Abstract:
An apparatus includes an operational amplifier, a switched capacitor network, an optical sensor, and a clock. The switched capacitor network is coupled to an input terminal of the operational amplifier and coupled to an output terminal of the operational amplifier. The optical sensor includes a sensor output coupled to the switched capacitor network. The clock is coupled to at least one switch of the switched capacitor network. The clock is configured to activate the at least one switch to provide an integrated output at the output terminal corresponding to the sensor output.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 61/061,454, filed Jun. 13, 2008, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present subject matter relates to an apparatus which is capable of integration while allowing periodic readout and reset functions, and more particularly to an integrator which is capable of integrating an input charge and enabling a readout and reset of the integrator while minimizing switching noise. 
       BACKGROUND 
     Integrator Circuits 
       [0003]    An integrator utilizing an operational amplifier requires a capacitive element with capacitance C to act as a feedback path from the output of the operational amplifier to its inverting input. A resistive element with resistance R is connected in series between the input voltage to be integrated and said inverting input of the operational amplifier. The time constant for such an integrator is simply RC. All operational amplifiers inherently have voltage offsets present on their input and output terminals due to finite component mismatches. The magnitude of each of these voltage offsets is a unique characteristic of each individual operational amplifier and is a source of error in each operational amplifier output signal. Integrators fabricated utilizing MOS techniques have been constructed utilizing switched capacitors in place of resistive elements. Switched capacitor integrators constitute an improvement over integrators utilizing resistive elements due to the fact that resistance values of diffused resistors are not highly controllable in MOS circuits while the ratios of capacitance values are more controllable. 
       Optical-Based Physiological Sensor Devices 
       [0004]    There exists a wide range of devices that depend upon the transmission of optical signals to monitor or measure various biological or environmental parameters of a patient. For example, various forms of blood oximetry devices employ the transmission and reception of signals in the measurement of one or more biological or environmental parameters of a patient. 
         [0005]    Blood oximetry devices are commonly used to monitor or measure the oxygen saturation levels of blood in a body organ or tissues, including blood vessels, or the oxidative metabolism of tissues or organs. An example of an optical oximeter is disclosed in U.S. Pat. No. Re 33,643, entitled “Single Channel Pulse Oximeter.” These devices are also often capable of and are used to determine pulse rate and volume of blood flow in organs or tissues, or to monitor or measure other biological or environmental parameters. 
         [0006]    A blood oximetry device measures the levels of the components of one or more signals of one or more frequencies as transmitted through or reflected from tissue or an organ to determine one or more biological or environmental parameters, such as blood oxygenation level and blood volume or pulse rate of a patient. 
         [0007]    Blood oximetry devices may also be constructed as directly connected devices, that is, devices that are directly connected to a patient and that directly present the desired information or directly record the information, and as remote devices, that is, devices attached to a patient and transmitting the measurements to a remote display, monitoring or data collection device. 
         [0008]    Blood oximetry devices measure blood oxygen levels, pulse rate and volume of blood flow by emitting radiation in a frequency range, such as the red or near infrared range, wherein the transmission of the radiation through or reflectance of the radiation from the tissues or organ is measurably affected by the oxygen saturation levels and volume of the blood in the tissues or organ. A measurement of the signal level transmitted through a tissue or organ or reflected from a tissue or organ may then provide a measurement or indication of the oxygen saturation level in the tissue or organ. The transmitted or reflected signals may be of different frequencies which are typically affected in measurably different ways or amounts by various parameters or factors or components of the blood. 
         [0009]    Parameters represented by transmitted or reflected signals may be represented by different and related or unrelated parameters of the received signals. For example, a signal transmitted through or reflected from tissue or an organ to measure, for example, blood oxygenation or flow, may have a constant or “dc” component due to the steady state volume of blood in the tissue or organ and a time varying or “ac” component indicative of the time varying volume of blood flowing through the tissue or organ due to the heart beat of the body. Each signal component may provide different information, and may provide information that may be used together to generate or determine further information. 
       SUMMARY 
       [0010]    The present subject matter is directed to a switched capacitor integrator finding particular suitability within a physiological sensor. The switched capacitor provides an improved solution to reducing the overhead of components while allowing application to custom or reconfigurable environments. Errors in gain variation are substantially reduced as the effect of clock drifts or jitters is minimized. Pulse oximetry is one application where embodiments of the present subject matter are particularly suitable. 
         [0011]    The foregoing has outlined rather broadly the features and technical advantages of the present subject matter in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the subject matter. The novel features which are believed to be characteristic of the subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic illustration of an integrator. 
           [0013]      FIG. 2  is a schematic illustration of a switched capacitor resistor equivalent. 
           [0014]      FIG. 3  is a schematic illustrations of a circuit equivalent to the integrator shown in  FIG. 1  utilizing switched capacitor resistor equivalents. 
           [0015]      FIG. 4  is an illustration of periodic clock signals suitable for use with the circuit of  FIG. 3 . 
           [0016]      FIG. 5  is an illustration of an embodiment of a switched capacitor integrator in accordance with one example. 
           [0017]      FIG. 6  is a diagrammatic representation of an embodiment of the present subject matter utilizing a mixed signal processor to control LED drive discretes and sensor LEDs. 
           [0018]      FIG. 7  is an example of a front end signal path of a device implementing the present subject matter. 
           [0019]      FIG. 8  is an illustration of a switched capacitor integrator in accordance with one embodiment of the present subject matter. 
           [0020]      FIG. 9  is an illustration of clock signals, Φ 1  and Φ 2 , suitable for use with the circuit of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
     Integrators 
       [0021]    An integrator is shown in  FIG. 1 . Operational amplifier  10  is used in the inverting mode with capacitor  11  supplying negative feedback from operational amplifier output  12  to inverting input  6 . The input voltage to be integrated is applied to the inverting input  6  of operational amplifier  10  through resistor  9  from terminal  8 . If resistor  9  has a resistance value of R and capacitor  11  has a capacitance value C, the time constant, T, for this integrator is given by the equation: 
         [0000]      T=RC 
         [0022]    Switch  13  is connected in parallel across capacitor  11  in order to initialize the integrator by discharging capacitor  11 . An ideal operational amplifier  10  will always have inverting input  6  at the same potential as noninverting input  5 , which is connected to ground in the circuit of  FIG. 1 . An ideal operational amplifier will therefore have its output terminal  12  at ground potential as well. Thus, after initialization has been completed by discharging capacitor  11  through closed switch  13 , an ideal operational amplifier connected as shown in  FIG. 1  may begin integrating the voltage applied at terminal  8 , and the result of the integration will appear at output terminal  12  of operational amplifier  10 . 
         [0023]    When embodying the integrator of  FIG. 1  in an integrated circuit, the resistor and capacitor of the integrator have significant accuracy errors. These errors vary substantially with the operation environment, such as manufacturing process, temperature and use time, making it difficult to obtain accurate and reliable frequency characteristics. Therefore, in order to solve the above problem of the integrated circuit, there has been introduced a switched capacitor circuit illustrated in  FIG. 3 . Such a switched-capacitor circuit can be readily integrated on a single chip through the use of modern MOS manufacturing processes and has advantages of removing resistors and reducing power consumption. 
         [0024]    As mentioned, in the construction of MOS semiconductor devices, values of resistors and capacitors are not highly controllable. Thus in the integrator circuit shown in  FIG. 1  with the time constant equal to RC, circuits constructed utilizing MOS techniques will result in highly uncontrollable time constants. 
         [0025]    In practice, resistors are generally formed by diffusion, resulting in resistance values and resistance ratios which are not highly controllable. Capacitors, on the other hand, are formed by utilizing layers of conductive material, such as metal or polycrystalline silicon, as capacitor plates. Each plate of conductive materials is separated by a layer of electrical insulation material, such as SiO 2  or silicon nitride, serving as a dielectric, from another conductive layer or from a conductive substrate. While capacitor areas are quite controllable, dielectric thickness is not. Thus, while capacitance values are not highly controllable, ratios of capacitance values are, since dielectric thickness is quite uniform across a single semiconductor die. 
         [0026]    A switched capacitor resistor equivalent is shown in  FIG. 2 . Terminals  15  and  19  are available as equivalents to the terminals available on a resistor. Capacitor  18  has a capacitance value of C. Switch  16  is connected in series between input terminal  15  and capacitor  18 , and controls when the input voltage is applied to capacitor  18  from terminal  15 . 
         [0027]    Switch  17  is connected in series between output terminal  19  and capacitor  18 , and controls when the voltage stored in capacitor  18  is applied to output terminal  19 . In practice, switches  16  and  17  are controlled by two clock generators having the same frequency of operation but generating non-overlapping control pulses. When the clock controlling switch  16  goes high, switch  16  closes, thus causing capacitor  18  to be charged to the input voltage applied to terminal  15 . Because the two clock generators are non-overlapping, switch  17  is open during this charge cycle. Switch  16  then opens. Then switch  17  closes, while switch  16  remains open, thus applying the voltage stored on capacitor  18  to terminal  19 . This resistor equivalent circuit of  FIG. 2  simulates a resistor having resistance value R by the following equation: 
         [0000]    
       
      
       R=t/CR  
      
     
         [0000]    where t is the period of switches  16  and  17 , in seconds, and CR is the capacitance of resistor equivalent capacitor  18 . From these equations we can see that the time constant for the integrator of  FIG. 1  utilizing a switched capacitor as a resistor equivalent will be: 
         [0000]    
       
      
       T=C/CR  
      
     
         [0028]    Since the time constant of an integrator utilizing a switched capacitor as a resistor equivalent is dependent on the ratio of capacitors, it is possible to construct many devices having a uniform capacitance ratio and thus uniform time constants. 
         [0029]    A circuit equivalent to the integrator shown in  FIG. 1  utilizing switched capacitor resistor equivalents is shown in  FIG. 3 . Capacitor  31  having capacitance value of C 1  provides negative feedback from output terminal  43  to inverting input terminal  44  of operational amplifier  48 . Switch  26  is connected in parallel across capacitor  31  to provide means for discharging capacitor  31  and thus reinitializing the integrator. The non-inverting input terminal of operational amplifier  48  is connected to ground. Capacitor  32  together with switches  21 ,  22 ,  23  and  24  provide the switched capacitor resistor equivalent. Capacitor  32  has a capacitance value of C 2 . Capacitors  33  and  34  are connected between node  41  and ground and between node  40  and ground, respectively, in order to attenuate the effects of noise impulses generated when switches  21 ,  22 ,  23  and  24  open. Capacitor  35  is connected between node  42  and ground in order to further attenuate the effects of noise impulses generated when switch  24  opens. 
         [0030]    The operation of the circuit of  FIG. 3  requires three separate control signals. Periodic clock signals suitable for this purpose are shown in  FIG. 4 . Φ 3  is used to drive switch  26 . For each positive going pulse of Φ 3 , switch  26  is closed, thereby discharging capacitor  31  and reinitializing the integrator. The frequency of Φ 1  is equal to an integral multiple of that of Φ 3 . As shown in  FIG. 4  however, while Φ 2  has the same frequency as Φ 1 , it is delayed in such a manner that Φ 1  and Φ 2  are nonoverlapping clock signals of the same frequency. 
         [0031]    During operation of the circuit of  FIG. 3 , both Φ 1  and Φ 3  go high at the same time as shown in  FIG. 4 . Φ 3  controls switch  26  such that a positive going pulse on Φ 3  will cause switch  26  to close, thus discharging capacitor  31  and reinitializing the integrator. Φ 1  controls switches  21  and  23  such that a positive going pulse on Φ 1  causes switches  21  and  23  to close. Φ 2  controls switches  22  and  24  such that a positive going pulse on Φ 2  causes switches  22  and  24  to close. During the reinitialization period of the integration cycle, Φ 1  is high, Φ 2  is low and Φ 3  is high. Thus switch  26  is closed, switches  21  and  23  are closed and switches  22  and  24  are open. Switch  26  shorts out capacitor  31  causing it to discharge. Furthermore, the voltage appearing at output terminal  43  of operational amplifier  48  is connected to the inverting input terminal of operational amplifier  48  forcing the voltage on inverting terminal  44 , and thus charging capacitor  35  to V OFF , the magnitude of the offset voltage of operational amplifier  48 . At the same time capacitor  32  is charged to V IN , the input voltage is applied to terminal  20 . 
       Application of Integrators in Medical Devices 
       [0032]    An embodiment of a switched capacitor integrator is disclosed herein with reference to an oximeter system  50  of  FIG. 5 . Device  50  includes a light source  51  which contains one or more light emitters  52  for generating corresponding light signals  53 . Light signals  53  are transmitted through or reflected from a tissue field, such as finger  54 , an organ or other body parts having parameters  55  which are to be measured or monitored. It is envisioned that embodiments of the present subject matter would be suitable in other physiologic data acquisition devices. As a result, the subject matter is not limited to the application of pulse oximeters. 
         [0033]    The light signals  53  that are transmitted through or reflected from the tissue field  54  are received as modulated signals  56  by sensors  57 . Sensors  57  in turn provide received signals  58  that correspond to and represent modulated signals  56  and the components and characteristics of modulated signals  56  due to modulations and modifications imposed on or induced in emitted signals  53  due to parameters  55 . 
         [0034]    Received signals  58  contain information relating to parameters  55  of the tissue field  54 , and that information can be extracted or otherwise obtained from received signals  58  by appropriate signal processing. Such processing may include, for example, comparing components of the received signals  58  with those of light signals  53  or detecting and extracting components of received signals  58 , such as the “dc” and “ac” components of the signal or signals. 
         [0035]    The processing of received signals  58  to obtain the desired information comprising or pertaining to parameters  55  is performed by a signal processor  59 , which provides parameter outputs which may be displayed, stored for later display or subsequent processing, or transmitted to another facility or system. 
         [0036]    The specific process and algorithms by which received signals  58  are processed to generate parameter outputs representing the desired information are dependent upon the specific parameters  55  and tissue fields  54  of interest. These factors, elements and processes are, however, well known to and understood by those of skill in the relevant arts and the adaptation of the present subject matter to different ones and different combinations of these factors, elements and processes will be well understood by those of skill in the relevant arts. As such, these elements need not and will not be discussed in further detail herein. 
         [0037]      FIG. 6  is a diagrammatic representation of an embodiment of the present subject matter utilizing a mixed signal processor  60  to control LED drive discretes  62  and sensor LEDs  64  via, for example, optional LED drive cable  66 . Processor  60  also receives parameter signals  67  from analog front end discretes  68  as received from photodetector  70 . Processor  60  may be in communication with another processor and/or remote device, via for example channel  71 . Processor  60  provides timing signal  72  and control signals  73  to sensor LED drive discretes  62 . 
         [0038]    In one embodiment of the present subject matter, processor  60  includes an application-specific-integrated-circuit (ASIC). Advantages of an ASIC-based device include significant cost savings as fewer discrete components are required, minimizing the opportunity of reverse engineering, reduced assembly and test time, increased flexibility of component placement, and potential power savings. In alternative embodiments, processor  60  may include a variety of analog and/or digital components as appreciated by one of ordinary skill in the art. 
         [0039]      FIG. 7  is an example of a front end signal path of a device implementing an example of the present subject matter. The front end includes an input current-to-log amplifier  80 , and ambient light current track/hold amplifier  82  together receiving an input signal from a sensor. The front end signal path also includes an anti-alias filter  83 , an integration amplifier  84 , a dual channel high pass filter  85 , a multiplexor  86 , a voltage amplifier  87  and a track/hold DC voltage amplifier  88 . Outputs of the front end include DC out and AC out. Integration amplifier  84  operates to integrate an input signal. Additional disclosure is provided in applicant&#39;s pending U.S. Provisional Application Ser. No. 61/058,390, entitled “LED Control Utilizing Ambient Light or Signal Quality,” and being incorporated by reference herein. 
         [0040]    Referring to  FIG. 8 , there is illustrated a switched capacitor integrator in accordance with an embodiment of the present subject matter. The switched-capacitor integrator comprises a switch unit  100  for supplying a first or a second input voltage, Signal A or Signal B, to a first terminal  101  of capacitor  102 , and for periodically supplying a third input voltage, Signal C, to a second terminal  103  of capacitor  102 . 
         [0041]    Switches  104 ,  105 ,  106 , and  107  operate in response to clock signals, Φ 1  and Φ 2 , such as shown in  FIG. 9 . Another switch  108  is connected between the terminal  109  and the terminal  114  of a reset capacitor  110 . Switch  108  operates in response to clock signal CLEAR. Terminal  109  is also conductively coupled to the inverting input of op amp  112 . The other terminal of reset capacitor  110  is conductively connected to the output terminal  114 . Signal C is also supplied at the non-inverting input of opamp  112 . An integration value is provided at the output terminal  114 . 
         [0042]    Signal A is defined as a main input signal, that is the signal for which the integrator circuit operates. Signal A may originate from a variety of sources depending on the function and type of physiological sensor incorporating the switched capacitor network. Signal B may be a function of Signal C. For example, Signal B=log(Signal C). Signal A may provide a voltage referenced to Signal B. 
         [0043]    As mentioned before switches  104 ,  105 ,  106 , and  107  operate in response to Φ 1  and Φ 2 , which are the non-overlapping two-phase clock signals. The switches  105  and  107  operate in response to the first phase clock signal Φ 1  and the switches  104  and  106  operate in response to the second phase clock signal Φ 2 . 
         [0044]    When the second phase clock signal Φ 2  is enabled and, thus, the switches  104  and  106  are on, a charge is stored on capacitor  102 . The charge applied across input capacitor  102  is the voltage difference between Signals C and B. 
         [0045]    When the actuated clock signal changes from Φ 2  to Φ 1 , the amount of charge stored in the capacitor  102  cannot change suddenly from and, therefore, the input capacitor  102  maintains an instant voltage. However, since the input voltage changes to a voltage of Signal A at the moment when the actuated clock signal becomes Φ 1 , the voltage at the inverting terminal changes as a function of Signal A. 
         [0046]    In a broad sense, the switched capacitor integrator includes an input capacitor and a plurality of switches controlling the voltages presented to a first terminal of the input capacitor. The voltages may be presented as Signals A and B. The switched capacitor integrator includes other switches controlling the voltage at the second terminal of the input capacitor. The second terminal is connected to a common terminal including a reset switch, a reset capacitor and an inverting input of an opamp. During one phase of operation, the terminals of the input capacitor are presented with the voltages of Signals B and C. During another phase of operation, one terminal of the input capacitor is presented with Signal A and the other terminal is conductively coupled to the inverting input of the opamp. 
         [0047]    One potential method of operating the switched capacitor integrator includes defining a pair of clock signals, providing an input capacitor and a plurality of switches controlled in response to the pair of clock signals, wherein during a first phase of operation the input capacitor is charged to the difference between Signal B and C and during a second phase of operation one terminal of the input capacitor is connected to the main input signal, Signal A, and the other terminal is connected to the inverting input of the opamp. Signal C is always present at the noninverting terminal of the opamp. A reset capacitor and reset switch are connected between the inverting input and the opamp output. The reset capacitor is periodically reset in response to a reset signal. In one exemplary method of operation, Signals C and B are functions of each other. 
         [0048]    Although the present subject matter and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the subject matter. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present subject matter, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present subject matter.