Patent Publication Number: US-2022211303-A1

Title: Method and System for Providing Calibration of an Analyte Sensor in an Analyte Monitoring System

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/404,613 filed May 6, 2019, which is a continuation of U.S. patent application Ser. No. 15/828,410 filed Nov. 30, 2017, now U.S. Pat. No. 10,278,630, which is a continuation of U.S. patent application Ser. No. 15/209,743 filed Jul. 13, 2016, now U.S. Pat. No. 9,833,181, which is a continuation of U.S. patent application Ser. No. 13/766,673 filed Feb. 13, 2013, now U.S. Pat. No. 9,408,566, which is a continuation of U.S. patent application Ser. No. 12/624,377 filed Nov. 23, 2009, now U.S. Pat. No. 8,376,945, which is a continuation of U.S. patent application Ser. No. 11/463,582 filed Aug. 9, 2006, now U.S. Pat. No. 7,653,425, entitled “Method and System for Providing Calibration of an Analyte Sensor in an Analyte Monitoring System”, the disclosures of each of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Analyte, e.g., glucose monitoring systems including continuous and discrete monitoring systems generally include a small, lightweight battery powered and microprocessor controlled system which is configured to detect signals proportional to the corresponding measured glucose levels using an electrometer, and RF signals to transmit the collected data. One aspect of certain analyte monitoring systems include a transcutaneous or subcutaneous analyte sensor configuration which is, for example, partially mounted on the skin of a subject whose analyte level is to be monitored. The sensor cell may use a two or three-electrode (work, reference and counter electrodes) configuration driven by a controlled potential (potentiostat) analog circuit connected through a contact system. 
     To obtain accurate data from the analyte sensor, calibration is necessary. Typically, blood glucose measurements are periodically obtained using, for example, a blood glucose meter, and the measured blood glucose values are used to calibrate the sensors. Indeed, the patient must calibrate each new analyte sensor using for example, capillary blood glucose measurements. This may be inconvenient for the patient. 
     In view of the foregoing, it would be desirable to have a method and system for calibrating analyte sensors of an analyte monitoring system that does not inconveniently require periodic blood glucose measurements for sensor calibration. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, in accordance with the various embodiments of the present invention, there is provided a method and system for providing substantially automatic and substantially real time calibration of analyte sensors for use in an analyte monitoring system. 
     These and other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the embodiments, the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a data monitoring and management system for practicing one embodiment of the present invention; 
         FIG. 2  is a block diagram of the transmitter unit of the data monitoring and management system shown in  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 3  is a block diagram of the receiver/monitor unit of the data monitoring and management system shown in  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating the analyte sensor sensitivity estimation procedure in accordance with one embodiment of the present invention; 
         FIG. 5  is a flowchart illustrating the analyte sensor parameter estimation procedure in accordance with one embodiment of the present invention; 
         FIG. 6  is a flowchart illustrating an analyte sensor sensitivity estimation procedure in accordance with another embodiment of the present invention; 
         FIG. 7A  illustrates the transmission of the control signal from the transmitter processor in accordance with one embodiment of the present invention; 
         FIG. 7B  illustrates the measured response to the control signal from the transmitter processor shown in  FIG. 7A  in accordance with one embodiment of the present invention; 
         FIG. 8  is a tabular illustration of a lookup table for sensor sensitivity for use with the calibration procedure in accordance with one embodiment of the present invention; and 
         FIG. 9  is a flowchart illustrating the analyte sensor sensitivity estimation procedure in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As described in detail below, in accordance with the various embodiments of the present invention, there is provided a method and system for determining sensor sensitivity of an analyte sensor which may be used to calibrate the analyte sensor in the analyte monitoring system. In particular, within the scope of the present invention, there is provided method and system for automatically calibrating subcutaneous or transcutaneously positioned analyte sensors such that the frequency of capillary blood glucose measurement for calibration of the sensors may be minimized. 
     More specifically,  FIG. 1  illustrates a data monitoring and management system such as, for example, analyte (e.g., glucose) monitoring system  100  in accordance with one embodiment of the present invention. The subject invention is further described primarily with respect to a glucose monitoring system for convenience and such description is in no way intended to limit the scope of the invention. It is to be understood that the analyte monitoring system may be configured to monitor a variety of analytes, e.g., lactate, and the like. 
     Analytes that may be monitored include, for example, acetyl choline, amylase, bilirubin, cholesterol, chorionic gonadotropin, creatine kinase (e.g., CK-MB), creatine, DNA, fructosamine, glucose, glutamine, growth hormones, hormones, ketones, lactate, peroxide, prostate-specific antigen, prothrombin, RNA, thyroid stimulating hormone, and troponin. The concentration of drugs, such as, for example, antibiotics (e.g., gentamicin, vancomycin, and the like), digitoxin, digoxin, drugs of abuse, theophylline, and warfarin, may also be monitored. 
     The analyte monitoring system  100  includes a sensor  101 , a transmitter unit  102  coupled to the sensor  101 , and a receiver unit  104  which is configured to communicate with the transmitter unit  102  via a communication link  103 . The receiver unit  104  may be further configured to transmit data to a data processing terminal  105  for evaluating the data received by the receiver unit  104 . Moreover, the data processing terminal in one embodiment may be configured to receive data directly from the transmitter unit  102  via a communication link  106  which may optionally be configured for bi-directional communication. 
     Only one sensor  101 , transmitter unit  102 , receiver unit  104 , communication link  103 , and data processing terminal  105  are shown in the embodiment of the analyte monitoring system  100  illustrated in  FIG. 1 . However, it will be appreciated by one of ordinary skill in the art that the analyte monitoring system  100  may include one or more sensor  101 , transmitter unit  102 , receiver unit  104 , communication link  103 , and data processing terminal  105 . Moreover, within the scope of the present invention, the analyte monitoring system  100  may be a continuous monitoring system, or semi-continuous, or a discrete monitoring system. In a multi-component environment, each device is configured to be uniquely identified by each of the other devices in the system so that communication conflict is readily resolved between the various components within the analyte monitoring system  100 . 
     In one embodiment of the present invention, the sensor  101  is physically positioned in or on the body of a user whose analyte level is being monitored. The sensor  101  may be configured to continuously sample the analyte level of the user and convert the sampled analyte level into a corresponding data signal for transmission by the transmitter unit  102 . In one embodiment, the transmitter unit  102  is mounted on the sensor  101  so that both devices are positioned on the user&#39;s body. The transmitter unit  102  performs data processing such as filtering and encoding on data signals, each of which corresponds to a sampled analyte level of the user, for transmission to the receiver unit  104  via the communication link  103 . 
     In one embodiment, the analyte monitoring system  100  is configured as a one-way RF communication path from the transmitter unit  102  to the receiver unit  104 . In such embodiment, the transmitter unit  102  transmits the sampled data signals received from the sensor  101  without acknowledgement from the receiver unit  104  that the transmitted sampled data signals have been received. For example, the transmitter unit  102  may be configured to transmit the encoded sampled data signals at a fixed rate (e.g., at one minute intervals) after the completion of the initial power on procedure. Likewise, the receiver unit  104  may be configured to detect such transmitted encoded sampled data signals at predetermined time intervals. Alternatively, the analyte monitoring system  100  may be configured with a bi-directional RF (or otherwise) communication between the transmitter unit  102  and the receiver unit  104 . 
     Additionally, in one aspect, the receiver unit  104  may include two sections. The first section is an analog interface section that is configured to communicate with the transmitter unit  102  via the communication link  103 . In one embodiment, the analog interface section may include an RF receiver and an antenna for receiving and amplifying the data signals from the transmitter unit  102 , which are thereafter, demodulated with a local oscillator and filtered through a band-pass filter. The second section of the receiver unit  104  is a data processing section which is configured to process the data signals received from the transmitter unit  102  such as by performing data decoding, error detection and correction, data clock generation, and data bit recovery. 
     In operation, upon completing the power-on procedure, the receiver unit  104  is configured to detect the presence of the transmitter unit  102  within its range based on, for example, the strength of the detected data signals received from the transmitter unit  102  or a predetermined transmitter identification information. Upon successful synchronization with the corresponding transmitter unit  102 , the receiver unit  104  is configured to begin receiving from the transmitter unit  102  data signals corresponding to the user&#39;s detected analyte level. More specifically, the receiver unit  104  in one embodiment is configured to perform synchronized time hopping with the corresponding synchronized transmitter unit  102  via the communication link  103  to obtain the user&#39;s detected analyte level. 
     Referring again to  FIG. 1 , the data processing terminal  105  may include a personal computer, a portable computer such as a laptop or a handheld device (e.g., personal digital assistants (PDAs)), and the like, each of which may be configured for data communication with the receiver via a wired or a wireless connection. Additionally, the data processing terminal  105  may further be connected to a data network (not shown) for storing, retrieving and updating data corresponding to the detected analyte level of the user. 
     Within the scope of the present invention, the data processing terminal  105  may include an infusion device such as an insulin infusion pump or the like, which may be configured to administer insulin to patients, and which may be configured to communicate with the receiver unit  104  for receiving, among others, the measured analyte level. Alternatively, the receiver unit  104  may be configured to integrate an infusion device therein so that the receiver unit  104  is configured to administer insulin therapy to patients, for example, for administering and modifying basal profiles, as well as for determining appropriate boluses for administration based on, among others, the detected analyte levels received from the transmitter unit  102 . 
     Additionally, the transmitter unit  102 , the receiver unit  104  and the data processing terminal  105  may each be configured for bi-directional wireless communication such that each of the transmitter unit  102 , the receiver unit  104  and the data processing terminal  105  may be configured to communicate (that is, transmit data to and receive data from) with each other via the wireless communication link  103 . More specifically, the data processing terminal  105  may in one embodiment be configured to receive data directly from the transmitter unit  102  via the communication link  106 , where the communication link  106 , as described above, may be configured for bi-directional communication. 
     In this embodiment, the data processing terminal  105  which may include an insulin pump, may be configured to receive the analyte signals from the transmitter unit  102 , and thus, incorporate the functions of the receiver unit  104  including data processing for managing the patient&#39;s insulin therapy and analyte monitoring. In one embodiment, the communication link  103  may include one or more of an RF communication protocol, an infrared communication protocol, a Bluetooth® enabled communication protocol, an 802.11x wireless communication protocol, or an equivalent wireless communication protocol which would allow secure, wireless communication of several units (for example, per HIPAA requirements) while avoiding potential data collision and interference. 
       FIG. 2  is a block diagram of the transmitter unit of the data monitoring and detection system shown in  FIG. 1  in accordance with one embodiment of the present invention. Referring to the Figure, the transmitter unit  102  in one embodiment includes an analog interface  201  configured to communicate with the sensor  101  ( FIG. 1 ), a user input  202 , and a temperature detection section  203 , each of which is operatively coupled to a transmitter processor  204  such as a central processing unit (CPU). There are provided four contacts (not shown), three of which are electrodes—work electrode, guard contact, reference electrode, and counter electrode, each operatively coupled to the analog interface  201  of the transmitter unit  102  for connection to the sensor  101  ( FIG. 1 ). In one embodiment, each of the work electrode, guard contact, reference electrode, and counter electrode may be made using a conductive material that is either printed or etched, for example, such as carbon which may be printed, or metal foil (e.g., gold) which may be etched. 
     Further shown in  FIG. 2  are a transmitter serial communication section  205  and an RF transmitter  206 , each of which is also operatively coupled to the transmitter processor  204 . Moreover, a power supply  207  such as a battery is also provided in the transmitter unit  102  to provide the necessary power for the transmitter unit  102 . Additionally, as can be seen from the Figure, clock  208  is provided to, among others, supply real time information to the transmitter processor  204 . 
     In one embodiment, a unidirectional input path is established from the sensor  101  ( FIG. 1 ) and/or manufacturing and testing equipment to the analog interface  201  of the transmitter unit  102 , while a unidirectional output is established from the output of the RF transmitter  206  of the transmitter unit  102  for transmission to the receiver unit  104 . In this manner, a data path is shown in  FIG. 2  between the aforementioned unidirectional input and output via a dedicated link  209  from the analog interface  201  to serial communication section  205 , thereafter to the processor  204 , and then to the RF transmitter  206 . As such, in one embodiment, via the data path described above, the transmitter unit  102  is configured to transmit to the receiver unit  104  ( FIG. 1 ), via the communication link  103  ( FIG. 1 ), processed and encoded data signals received from the sensor  101  ( FIG. 1 ). Additionally, the unidirectional communication data path between the analog interface  201  and the RF transmitter  206  discussed above allows for the configuration of the transmitter unit  102  for operation upon completion of the manufacturing process as well as for direct communication for diagnostic and testing purposes. 
     As discussed above, the transmitter processor  204  is configured to transmit control signals to the various sections of the transmitter unit  102  during the operation of the transmitter unit  102 . In one embodiment, the transmitter processor  204  also includes a memory (not shown) for storing data such as the identification information for the transmitter unit  102 , as well as the data signals received from the sensor  101 . The stored information may be retrieved and processed for transmission to the receiver unit  104  under the control of the transmitter processor  204 . Furthermore, the power supply  207  may include a commercially available battery. 
     The transmitter unit  102  is also configured such that the power supply section  207  is capable of providing power to the transmitter for a minimum of about three months of continuous operation after having been stored for about eighteen months in a low-power (non-operating) mode. In one embodiment, this may be achieved by the transmitter processor  204  operating in low power modes in the non-operating state, for example, drawing no more than approximately 1 μA of current. Indeed, in one embodiment, the final step during the manufacturing process of the transmitter unit  102  may place the transmitter unit  102  in the lower power, non-operating state (i.e., post-manufacture sleep mode). In this manner, the shelf life of the transmitter unit  102  may be significantly improved. 
     Moreover, as shown in  FIG. 2 , while the power supply unit  207  is shown as coupled to the processor  204 , and as such, the processor  204  is configured to provide control of the power supply unit  207 , it should be noted that within the scope of the present invention, the power supply unit  207  is configured to provide the necessary power to each of the components of the transmitter unit  102  shown in  FIG. 2 . 
     Referring back to  FIG. 2 , the power supply section  207  of the transmitter unit  102  in one embodiment may include a rechargeable battery unit that may be recharged by a separate power supply recharging unit (for example, provided in the receiver unit  104 ) so that the transmitter unit  102  may be powered for a longer period of usage time. Moreover, in one embodiment, the transmitter unit  102  may be configured without a battery in the power supply section  207 , in which case the transmitter unit  102  may be configured to receive power from an external power supply source (for example, a battery) as discussed in further detail below. 
     Referring yet again to  FIG. 2 , the temperature detection section  203  of the transmitter unit  102  is configured to monitor the temperature of the skin near the sensor insertion site. The temperature reading is used to adjust the analyte readings obtained from the analog interface  201 . The RF transmitter  206  of the transmitter unit  102  may be configured for operation in the frequency band of 315 MHz to 322 MHz, for example, in the United States. Further, in one embodiment, the RF transmitter  206  is configured to modulate the carrier frequency by performing Frequency Shift Keying and Manchester encoding. In one embodiment, the data transmission rate is 19,200 symbols per second, with a minimum transmission range for communication with the receiver unit  104 . 
     Additional detailed description of the continuous analyte monitoring system, its various components including the functional descriptions of the transmitter are provided in U.S. Pat. No. 6,175,752 issued Jan. 16, 2001 entitled “Analyte Monitoring Device and Methods of Use”, and in U.S. patent application Ser. No. 10/745,878 filed Dec. 26, 2003, now U.S. Pat. No. 7,811,231, entitled “Continuous Glucose Monitoring System and Methods of Use”, each assigned to the Assignee of the present application. 
       FIG. 3  is a block diagram of the receiver/monitor unit of the data monitoring and management system shown in  FIG. 1  in accordance with one embodiment of the present invention. Referring to  FIG. 3 , the receiver unit  104  includes a blood glucose test strip interface  301 , an RF receiver  302 , an input  303 , a temperature detection section  304 , and a clock  305 , each of which is operatively coupled to a receiver processor  307 . As can be further seen from the Figure, the receiver unit  104  also includes a power supply  306  operatively coupled to a power conversion and monitoring section  308 . Further, the power conversion and monitoring section  308  is also coupled to the receiver processor  307 . Moreover, also shown are a receiver serial communication section  309 , and an output  310 , each operatively coupled to the receiver processor  307 . 
     In one embodiment, the test strip interface  301  includes a glucose level testing portion to receive a manual insertion of a glucose test strip, and thereby determine and display the glucose level of the test strip on the output  310  of the receiver unit  104 . This manual testing of glucose can be used to calibrate sensor  101 . The RF receiver  302  is configured to communicate, via the communication link  103  ( FIG. 1 ) with the RF transmitter  206  of the transmitter unit  102 , to receive encoded data signals from the transmitter unit  102  for, among others, signal mixing, demodulation, and other data processing. The input  303  of the receiver unit  104  is configured to allow the user to enter information into the receiver unit  104  as needed. In one aspect, the input  303  may include one or more keys of a keypad, a touch-sensitive screen, or a voice-activated input command unit. The temperature detection section  304  is configured to provide temperature information of the receiver unit  104  to the receiver processor  307 , while the clock  305  provides, among others, real time information to the receiver processor  307 . 
     Each of the various components of the receiver unit  104  shown in  FIG. 3  is powered by the power supply  306  which, in one embodiment, includes a battery. Furthermore, the power conversion and monitoring section  308  is configured to monitor the power usage by the various components in the receiver unit  104  for effective power management and to alert the user, for example, in the event of power usage which renders the receiver unit  104  in sub-optimal operating conditions. An example of such sub-optimal operating condition may include, for example, operating the vibration output mode (as discussed below) for a period of time thus substantially draining the power supply  306  while the processor  307  (thus, the receiver unit  104 ) is turned on. Moreover, the power conversion and monitoring section  308  may additionally be configured to include a reverse polarity protection circuit such as a field effect transistor (FET) configured as a battery activated switch. 
     The serial communication section  309  in the receiver unit  104  is configured to provide a bi-directional communication path from the testing and/or manufacturing equipment for, among others, initialization, testing, and configuration of the receiver unit  104 . Serial communication section  104  can also be used to upload data to a computer, such as time-stamped blood glucose data. The communication link with an external device (not shown) can be made, for example, by cable, infrared (IR) or RF link. The output  310  of the receiver unit  104  is configured to provide, among others, a graphical user interface (GUI) such as a liquid crystal display (LCD) for displaying information. Additionally, the output  310  may also include an integrated speaker for outputting audible signals as well as to provide vibration output as commonly found in handheld electronic devices, such as mobile telephones presently available. In a further embodiment, the receiver unit  104  also includes an electro-luminescent lamp configured to provide backlighting to the output  310  for output visual display in dark ambient surroundings. 
     Referring back to  FIG. 3 , the receiver unit  104  in one embodiment may also include a storage section such as a programmable, non-volatile memory device as part of the processor  307 , or provided separately in the receiver unit  104 , operatively coupled to the processor  307 . The processor  307  is further configured to perform Manchester decoding as well as error detection and correction upon the encoded data signals received from the transmitter unit  102  via the communication link  103 . 
     Referring back to the Figures, as described in further detail below, in one embodiment of the present invention, the transmitter processor  204  may be configured to transmit a control signal to the analog interface  201  to determine the poise voltage between the work electrode and the reference electrode of the sensor  101 , each of which are operatively coupled to the analog interface  201  of the transmitter unit  102 . 
     More specifically, in one embodiment, a control processor component of the transmitter unit  102  processor  204  is configured to provide a perturbation control signal to the analog interface  201 . The analog interface  201  is configured to translate the received perturbation control signal to a perturbation that affects the sensor response. For example, the control signal in one embodiment may be configured to control the voltage level that is applied to the sensor  101  between the work and reference electrodes (i.e., the poise voltage). In one embodiment, the analog interface  201 of the transmitter unit  102  is configured to translate the sensor response to the perturbation to a corresponding response signal that is acquired by the signal processing component of the processor  204  of the transmitter unit  102 . The signal processing component of the processor  204  in the transmitter unit  102  in one embodiment may be configured to determine the desired sensor parameter estimation which is transmitted to the receiver unit  104 . Alternatively, the signal processing component of the processor  204  in the transmitter unit  102  may be configured to preprocess the data, which are then transmitted to the receiver unit for sensor parameter estimation determination. 
     More specifically,  FIG. 4  is a flowchart illustrating analyte sensor sensitivity estimation procedure in accordance with one embodiment of the present invention. Referring to  FIG. 4 , at step  410 , the transmitter processor  204  ( FIG. 2 ) in one embodiment is configured to provide a control signal to the analog interface  201  (for example a poise voltage control circuit) of the transmitter unit  102 . In one aspect, the control signal provides a perturbation input to determine the poise voltage between the work electrode and the reference electrode of the sensor  101 . In one aspect, the poise voltage may be in the range of approximately −600 mV and 600 mV, and the analog interface  201  may be configured to control the poise voltage and apply the poise voltage to the electrodes of the sensor  101 . 
     One embodiment of the control signal perturbations is shown in  FIG. 7A  which illustrates the control signal from the transmitter processor  204  so as to provide a poise voltage waveform that is a square wave of 50% duty cycle with a one minute time period interval. In one embodiment, the poise voltage square wave amplitude may be switched from 40 mV to −600 mV from, for example, the normal operating poise voltage to a predetermined level such as −600 mV which effectively shuts down the current signal on the work electrode. 
     Referring back to  FIG. 4 , at step  420 , the analog interface  201  in one embodiment is configured to determine a measured response to the received control signal, for example, a voltage signal which is substantially proportional to the current signal level on the work electrode of the sensor  101 . An aspect of the measured response is illustrated in  FIG. 7B . As shown, in one aspect, the current signal level is associated with the analyte level of the patient and may be modulated by the poise voltage perturbations driven by the control signal from the transmitter processor  204 . Thereafter at step  430 , the transmitter processor  204  may be optionally configured to synchronize the measured response from the analog interface  201  with the control signal. The transmitter processor  204  may be further configured to store the measured response and the associated control signal in a storage unit (not shown) such as a memory device. 
     Referring again to  FIG. 4 , the transmitter processor  204  in one embodiment is configured to determine the difference or variance in the measured response based on the control signal  440 , and the sensor sensitivity may be determined based on the determined difference in measured response  450 . That is, in one embodiment, the difference in measured response is compared to a look up table stored, for example, in the transmitter processor  204  memory unit which includes calculated measured response difference for the sensor based on characteristics of the sensor  101 . 
     By way of an example, for a measured response difference of  47  analog to digital counts, the lookup table for sensor sensitivity ( FIG. 8 ) indicates 34.5 pA/(mg/dL) for the sensor. Then, the determined sensor sensitivity may be applied to the work electrode current to determine the corresponding calibrated analyte value. That is, the calibrated analyte value may be determined by dividing the work electrode current signal by the sensor sensitivity. 
       FIG. 5  is a flowchart illustrating the analyte sensor parameter estimation procedure in accordance with one embodiment of the present invention. Referring to  FIG. 5 , at step  510 , a perturbation control signal is applied to the sensor  101  ( FIG. 1 ), and then the response to the perturbation control signal is measured at step  520 . Based on the measured response to the perturbation control signal, at step  530 , the sensor parameter(s) is estimated and at step  540 , the analyte level is estimated based on the measured response to the perturbation control signal. In one embodiment, the procedure shown in  FIG. 5  is repeated continuously. 
     In accordance with the various embodiments of the present invention, different estimates may be determined including, for example, estimation of sensor properties such as sensitivity and response time, the analyte level, and analyte level validity/accuracy. In one embodiment, there are several mechanisms that may be used to perturb the sensor  101  ( FIG. 1 ), for example, the variable poise voltage. In a further aspect, the one or more of the perturbation control signals may include, for example, square waves. Also, in one aspect, the one or more physical sensor responses that are measured may include, for example, work electrode current variation due to poise voltage perturbation. In addition, signal processing may be used in one embodiment to estimate the sensor parameter or analyte level from the sensor response to the perturbation as described above. 
       FIG. 6  is a flowchart illustrating an analyte sensor sensitivity estimation procedure in accordance with another embodiment of the present invention. Referring to  FIG. 6 , a control signal is applied  610 , for example, to the analog interface  201  of the transmitter unit  102  ( FIG. 1 ). That is, in one embodiment, the processor  204  of the transmitter unit  102  may be configured to provide a control signal to a poise voltage control circuit (for example, incorporated in the processor  204  of the transmitter unit  102  as shown in  FIG. 2 , but which may, in one embodiment, be separately provided within the transmitter unit  102 ) of the transmitter unit  102 . 
     In one aspect, the control signal may be configured to provide a perturbation input signal to determine the poise voltage between the work electrode and the reference electrode of the sensor  101 . In one embodiment, the poise voltage may be in the range of approximately −600 mV and 600 mV, and the analog interface  201  may be configured to control the poise voltage and apply the poise voltage to the electrodes of the sensor  101 . 
     As described in further detail below, an embodiment of the control signal perturbations is shown in  FIG. 7A  which illustrates the control signal from the processor  204  ( FIG. 2 ) to provide a poise voltage waveform that is a square wave of 50% duty cycle with a one minute time period interval. Referring to  FIG. 7A , in one embodiment, the poise voltage square wave amplitude may be switched from 40 mV to −600 mV from, for example, the normal operating poise voltage to a predetermined level such as - 600  mV which effectively shuts down the current signal on the work electrode. 
     Referring back to  FIG. 6 , the analog interface  201  in one embodiment is configured to determine a measured response to the received control signal  620 , for example, a voltage signal which is substantially proportional to the current signal level on the work electrode of the sensor  101  ( FIG. 1 ). As discussed in further detail below, one embodiment of the measured response is shown in  FIG. 7B . Referring to  FIG. 7B , in one embodiment, the average signal level for half of the duty cycle is associated with the analyte level of the patient, but the transient within the half-duty cycle period, caused by the poise voltage perturbations driven by the control signal from the transmitter processor  204 , is associated with the sensitivity parameter of the sensor  101 . The transmitter processor  204  may be further configured to store the measured response and the associated control signal in a storage unit (not shown) such as a memory device. 
     Referring again to  FIG. 6 , the transmitter processor  204  in one embodiment is configured to determine the amplitude difference of the transient from the start of the half-duty cycle to the end (referred to sometimes as the “on” period) in the measured response  630 , and the sensor sensitivity may be determined based on the determined difference in the response  640 . That is, in one embodiment, the difference in measured response is compared to a predetermined sensor parameter such as sensor sensitivity that may be stored in a look up table, for example, in the transmitter processor  204  memory unit. In one aspect, the look up table may include a calculated measured response difference for the sensor  101  and corresponding sensor sensitivities based on characteristics of the sensor  101 . 
     In one embodiment, the transmitter may be configured to determine this sensitivity value once per minute, and to transmit the sensitivity value it to the receiver unit  104  ( FIG. 1 ) in addition to data or signal corresponding to the work current signal level, determined at the end of the “on” period, and skin temperature. 
     In one embodiment, the receiver unit  104  ( FIG. 1 ) may be configured to apply the determined sensor sensitivity to the temperature compensated work electrode current signal in order to determine the corresponding calibrated analyte value or level. That is, the calibrated analyte value may be determined by dividing the temperature compensated work electrode current signal by the determined sensor sensitivity. In one aspect, a time-series of the calibrated analyte values may be acquired by the receiver unit  104  ( FIG. 1 ) in real-time, and may be used to determine analyte rate-of-change and other analyte signal metrics and/or statistics. In addition, the calibrated analyte values may also be used to drive alarms or alerts that inform the patient whose analyte is being monitored of analyte level conditions that require attention. In addition, in accordance with one aspect of the present invention, the receiver unit  104  may be configured to determine whether the sensor sensitivity range is within a valid range. 
       FIG. 7A  illustrates the transmission of the control signal from the transmitter processor in accordance with one embodiment of the present invention. More particularly,  FIG. 7A  illustrates the poise voltage square wave with 50% duty cycle with one minute time periods is shown, where the poise voltage square wave amplitude is switched from 40 mV to −600 mV as in normal operating mode.  FIG. 7B  illustrates the measured response to the control signal from the transmitter processor shown in  FIG. 7A  in accordance with one embodiment of the present invention. More specifically, the measured response which is associated with the analyte level measured by the sensor  101  from the interstitial fluid of a patient as modulated by the control signal from the transmitter processor  204  is illustrated with one minute time periods. 
       FIG. 8  is a tabular illustration of a lookup table for sensor sensitivity for use with the calibration procedure in accordance with one embodiment of the present invention. More specifically, in one embodiment, the lookup table shown in  FIG. 8  is stored in a memory unit (not shown) of the transmitter unit  102  (or alternatively, in the transmitter processor  204 ) and may be accessed by the transmitter processor  204  to retrieve a corresponding sensitivity value associated with the determined measured response difference. 
       FIG. 9  is a flowchart illustrating the analyte sensor sensitivity estimation procedure in accordance with another embodiment of the present invention. Referring to  FIG. 9 , in one embodiment, a control signal from the transmitter processor  204  ( FIG. 2 ) is provided  910  to the analog interface  201  of the transmitter unit  102 , and a response to the applied control signal is determined  920 . Thereafter, the difference or variance in the determined response to the control signal between the beginning and end of the half duty cycle is determined  930 . As can be seen, in one embodiment, steps  910  to  930  are substantially similar to steps  610  to  630 , respectively described above. 
     Referring back to  FIG. 9 , after determining the measured response variance or difference between the beginning and end of the half duty cycle, it is determined whether the number of transmitted or applied control signals exceed a predetermined number or count  940 . If it is determined that the number of transmitted or applied control signals do not exceed the predetermined number or count, then a control signal counter (for example, provided in the transmitter unit  102 ) is incremented by one count  960 , and the routine returns to the beginning where another control signal is provided to the analog interface  201  of the transmitter unit  102 . 
     On the other hand, if it is determined that the number of transmitted or applied control signals exceeds the predetermined number or count, then the sensor sensitivity may be determined based on the determined difference in the response  950 . That is, as discussed above, the difference in measured response in one embodiment is compared to a predetermined sensor parameter such as sensor sensitivity that may be stored in a look up table, for example, in the transmitter processor  204  memory unit. In one aspect, the look up table may include a calculated measured response difference for the sensor and corresponding sensor sensitivities based on characteristics of the sensor. Furthermore, as discussed above, in one embodiment, the calibrated analyte value or level may be determined by, for example, dividing the corresponding sensor signal (e.g., work electrode current signal) level by the determined sensor sensitivity value. 
     Within the scope of the present invention, the perturbations to the analyte sensors may be provided by, for example, altering the poise voltage in time. Alternatively, an additional electrical current signal may be provided to the sensor work or counter electrodes via an AC coupling, where the level of the additional electrical current signal may be varied in time by the control signal in a manner similar as discussed above. Still in accordance with another embodiment, the work/counter electrode current path may be opened and closed in a time varying manner controlled by the control signal. Yet still another embodiment may provide a variable resistance in the work/counter electrode current path, where the variable resistance is varied in time as controlled by the control signal. 
     In another aspect of the present invention, the transcutaneously positioned sensor may be perturbed with a mechanical transducer controlled in time and amplitude by a predetermined control signal. In one embodiment, mechanical transducers may include those that can provide physical signals of vibration, acoustics, thermal or electro-magnetic media, for example. Broadly, any suitable mechanism to apply perturbations to the transcutaneously positioned sensor may be used to the extent that the measured response may be analyzed by the signal processing component such as, for example, the transmitter unit processor  204  to estimate one or more sensor properties based on the signal response induced by the perturbations. For example, vibration perturbations may induce fluctuations in the sensor membrane that could be detected in the measured response transients, which may be correlated with membrane thickness and thus provide a measure of the sensitivity of the sensor. 
     In addition, in accordance with the various embodiments of the present invention, there are provided a variety of time-varying controls signals that may be applied, along with a variety of techniques used to analyze the measured response and estimate the sensor parameter of interest. Some of these control signals may be appropriate to induce a measured response that is more informative about a specific sensor parameter than other control signals, and some control signals may be more practical to implement than others. As discussed previously, a square-wave control signal may be employed in one embodiment. Variations in this type of control signal may be suitably used where the positive and negative amplitudes are at different levels, the duty cycle is other than 50%, or the period is other than 1 minute. 
     In another embodiment of the present invention, a feedback mechanism may be provided where the duty cycle is varied to achieve a desired response, such as a specific transient response time. In this case, the final duty cycle is the parameter that is correlated with the sensor parameter to be estimated. This feedback technique may be extended to other types of control signals, mentioned below, and other characteristics of the signal such as phase, amplitude and frequency may be varied to achieve a desired response. 
     Alternatively, a sine wave may be used as the control signal discussed above rather than a square wave. Still alternatively, a series of sine waves at different frequencies, or a chirp signal may be used as control signals in one embodiment of the present invention. The measured response of these perturbation signals may then be analyzed using standard spectral analysis techniques. Based on the spectral analysis, metrics may be determined that are correlated with the sensor parameter to be estimated. 
     In accordance with yet another embodiment, an impulse signal, or a series of impulse signals may be alternatively used as control signals. The measured response of these perturbation signals may be analyzed using known impulse response analysis techniques. For example, the maximum height of the measured response may be used to determine the associated sensor sensitivity. Alternatively, other signal metrics such as the time to reach the maximum height of the measured response, the area under the curve of the measured response, the slope of the measured response may be correlated with the sensor parameter to be estimated. 
     In still another embodiment, pseudo-random modulation similar to those used in spread-spectrum communication systems may be used as the control signals. The measured response of these perturbation signals may be analyzed using known spread-spectrum analysis techniques. Based on this analysis, metrics may be determined that are correlated with the sensor parameter to be estimated. In addition, the response signal may be demodulated using spread-spectrum techniques to recover the analyte level. 
     For some of the control signal/response measurement analysis techniques discussed above, the relative phase between the control signal and the measured response may be used to analyze the measured response to the perturbation. For some of the control signal/response measurement analysis techniques discussed above, multiple metrics may be determined. One or more of these metrics may be used to estimate the sensor parameter of interest. For example, in one embodiment, a multidimensional table lookup may be used where one dimension includes the sensor parameter of interest, and the other dimensions may each be associated with a different metric that characterizes the measured response. More specifically, by way of illustration, in the impulse response approach described above, both the maximum height and the time to reach the height of the measured response may be determined. In this case, a three dimensional lookup table may be used. 
     As discussed above, in one embodiment, a lookup table may be used to correlate a metric associated with the measured response with a sensor parameter of interest (for example, sensitivity). Alternatively, a mathematical function that relates the measured response metric with the sensor parameter may be used. The sensor parameter may then be determined based on the measured response metric as an input. In another aspect, the estimate of the sensor parameter may be determined for many measurements using, for example, the least squares approach. 
     In addition, within the scope of the present invention, the control signal may be transmitted to the analog interface  201  at predetermined time periods during the life of the sensor. Alternatively, the transmitter processor  204  may be configured to transmit the control signal only during the time periods when sensor calibration is desired or if some other factor, such as a detection of sensitivity instability, determines that sensor calibration is required. 
     Moreover, in one embodiment, other system parameters in addition to sensitivity may be associated with the measured response from the analog interface  201  in response to the control signal from the transmitter processor  204 . These include, but are not limited to, sensor response time, sensor response linearity, sensitivity stability and sensor failure. Accurately estimated sensor response time can be useful for incorporation into algorithms that compensate for errors due to lag in the analyte measurement system. Knowledge of the non-linearity in the sensor response (non-linearity means that the sensitivity is not constant over the entire range to measured response) allows for compensation of errors caused by this non-linearity. 
     Detection of sensitivity instability (that is, detection when the sensitivity has changed value) may be used to accurately determine the new sensitivity. For example, if instability has been detected by the signal processing component, it can direct the control processing component such as the transmitter unit processor  204  to initiate a control signal that is more appropriate to accurately estimating the sensitivity. Also, detecting a sudden, substantial change in sensitivity may be used to identify that a sensor may have failed. 
     While the control signal may be used to determine the sensor sensitivity, in one embodiment, the resulting modulation in the measured response may be removed by, for example, one or more signal filters to recover the glucose signal. In one aspect, a standard signal filter may be used to remove the high frequency content of the signal due to modulation by the perturbation control signal, and recover the lower frequency content that represents the analyte level. In another aspect, the modulation may be deconvolved using the control signal, the calculated sensor response and the estimated sensitivity. 
     Furthermore, there are several approaches to measure a sensor&#39;s response to the perturbation signals in order to estimate desired properties or characteristics of the sensor. For example, in one embodiment, the electrical current that flows through the work (and counter) electrode may be measured. Alternatively, the perturbation response in the counter electrode voltage may be alternatively measured. The measured counter voltage response may be analyzed using same or similar techniques as the measured work current response. In another embodiment, both work current and counter voltage responses may be measured and analyzed. 
     In the manner described above, within the scope of the present invention, there is provided method and system for performing calibration of analyte sensors based on the sensor dynamic behavior and on a substantially real time basis such that sensor calibrations based on blood glucose measurements may be minimized further to improve the accuracy of the analyte sensor data. 
     In accordance with the various embodiments of the present invention, the transmitter processor  204  may include a microcontroller, or alternatively, may be implemented with digital logic such as a gate array or similar logic devices. In addition, in one embodiment, the measured response variance as well as the estimated sensor sensitivity determined by the transmitter processor  204  may be transmitted to the receiver unit  104  ( FIG. 1 ) in the analyte monitoring system  100  in addition to the analyte sensor measurements (for example, the work electrode current measurements detected by the sensor  101 ). 
     In a further aspect, some of the processing may be performed by the receiver unit  104  ( FIG. 1 ) rather than by the transmitter processor  204  such that the transmitter unit  102  may be configured to periodically transmit the measured response variance to the receiver unit  104 , and the receiver unit processing and storage unit  307  ( FIG. 3 ) may be configured to perform the sensor sensitivity determination based on the lookup table which may be stored in a memory device (not shown) in the receiver unit  104 . 
     A method of calibrating an analyte sensor in one embodiment includes applying a control signal, detecting a measured response to the control signal, determining a variance in the detected measured response, and estimating a sensor sensitivity based on the variance in the detected measured response. 
     The level of the control signal may in one embodiment vary in time. 
     In one aspect, the control signal may include a square wave signal, where the square wave signal may be applied to a poise voltage. 
     In a further aspect, detecting the measured response may include determining a work electrode current signal. 
     In still another aspect, the variance may be determined based on comparing the difference between the beginning and end of the half duty cycle of the measured response to the control signal. 
     Moreover, estimating the sensor sensitivity may include retrieving a predetermined sensor sensitivity corresponding to the determined variance in the detected measured response. 
     The method may also include determining a validity of the estimated sensor sensitivity. 
     In addition, the method may also include determining analyte level based on the estimated sensor sensitivity. 
     The sensor in one embodiment may include an analyte sensor. 
     An analyte sensor calibration device in accordance with another embodiment includes a processor configured to apply a control signal, detect a measured response to the control signal, determine a variance in the detected measured response, and estimate a sensor sensitivity based on the variance in the detected measured response. 
     The processor may be configured to vary the level of the control signal with time. 
     In another aspect, the processor may be configured to apply a square wave signal to a poise voltage. 
     The processor in a further aspect may be configured to determine a work electrode current signal of an analyte sensor operatively coupled to the processor. 
     Moreover, the processor may be configured to determine the variance based on comparing the difference between the beginning and end of the half duty cycle of the measured response to the control signal. 
     In addition, the processor in a further aspect may be configured to retrieve a predetermined sensor sensitivity corresponding to the determined variance in the detected measured response. 
     The processor may be operatively coupled to a data receiver unit configured to determine a validity of the estimated sensor sensitivity, where the data receiver unit may be configured to determine an analyte level based on the estimated sensor sensitivity. 
     The various processes described above including the processes performed by the transmitter processor  204  in the software application execution environment in the transmitter unit  102  including the processes and routines described in conjunction with  FIGS. 4-6 and 9 , may be embodied as computer programs developed using an object oriented language that allows the modeling of complex systems with modular objects to create abstractions that are representative of real world, physical objects and their interrelationships. The software required to carry out the inventive process, which may be stored in the memory (not shown) of the transmitter unit  102  may be developed by a person of ordinary skill in the art and may include one or more computer program products. 
     Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.