) does not extend beyond the dynamic range of the ADCs of the system. The MCU  520  stores the signal information for S 1 (t), S 2 (t), the gain, and the offset(t), and this signal information is associated with a time period for which it is relevant. The different time periods (with the offset values shown in the stage  1  ADC chart) are illustrated with vertical lines separating each time period. The MCU  520  may then perform processing to reconstruct the PPG signal. Referring to  FIG. 8 , the MCU  520  may perform processing to construct a PPG signal S 3 (t) which may be calculated from S 1 (t), S 2 (t), the gain, and the offset(t). In general, S 3 (t) is equivalent to S 1 (t) multiplied by the gain, or S 2 (t) plus the gain multiplied by the offset(t). The resulting signal (S 3 (t)) allows the MCU  520  to perform processing on a signal with more resolution and without need for expensive high-resolution ADCs or oversampling to resolve the AC component of the PPG signal. In this manner, embodiments provide more resolution for PPG signals because the DC component of the PPG signal can be subtracted before amplification. 
         [0044]    Further, embodiments allow a monitoring device to be operated with different types of biometric measurements. For example, devices consistent with the embodiments shown in  FIGS. 4 and 5  may be operated to obtain measurements for both heart rate (which may use photodiodes operating in the green wavelength) as well as SpO2 (which may use photodiodes operating in the red and infrared wavelengths). Pursuant to some embodiments, the DC subtraction component  514  operates independently of which mode of operation is used. In some embodiments, when the device is operated in a mode to obtain heart rate measurements, a photodiode  502  operating in the green wavelength is enabled (or selected as the PPG signal source by the multiplexor  510 ) and the offset control signal is set to equal zero. When the device is operated in a mode to obtain SpO2 measurements, photodiodes  504  operating in the red and infrared wavelengths are enabled (or selected as the PPG signal source by the multiplexor  510 ) and the offset control signal is determined as described above. Further, depending on the selected mode of operation, the MCU  520  may be enabled to reconstruct a signal S 3 (t) (as shown in  FIG. 8 ) or the signal reconstruction may be disabled. For example, when the device is operated in a SpO2 measurement mode of operation the offset control signal is determined as described above and the MCU  520  is configured to perform a signal reconstruction. When the device is operated in a heart rate measurement mode of operation, the offset control signal is set to zero and the MCU  520  is configured to not perform a signal reconstruction. In this manner, embodiments allow multiple modes of operation using the same components, without expensive high-resolution ADCs to resolve the AC component of signals (particularly for SpO2 modes of operation). 
         [0045]    The foregoing diagrams represent logical architectures for describing processes according to some embodiments, and actual implementations may include more or different components arranged in other manners. Other topologies may be used in conjunction with other embodiments. Moreover, each system described herein may be implemented by any number of devices in communication via any number of other public and/or private networks. Two or more of such computing devices may be located remote from one another and may communicate with one another via any known manner of network(s) and/or a dedicated connection. Each device may include any number of hardware and/or software elements suitable to provide the functions described herein as well as any other functions. For example, any computing device used in an implementation of some embodiments may include a processor to execute program code such that the computing device operates as described herein. 
         [0046]    The modules and components described herein may be implemented in hardware, a processor executing firmware and/or software instructions, or any combination thereof. For instance, memory portions of processing modules may be implemented in one or more memory devices such as one or more of a magnetic disk, an optical disk, a random access memory (RAM), a video RAM, a Flash memory, etc. Similarly, processing units of processing or control modules (such as, for example, the light source control module  412  of  FIG. 4 ) may be implemented using one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), a programmable logic device (PLD), etc. If the module, processor and/or processing units are implemented using a processor executing firmware and/or software instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, in a RAM or ROM or Flash memory, a memory of a processor (e.g., a cache memory), etc. The processor executing firmware and/or software instructions may comprise a general purpose processor or a special purpose processor such as a digital signal processor (DSP), a graphics processor, or the like. 
         [0047]    Those in the art will appreciate that various adaptations and modifications of the above-described embodiments can be configured without departing from the scope and spirit of the claims. Therefore, it is to be understood that the claims may be practiced other than as specifically described herein.