Abstract:
An electrosurgical system for performing an electrosurgical procedure is provided and includes an electrosurgical generator and a calibration computer system. The electrosurgical generator includes one or more processors and a measurement module including one or more log amps that are in operative communication with the processor. The calibration computer system configured to couple to a measurement device and is configured to measure parameters of an output signal generated by the electrosurgical generator. The calibration computer system is configured to compile the measured parameters into one or more data look-up tables and couple to the electrosurgical generator for transferring the data look-up table(s) to memory of the electrosurgical generator. The microprocessor is configured to receive an output from the log amp(s) and access the data look-up table(s) from memory to execute one or more control algorithms for controlling an output of the electrosurgical generator.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to an electrosurgical generator. More particularly, the present disclosure relates to electrosurgical generators having a gain calibrated RF logarithmic amplifier sensor. 
         [0003]    2. Description of Related Art 
         [0004]    RF generators configured for use with electrosurgical instruments are known in the art. In certain instances, one or more logarithmic amplifier, sometimes referred to as “log amps,” may be incorporated into the circuitry of the RF generators to compress a large dynamic input range to a more manageable output for a digital signal processor of the RF generator. For example, a typical log amp can have an input range that varies by 10,000 times, e.g., from about 100 μV to about 1V. Conversely, a log amp output, typically, varies by 3 times, e.g., from about 1V to about 3V. To accomplish these log amp input and output ratios, log amps typically have a very high gain bandwidth, which is fairly constant. Due to internal construction of the log amps, however, the log amps have non-linear gain dependency at their inputs, which, in turn, results in decreased accuracy of log amp sensors associated with the log amps. 
       SUMMARY 
       [0005]    In view of the foregoing, there exists a need for RF generators having one or more gain calibrated log amp sensors configured to compensate for gain variance at the input of the log amp. 
         [0006]    Aspects of the present disclosure are described in detail with reference to the drawing wherein like reference numerals identify similar or identical elements. As used herein, the term “distal” refers to the portion that is being described which is further from a user, while the term “proximal” refers to the portion that is being described which is closer to a user. 
         [0007]    An aspect of the present disclosure provides an electrosurgical system for performing an electrosurgical procedure. The electrosurgical system including an electrosurgical generator that includes at least one processor and a measurement module including at least one log amp in operative communication with the processor. A calibration computer system is configured to couple to a measurement device and is configured to measure parameters of an output signal generated by the electrosurgical generator. The calibration computer system is configured to compile the measured parameters into at least one data look-up table, and couple to the electrosurgical generator for transferring the at least one data look-up table to memory of the electrosurgical generator. The microprocessor is configured to receive an output from the at least one log amp and access the at least one data look-up table from memory to execute at least one control algorithm for controlling an output of the electrosurgical generator. 
         [0008]    The at least one log amp may be configured to receive an output from a sensor that is configured to sense an output of the electrosurgical generator. 
         [0009]    The measurement device may be a device that measures RMS current. In this instance, the measurement device may include a current toroid that is configured to measure an output current of the electrosurgical device. 
         [0010]    The output from the at least one log amp may be a voltage output. 
         [0011]    The parameters contained in the data look-up table may include gain of the at least one log amp, output voltage of the at least one log amp, input voltage of the at least one log amp, output current of the electrosurgical generator and output voltage of the electrosurgical generator. 
         [0012]    The at least one control algorithm may be configured to calculate a slope and gain of the log amp. The slope and gain of the log amp may be calculated through a dynamic operating range of the log amp. The log amp may be configured to allow a user to vary a slope thereof to obtain an optimum slope through the dynamic operating range. 
         [0013]    The control algorithm may utilize a piece-wise linear fit of the gain relative to the measured output of the log amp to correct gain variations of the log amp. Moreover, the control algorithm may utilize an averaging technique of the gain relative to the measured output of the log amp to correct gain variations of the log amp. Further, the control algorithm may utilize a polynomial curve fitting technique of the gain relative to the measured output of the voltage log amp to correct the variations in gain. 
         [0014]    The measurement module may include a voltage and current sensor that are configured to measure respective voltage and current on a patient side of an output transformer of the measurement module. The voltage and current sensors may be isolated from a patient and referenced to ground of the electrosurgical generator by isolating and/or reducing a voltage sense line with the use of one or more capacitors. 
         [0015]    An aspect of the instant disclosure provides a computer calibration system for calibrating measurement circuitry of an electrosurgical device. The computer calibration system includes a measurement device that is configured to measure an output of the electrosurgical device. A memory that stores measurement data includes a plurality of current and voltage values of at least one log amp of the measurement circuitry, a plurality of gain values of the log amp, a plurality of current and voltage values of the electrosurgical device and a plurality of impedance values of a load coupled to the electrosurgical device. A processor may be configured to execute one or more control algorithms to compile the measurement data into at least one data look-up table. A communication interface may be configured for transferring the at least one data look-up table to memory of the electrosurgical device when the computer calibration system is coupled to the electrosurgical generator. 
         [0016]    The at least one log amp may be configured to receive an output from a sensor that is configured to sense an output of the electrosurgical device. 
         [0017]    The measurement device may be configured to measure RMS current. In this instance, the measurement device may include a current toroid that is configured to measure an output current of the electrosurgical device. 
         [0018]    An aspect of the instant disclosure provides a method for calibrating measurement circuitry of an electrosurgical device. An output of the electrosurgical device is measured. An output of at least one log amp of the measurement circuitry of the electrosurgical device is measured. A data look-up table including measurement data obtained from the measuring steps is compiled. A slope and gain of the at least one log amp is calculated based on the measurement data obtained from the measuring step. At least one control algorithm utilizing the calculated slope and gain is executed. A gain of the log amp based on an output of the log amp to calibrate the measurement circuitry is recalculated. 
         [0019]    The at least one control algorithm may utilize a piece-wise linear fit of the gain relative to the measured output of the log amp to correct gain variations of the log amp. Moreover, the at least one control algorithm may utilize either an averaging technique or a polynomial curve fitting technique of the log amp gain relative to the measured output of the voltage log amp to correct the variations in gain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Various embodiments of the present disclosure are described hereinbelow with references to the drawings, wherein: 
           [0021]      FIG. 1  is a perspective view of an electrosurgical system including a generator, calibration computer system, and electrosurgical instrument(s) according to an embodiment of the present disclosure; 
           [0022]      FIG. 2  is a block diagram of the electrosurgical system depicted in  FIG. 1 ; 
           [0023]      FIG. 3  is a schematic diagram of an RF output stage of the generator depicted in  FIGS. 1 and 2 ; 
           [0024]      FIG. 4  is graphical illustration of an output voltage of a log amp plotted against an input voltage of the log amp across the dynamic range of the log amp; 
           [0025]      FIG. 5  is a block diagram of the electrosurgical system depicted in  FIG. 1  coupled to a measuring device and a test load; and 
           [0026]      FIG. 6  is a table illustrating tabulated measured and calculated data taken across a dynamic range of a log amp depicted in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Detailed embodiments of the present disclosure are disclosed herein; however, the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. 
         [0028]      FIG. 1  is a perspective view of an electrosurgical system  2  that includes a calibration system  16  according to embodiments of the present disclosure. System  2  includes an electrosurgical generator  8  including electronic circuitry that generates radio frequency power for various electrosurgical procedures (e.g., sealing, cutting, coagulating, or ablating). The electrosurgical generator  8  may include a plurality of connectors to accommodate various types of electrosurgical instruments, e.g., a forceps  4 , pencil  5 , etc., that are configured to deliver the electrosurgical energy to tissue during a surgical procedure. In the illustrated embodiment, each of the forceps  4  and pencil  5  includes one or more electrodes that are configured to provide electrosurgical energy to treat tissue. For example, forceps  4  may be configured as a bipolar electrosurgical forceps including jaw members  10  and  12  ( FIG. 1 ) having a respective electrode  24 ,  26  ( FIGS. 1 and 2 ) that are each connectable to the electrosurgical generator  8  via a cable  14  ( FIG. 1 ). Alternately, the forceps  4  may be a monopolar electrosurgical forceps, in which case one of the jaw members  10  and  12  includes an active electrode, and a return electrode is operably supported on a return pad (not shown) that is configured to contact a patient. In the illustrated embodiment, pencil  5  includes an active electrode  13 , and a return electrode is operably supported on a return pad (not shown) that is configured to contact a patient. For the remainder of the discussion, electrosurgical system  2  and calibration system  16  are described in terms of use with a bipolar electrosurgical forceps  4 . 
         [0029]      FIG. 2  is a block diagram of an electrosurgical system  2 , which includes the generator  8 , forceps  4  and calibration computer system  16  of  FIG. 1 . Briefly, generator  8  includes a controller  18 , a high voltage power supply  20 , and a radio frequency output stage  22 , which operate together to generate an electrosurgical signal to be applied to tissue through electrodes  24 ,  26  of forceps  4 . Controller  18  includes a digital signal processor (DSP)  28 , a main processor  30 , and a memory  32 . The controller  18  may be any suitable microcontroller, microprocessor, PLD, PLA, or other digital logic device. Memory  32  may be volatile, non-volatile, solid state, magnetic, or other suitable storage memory. 
         [0030]    The controller  18  may also include various circuitry that serves as an interface between the main processor  30  and other circuitry within the electrosurgical generator  8  (e.g., amplifiers and buffers). The controller  18  receives various feedback signals that are used by the main processor  30  and/or the DSP  28  to generate control signals to control various subsystems of the generator  8 , including the HVPS  20  and the RF output stage  22 . These subsystems are controlled to generate electrosurgical energy having desired characteristics for performing surgical procedures on tissue, which is represented in  FIG. 2  by a test load “L.” 
         [0031]    The generator  8  includes an AC/DC power supply  34  that receives power from an alternating current (AC) line source (not explicitly shown) and converts the AC line power into direct current (DC), and, in turn, provides the DC power to an energy conversion circuit  36 . The energy conversion circuit  36  then converts the DC power at a first energy level into DC power at a second, different energy level based upon control signals received from the controller  18 . The energy conversion circuit  36  supplies the DC power at the second, different energy level to the RF output stage  22 . The RF output stage  22  converts the DC power into a high-frequency alternating current (e.g., RF), which may then be applied to tissue. 
         [0032]    In accordance with the instant disclosure, the electrosurgical generator  8  includes a measurement module  21  ( FIGS. 2 and 3 ) that is configured to determine voltage, current, impedance, and power at a tissue site so that the controller  18  can use these measurements to control the characteristics of the electrosurgical output. The measurement module  21  includes a voltage sensor  38  and a current sensor  40  coupled to the output of the RF output stage  22  (FIG.  2 ). The voltage sensor  38  senses the voltage across the output of the RF output stage  22  and provides, via a log amp  46 , an analog signal representing the sensed voltage to an analog-to-digital converter (ADC)  42 , which converts the analog signal representing voltage into digital form ( FIGS. 2 and 3 ). Similarly, the current sensor  40  senses the current at the output of the RF output stage  22  and provides, via a log amp  48 , an analog signal representing the sensed current to another ADC  44 , which converts the analog signal representing current into digital form ( FIGS. 2 and 3 ). 
         [0033]    The DSP  28  receives the sensed voltage and sensed current data from the respective log amps  46  and  48  and uses it to calculate the impedance and/or the power at the tissue site. The main processor  30  of the controller  18  executes algorithms that use the sensed voltage, the sensed current, the impedance, and/or the power to control the HVPS  20  and/or the RF Output Stage  22 . For example, the main processor  30  may execute a PID control algorithm based upon the calculated power and a desired power level, which may be selected by a user, to determine the amount of electrical current that should be supplied by the RF output stage  22  to achieve and maintain the desired power level at the tissue site. As discussed in greater detail below, the DSP  28  utilizes one or more control algorithms to calibrate the received sensed voltage and current data to obtain a more accurate representation of the RMS voltage (Vrms) that is transmitted to the forceps  4 . 
         [0034]      FIG. 3  illustrates a block diagram of internal components of the measurement module  21  including the voltage and current sensors  38  and  40  connected to the RF output stage  22 . Voltage and current sensors  38  and  40  are measured on the patient side of an output transformer “T.” In the illustrated embodiment, both the voltage and current sensors  38  and  40  are duplicated for redundant sensing (only one of each is shown). The sensors  38 ,  40  are isolated from the patient and are referenced to ground “G” of the generator  8  ( FIG. 3 ). With respect to the voltage, this is accomplished by isolating and reducing a voltage sense line with the use of a pair of capacitors C 1  and C 2  ( FIG. 3 ). For the current, a center tap transformer configuration is utilized including a pair of blocking capacitors “BC” between the ground “G” of the generator  8  and transformer “T”. The blocking capacitors “BC” are used as a current sensing element instead of a traditional lossy element such as, for example, a resistor. The isolation and signal reduction for the current sense line is maintained using a pair of capacitors C 3  and C 4  ( FIG. 3 ). The capacitors C 1 -C 4  may be any suitable type of capacitors including without limitations polyphenylene sulphide (PPS) film capacitors, ceramic chip capacitors NPO (COG) and the like. In accordance with the instant disclosure, capacitors Cl-C 4  are ceramic chip capacitors NPO (COG) and are chosen for their voltage and thermal stability. The aforementioned capacitor configuration facilitates maintaining a consistent overall gain of the measurement module  21  and, in particular log amps  46  and  48 . 
         [0035]    Log amps  46  and  48  may be any suitable type of log amp. A suitable log amp may be an AD8310 manufactured by Analog Devices ( FIG. 3 ). This particular log amp was chosen because of its low current draw (e.g., approximately 8 mA), small footprint (e.g., approximately 4×5 mm), a suitable differential AC input that ranges from about −91 dBV to about 9 dBV, a suitable DC level output that ranges from about 0.4V to about 2.6V, and a large dynamic range that ranges from about −91 dB to about 4 dB. The AD8310 log amp utilizes minimal external components as compared to other log amps that are sometimes utilized in conventional generators. Other suitable log amps that may be utilized in accordance with the present disclosure include: AD8307, AD8318, AD8317, ADL5513, AD8363 (RMS) all manufactured by Analog Devices, HMC612LP manufactured by Hittite, LTC5507 manufactured by Linear Technology, and TL441 manufactured by Texas Instruments. 
         [0036]      FIG. 4  shows a typical performance characteristic of an output of the log amp  46 . In accordance with the instant disclosure, a slope is used to derive a linear slope equation in the form of y=mx+b, where m is the slope of the output of the log amp  46  and b is the intercept of the log amp  46  at a 0 dBV input. The X axis shows the input in dBV and the Y axis shows the corresponding output of the log amp  46 . Log amp  46  includes a slope input (not explicitly shown) that gives the flexibility to increase the slope of the thereof. In accordance with the instant disclosure, to balance between input versus output range, the slope was increased to 40 mV/dB; this was done by adding a 23.2 KΩ resistor between two or more pins of the log amp  46 . With the increased slope of 40 mv/dB the output of the log amp  46  is 1.2V and an intercept point of the log amp  46  is 2.95 at the 0 dBV point, see  FIG. 4 . The equation for the output of the log amp  46  is expressed as: 
         [0000]        V out=0.040 x+ 2.95   (1)
 
         [0000]    where x is the value in dB of the input signal. 
         [0037]    Equation (1) can be solved for x in volts (not dB) by rewriting equation (1) with dB: 
         [0000]        V out=Slope*[20*log( V in)]+2.95   (2)
 
         [0000]    the slope from equation (1) equals 0.04, Vin equals the input voltage to the log amp  46  and the  20  in equation (2) comes from the conversion of dBV to V which is: 
         [0000]      dBV=20*log( V )   (3)
 
         [0000]    solving for Vin: 
         [0000]        V in=10̂[( V Out−2.95)/(20*Slope)]  (4)
 
         [0000]    the power of 10 comes from the inverse of the log function. 
         [0038]    With reference again to  FIG. 3 , the voltage signal into log amp  46  is a voltage divider using the ratio of C 1  and C 2 . This voltage signal is equal to: 
         [0000]        V _RF= V _Out×( XC   2 /( XC   2   +XC   1 )   (5)
 
         [0000]    where XC=1/(2πfC), V_Out is the output voltage of the generator  8 , f is the operating frequency of the generator  8 , and C is the value of the capacitors. 
         [0039]    The current signal into the log amp  40  may be derived from measuring the voltage integrated across the blocking capacitors “BC”. The voltage across the blocking capacitor “BC” is related to the current through the blocking capacitor “BC” using the equation: 
         [0000]        dv=I×dt/C    (6)
 
         [0000]    where I is the output current of the generator  8 , C is the value of the blocking capacitor “BC”, dv is the change in voltage across the capacitor “BC”, and dt is the switching cycle time. 
         [0040]    In some embodiments, to reduce noise on the current signal, a differential pair (not explicitly shown) may be run from the blocking capacitors “BC” back to an input of the log amp  48 . In particular, a low pass differential filter (with a pole of suitable frequency, e.g., about 100 KHz) may be added in series with the log amp  48 . One or more low pass differential filters may also be added in series with the log amp  46  to facilitate reducing noise on the current signal. 
         [0041]    The aforementioned configuration of the log amps  46  and  48  and low pass filter facilitates maintaining a consistent overall gain of the measurement module  21 . 
         [0042]    To accurately control the electrosurgical energy applied to tissue, the controller  18  needs to accurately sense the voltage and current at the tissue. As noted above, however, the voltage sensed by the voltage sensor  38  and the current sensed by the current sensor  40  may be inaccurate because of the internal construction of the log amps  46  and  48 . In particular, log amps  46  and  48  may have an internal gain that is dependent on the input voltage such that the gain of the log amps  46  and  48  is not constant across a dynamic range of the log amps  46  and  48 . The dynamic range of the log amps  46  and  48  may be measured from a voltage minimum (or current minimum) to a voltage maximum (or current maximum). This gain variation present for both log amps  46  and  48  may result in decreased accuracy of log amp sensors  38  and  40 . In other words, the voltage and current measured at the RF output stage  22  by the voltage and current sensors  38 ,  40  may not equal the actual voltage and current at the load “L” (e.g., tissue) because of the non-linear gain dependency at the inputs of the log amps  46  and  48 . 
         [0043]    The electrosurgical system  2  is configured to calibrate sensors  38  and  40  to compensate for the gain losses of the log amps  46  and  48  that introduce errors into the sensor data provided by the sensors  38  and  40 . In particular, a slope of the log amps  42  and  44  and an overall gain of the measurement module  21  is calculated via the calibration computer system  16 . In accordance herewith, the overall gain of the measurement module  21  takes into account the voltages from the capacitor divider network, attenuation due to the filter, and the gain through the log amp  38  and/or  40 . For illustrative purposes, the overall gain of the measurement module  21  is obtained using the gain of the log amp  46  that relates to measurements sensed by the voltage sensor  38 . 
         [0044]    The slope and gain are calculated via calibration computer system  16 . Calibration computer system  16  is in connection with the controller  18  of the generator  8  and includes a processor  50 , a memory  52 , and a communications interface  54 . Calibration computer system  16  utilizes one or more suitable measuring devices to measure output voltage and current. In the illustrated embodiment, a RMS meter  56  in communication with a current toriod  58  is utilized to measure output current Irms sent to the test load “L” as shown in  FIG. 5 . The output voltage Vrms can be obtained by multiplying the output current by the resistance of the test load “L.” 
         [0045]    In some embodiments, the generator  8  is connected to the calibration computer system  16 , RMS meter  56  including the current toroid  58 , and a 100 ohm test load “L.” In this instance, for example, calibration computer system  16  configures the generator  8  to operate in an open loop mode, sets a target value(s), and turns on the generator  8  until the generator reaches the target value(s). In some embodiments, the power supply duty cycle of the generator  8  is varied to produce an RF output from 4 volts rms to 92 volts rms, e.g., through the dynamic range of the log amps  46  and  48 . Calibration computer system  16  records the data measured by the generator  8  and RMS current meter  56 , see table (1) in  FIG. 6 . In embodiments, this data can be loaded into one or more suitable templates, e.g., a formatted Excel template, for analysis of a load curve to plot error. 
         [0046]    Table (1) in  FIG. 6  lists the impedance of the test load “L”, e.g., a 100Ω load, measured Irms, V 1 raw (output voltage of log amp  46 ), I 1 raw (output current of log amp  48 ) and calculated Vrms which is obtained by multiplying a corresponding Irms by the test load “L” impedance (e.g., 0.893 A×100Ω=89.3 V). The processor  50  accesses measurement data of table (1) and stores the measurement data in the memory  32  of the generator  8 . This data is accessible by the DSP  28  to execute one or more control algorithms for controlling an output of the generator  8 . 
         [0047]    In some instances, the slope is calculated by dividing the dynamic range of the output of the voltage log amp  46  over the measured range of the output of the generator  8  in dB using the following equation: 
         [0000]      Slope=(Log out     —   max−Log out     —   min)/((20×log(Gen out     —   max))−(20×log(Gen out     —   min)))   (7)
 
         [0000]    where Log out     —   max is the maximum output value of the log amp  38  recorded (2.7979), Log out     —   min is the minimum value of the log amp  38  recorded (1.6479, (0.3735 in this column is used to verify the offset of the log amp  46  at no output of the generator  8 ; this offset should be between 0.3 to 0.5)), Gen out     —   max is the max output of the generator  8  (89.3) and Gen out     —   min is the min output of the generator  8  (3, (the 0 may be ignored)). It has been found that the slope of the log amp  46  is equal to about 39 mV/dB and is linear over the dynamic range, as shown in  FIG. 4 , for example. 
         [0048]    To measure the gain of the log amp  46  (e.g., overall gain of the measurement module  21 ), first the output voltage of the log amp  46  is translated to an input voltage, using the following equation: 
         [0000]        V in=10̂[( V out−2.95)/(20*Slope)]  (8)
 
         [0000]    where the “slope” is the slope of the voltage log amp  46  and Vout is the Vraw from table (1). The output of the log amp  46  equals 2.7979V at a calculated output voltage Vans from the generator  8  of 89.3V. Placing this into equation (4) and using the 39 mV for the slope, the input voltage is 0.638V, see Log V 1  column of table (1) in  FIG. 6 ; this is the actual voltage at the input of the log amp  46 . The gain can now be calculated using the equation: 
         [0000]      Gain V =Calculated —   V rms/ V in   (9)
 
         [0000]    in this instance the gain is equal to 139.88 (e.g., 89.3 v/0.638 v). The gain for each measured output voltage Vraw of the log amp  46  and corresponding output voltage Vrms of the generator  8  can be calculated in a similar manner and recorded. As can be appreciated, the more measurements taken through the dynamic range of the log amp  46  the more accurate the calculate gain will be. Multiplying equation (4) with the gain calculated in equation (9) gives an accurate representation of the output of the generator  8  referenced to the output of the log amp  46  when the generator  8  is coupled to a test load, see equation (10) below. 
         [0000]      Out=Gain*10̂[( V out−2.95)/(20*Slope)]  (10)
 
         [0049]    Equation (10) is stored in memory  32  and, in some instances, may be utilized in a control algorithm by the DSP to calculate the output of the generator  8  in a real time scenario. 
         [0050]    As can be appreciated, the Vrms is, however, not measured with an external device, e.g., an RMS meter  56 , in a real time scenario. As noted above, the gain of the log amp  46  (and/or log amp  48 ) is not constant across the dynamic range, see  FIG. 6  for example under the V 1  Gain column. The gain value changes from  132  to  140  in a non-linear relation. This gain variation is prevalent for both log amps  46  and  48 . Therefore, in a surgical environment, the DSP  28  utilizes one or more control algorithms to calculate the gain to calibrate the sensor  38  and/or sensor  40 . The DSP  28  uses the above data to fully automate the calibration of the sensor  38  (and/or sensor  40 ) by calculating the slope of the log amp  46  (and/or log amp  48 ) utilizing one or more of the aforementioned equations. The overall gain of the measurement module  21  is subsequently calculated and utilized by the DSP  28  to accurately calculate the Vrms, impedance and/or the power at the tissue site. 
         [0051]    In particular, and in one embodiment, the calibration computer system  16  utilizes a control algorithm that incorporates a piece-wise linear fit of the gain relative to the measured output of the log amp  46  to correct the variations in gain of the log amp  46 . In this instance, for example, the data of table (1) may be stored into memory  22  and accessed by the DSP  28 . When the output of the voltage log amp  46  detected by the DSP  28  falls between two known values the gain is calculated using a linear slope equation: 
         [0000]      Gain_Calc= mX+b    (11)
 
         [0000]    in this equation Gain_Calc is the calculated gain value, X is the V 1 raw data from table (1), m is the slope between the two known gain points, and b is a calculated offset. It should be noted that the “Gain_Calc” in equation (11) is not the same as the “Gain” in equation (10). Gain_Calc” is the gain between two gain values (e.g., 134.96 and 140.27) previously calculated and stored into memory  22 . For example, if the detected output from the log amp  46  is 2.10, which falls between an output of log amp  46  that equals 1.96 (see table (1) in  FIG. 5 ) and an output of the log amp  46  that equals 2.24 (see table (1) in  FIG. 5 ) the DSP  28  utilizes equation (11) below and the corresponding gains to calculate the Gain_Calc. 
         [0000]        m =(Gain2−Gain1)/(Log_out2−Log_out1)   (11)
 
         [0052]    To calculate m, Gain  1  is a first gain point (134.96), which corresponds to an output of the log amp  46  that equals  1 . 96  and Gain  2  is a second gain point (140.27), which corresponds to an output of the log amp  46  that equals 2.24 (see  FIG. 6 ). Log_out 1  is the output of the voltage log amp  46  in relation to the first gain point (1.96) and Log_out 2  is the output of the voltage log amp  46  in relation to the second gain point (2.24), see  FIG. 6 . In this example, m is equal to 18.96. 
         [0000]        b =Gain1− m *(Log_out1)   (12)
 
         [0053]    Continuing with the present example, and using equation (12) above, b is calculated to be equal to 97.8. 
         [0054]    Taking the values found for b and m and using them in equation (11), the corresponding output of the log amp  46  is equal to 2.10 and the gain is found to be equal to 137.62. 
         [0055]    The DSP  28  utilizes this gain value of 137.62 in equation (10) to accurately calculate Vrms, impedance and/or the power at the tissue site. The slope is assumed to equal 39 mV. The calculated value of the gain (137.62) is substituted into equation (10), Vrms is calculated to be (137.62)10̂[(2.10−2.95)/(20×0.039)], which is equal to Vrms=(137.62)10̂(−0.85/0.78), which is equal to Vrms=(137.62)10̂(−1.089), which is equal to Vrms=11.192. This value of Vrms falls between the previously measured Vrms values 17.1 and 7.3 that correspond to the outputs 2.24 and 1.96 of the log amp  46 . 
         [0056]    In some embodiments, the calibration computer system  16  utilizes a control algorithm that incorporates an averaging technique of the gain relative to the measured output of the log amp  46  to correct the variations in gain of the log amp  46 . In this instance, the log amp  46  gain values from the individual calibration points across the dynamic range are averaged together to get a mean gain for the error over the dynamic range of the voltage log amp  46 . In this instance, the DSP  28  calculates the mean gain value over the dynamic range and substitutes this gain in equation (10) and then calculates the Vrms (or other desired parameter at the tissue site). 
         [0057]    In some embodiments, the calibration computer system  16  utilizes a control algorithm that incorporates a polynomial curve fitting technique (“poly-fit” technique) of the gain relative to the measured output of the log amp  46  to correct the variations in gain of the log amp  46 . In this instance, one or more suitable polynomials may be utilized to represent the gain variations of the voltage log amp  46 . For example, a typical third order polynomial: 
         [0000]        V=a*V calc ̂3   +b*V calc ̂2   +c*V calc+ d    
         [0000]    where a, b, c, and d are the polynomial calibration coefficients may be utilized to represent the gain variations of the voltage log amp  46 . 
         [0058]    Operation of the electrosurgical system  2  is described with reference to the piece-wise linear fit algorithm. In this instance, it is assumed that the data of table (1) has been previously calculated for the log amp  46  and stored into memory  32  of the main processor  30 . 
         [0059]    In use, the DSP  28  receives an output, e.g., an output voltage, from the log amp  46 . DSP  28  runs through the piece-wise linear fit control algorithm to calculate a gain of the log amp  46 . Thereafter, the Vrms is calculated utilizing the calculated gain in equation (10). Once the Vrms is calculated, the DSP  28  can communicate control signals as needed to ensure that the generator  8  provides a correct amount of electrosurgical energy to the forceps  4 . 
         [0060]    The electrosurgical system  2  including the generator  8  and calibration computer system  16  provides an effective method of calibrating the sensors  38  and  40  to overcome the aforementioned drawbacks that are typically associated with conventional generators. 
         [0061]    From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. For example, various temperature ranges could utilize their own multi-point calibrations to compensate for any temperature dependency of the log amp  46  and/or log amp  48 . 
         [0062]    Moreover, it is within the purview of the instant disclosure to utilize a plurality of log amps  46  and a plurality of log amps  48 . As can be appreciated, this adds an extra layer of sensing capabilities, which, in turn may increase overall gain of the measurement module  21 . 
         [0063]    Further, the aforementioned control algorithms may utilized in combination with one another. For example, the DSP  28  may utilize a piece-wise linear control algorithm across a first portion of the dynamic range of the log amps  46  and  48  and may utilize the averaging (or poly-fit) control algorithm across a second, different portion of the dynamic range of the log amps  46  and  48 . 
         [0064]    While several embodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.