BACKGROUND 
   1. Field of the Invention 
   The present invention relates to signal integrators, and in particular, to signal integrators for integrating continuous and discrete signals. 
   2. Prior Art 
   Achieving linear integration of two different types of inputs, such as a continuous current and a discrete charge, is complicated by the slewing of the integrated signal caused by the effects of integrating discrete charges along with the continuous current. Compensating for these nonlinear effects, e.g., through the use of filters when possible, increase the size and complexity of the system, as well as power consumption, any and all of which are problematic for many applications, particularly in mobile devices or instrumentation systems. 
   Referring to  FIG. 1A , a conventional circuit for integrating continuous current and discrete charge receives a continuous output current I resulting from the application of an input voltage VIN to an input resistance R 1 . The discrete charge Q is received from an input capacitance C 1  which is alternately charged and discharged by the application of reference voltages VREF 1 , VREF 2  and a common mode voltage VCM (discussed in more detail below). This current I and charge Q are summed at a virtual ground VG formed at the inverting input electrode of an operational amplifier A 1  whose non-inverting input electrode is grounded. Due to the feedback capacitance Cf connected between the inverting input electrode and output electrode of the operational amplifier A 1  (in accordance with well known integrator principles), the summed current I and charge Q is accumulated on the feedback capacitance Cf forced by the virtual ground VG at the inverting input electrode of the operational amplifier A 1 . 
   The input capacitance C 1  is alternately charged and discharged by the alternate application of the reference voltages VREF 1 , VREF 2  and common mode voltage VCM during alternate time intervals, e.g., mutually exclusive, in accordance with switch control signals F 1 , F 2  having mutually opposing signal assertion and de-assertion phases. 
   Referring to  FIG. 1B , the input capacitance C 1  is disconnected from the virtual ground at the inverting input electrode of the operational amplifier A 1  during assertion of signal F 2  (reset phase), and its electrodes, or plates, are set to a reference voltage VREF 1  and the common mode voltage VCM. During assertion of signal F 1  (integrating phase), the top electrode of the input capacitance C 1  is connected to the virtual ground at the inverting input electrode of the operational amplifier A 1 , while the bottom electrode of the input capacitance C 1  is connected to the reference voltage VREF 2 . The resulting voltage difference VREF 1 -VREF 2  across the input capacitance C 1  produces the charge Q, which is transferred and accumulated on the feedback capacitance Cf. Because the voltage across the input capacitance cannot be changed instantly, there will be a voltage transient at the bottom electrode of the input capacitance C 1 , as well as at the top electrode and virtual ground VG. As is well known, the transconductance Gm of the operational amplifier A 1  is dependent upon its input voltage, i.e., voltage applied between its inverting and non-inverting electrodes. Accordingly, any voltage transient appearing across these input electrodes alters the transconductance Gm and, therefore, disturbs linear operation of the operational amplifier, in which, in this application, results in a disturbance in linear integration of the continuous input current I. As the voltage transient at the virtual ground VG increases, distortions in the integrated signal increase as well. 
   SUMMARY 
   In accordance with the presently claimed invention, a signal integrator and method are provided for linear integration of a continuous current and a discrete charge in which the discrete charge is provided for integration during multiple overlapping time intervals. 
   In accordance with one embodiment of the presently claimed invention, signal integration circuitry for integrating a continuous current and a discrete charge includes: 
   signal integration circuitry including a plurality of signal electrodes and responsive to a continuous current and a discrete charge by providing an integrated signal; 
   switched capacitive circuitry responsive to a plurality of voltages and a portion of a plurality of control signals by providing the discrete charge during a first one of a plurality of time intervals; and 
   first switch circuitry coupled between the switched capacitive circuitry and a portion of the plurality of signal electrodes and responsive to another portion of the plurality of control signals by conveying the discrete charge, wherein the discrete charge is conveyed to
         a first one of the portion of the plurality of signal input electrodes during the first one of the plurality of time intervals, and   each remaining one of the portion of the plurality of signal input electrodes during respective successively latter portions of the first one of the plurality of time intervals.       

   In accordance with another embodiment of the presently claimed invention, signal integration circuitry for integrating a continuous current and a discrete charge includes: 
   a signal electrode to convey a continuous current; 
   a plurality of electrodes to convey a plurality of voltages; 
   an operational amplifier circuit with input and output electrodes; 
   a feedback capacitance coupled between the input and output electrodes; 
   an input resistance coupled between the signal and input electrodes; 
   a plurality of additional resistances coupled to the input electrode; 
   first switch circuitry coupled to the plurality of electrodes; 
   second switch circuitry coupled to the first switch circuitry and the plurality of additional resistances; and 
   an input capacitance coupled to the first and second switch circuitries; 
   wherein 
   
       
       
         
           the first switch circuitry is responsive to a portion of a plurality of control signals by conveying the plurality of voltages during first and second time intervals, respectively, 
           the input capacitance is responsive to the plurality of voltages by providing a discrete charge during the first time interval, and 
           the second switch circuitry is responsive to another portion of the plurality of control signals by conveying the discrete charge to
           a first one of the plurality of additional resistances during the first time interval, and   each remaining one of the plurality of additional resistances during respective successively latter portions of the first time interval.   
         
         
       
     
  
   In accordance with another embodiment of the presently claimed invention, a method for integrating a continuous current and a discrete charge includes: 
   receiving a continuous current; 
   receiving a discrete charge during a first one of a plurality of time intervals; 
   converting the discrete charge to a first one of a plurality of conversion currents during the first one of the plurality of time intervals; 
   converting the discrete charge to each remaining one of the plurality of conversion currents during respective successively latter portions of the first one of the plurality of time intervals; and 
   integrating the continuous current and the plurality of conversion currents to provide an integrated signal. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic diagram of a conventional circuit for integrating continuous current and discrete charge. 
       FIG. 1B  is a signal timing diagram for the switches of  FIG. 1A . 
       FIG. 2A  is a schematic diagram of a circuit for integrating continuous current and discrete charge in accordance with one embodiment of the presently claimed invention. 
       FIG. 2B  is a signal timing diagram for the switches of  FIG. 2A . 
       FIG. 3A  is a schematic diagram of a circuit for integrating continuous current and discrete charge in accordance with another embodiment of the presently claimed invention. 
       FIG. 3B  is a signal timing diagram for the switches of  FIG. 3A . 
       FIG. 4  is a schematic diagram of a differential implementation of the circuit of  FIG. 2A  for integrating continuous current and discrete charge in accordance with another embodiment of the presently claimed invention. 
   


   DETAILED DESCRIPTION 
   The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
   Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
   Referring to  FIG. 2A , circuitry for integrating continuous current and discrete charge in accordance with one embodiment of the presently claimed invention adds additional input resistances Ra, Rb and corresponding switches F 1   a , F 1   b  for applying the discrete charge Q in multiple phases. For example, in this particular embodiment, the discrete charge Q is applied in two phases via the two additional input resistances Ra, Rb and corresponding switches F 1   a , F 1   b . (As discussed in more detail below, the charge Q can be applied in virtually any number of phases, as desired.). 
   Referring to  FIG. 2B , the integrating phase, i.e., during the assertion of switch control signal F 1 , is divided into two sub-phases F 1   a , F 1   b . Switch control signal F 1   a  is asserted during assertion of signal F 1  (and can be contemporaneous with assertion of signal F 1 ). Switch control signal F 1   b  is asserted during a latter portion of the assertion interval for signal F 1   a . Accordingly, switch F 1   a  is turned on with the discrete charge Q applied through resistor Ra during the entire integrating phase, while switch F 1   b  is turned on with the discrete charge Q applied through resistor Rb for the shorter, latter time interval during assertion of signal F 1   b . The first resistance Ra is greater than the second resistance Rb (Ra&gt;Rb). (The “on” resistances of each of the input switches F 1   a , F 1   b  are preferably significantly smaller than the input resistances Ra, Rb.). Therefore, the sum of the first resistance Ra and “on” resistance Rf 1   a  of switch F 1   a  will be greater than the sum of the second resistance Rb and “on” resistance Rf 1   b  of switch F 1   b . Since the sum of resistance Ra and “on” resistance Rf 1   a  of switch F 1   a  is greater, when switch F 1   a  is initially turned on at the beginning of the assertion of signal F 1   a , the transfer of the charge Q from the input capacitance C 1  appears more like a current source with a large internal resistance continuously injecting a small current into the virtual ground VG. Hence, the transient voltage appearing at the virtual ground VG is very small. 
   Later, during assertion of signal F 1   a  and upon assertion of signal F 1   b , due to the charge transfer during the earlier portion of the integrating phase, i.e., earlier during the assertion of signal F 1   a , the voltage difference across the input capacitance C 1  is smaller than it was at the beginning of the integrating phase, i.e., upon assertion of signal F 1   a . Accordingly, during assertion of signal F 1   b , i.e., with switch F 1   b  closed and the discrete charge Q now also being applied via the second input resistance Rb, even with the second resistance Rb being less than the first resistance Ra, the voltage at virtual ground VG will more closely follow the voltage at the bottom electrode of the input capacitance C 1 , and the voltage transient at virtual ground VG will be small. 
   With a well designed operational amplifier A 1 , i.e., an operational amplifier operating very nearly in accordance with ideal operating assumptions (which is increasingly common with current integrated operational amplifiers circuits), and appropriate resistance values for the input resistances Ra, Rb, it can be assured that the voltage transients appearing at the virtual ground VG will be significantly smaller, thereby ensuring a substantially linear integration of the continuous input current I. Additionally, a more complete transfer of the discrete charge Q from the input capacitance C 1  to the feedback capacitance Cf is achieved, thereby significantly reducing any settling error within the integrated output signal VOP. This, in turn, ensures a highly linear integration of the discrete charge Q from the input capacitance C 1 . 
   Referring to  FIGS. 3A and 3B , this switching technique for applying the discrete charge in multiple phases can be extended to more than two phases, or branches. In this example, four phases, or branches, are used for applying the discrete charge Q for integration. As discussed above, the first application of the discrete charge Q occurs during assertion of signal F 1   a , which occurs during (and may be coincident with) assertion of signal F 1  (integrating phase). During assertion of signal F 1   a , additional applications of the discrete charge Q occur during respective successively latter portions of the time interval defined by the assertion of signal F 1   a , as shown. Hence, the discrete charge Q from the input capacitance C 1  is applied via switch F 1   a  and resistance Ra during assertion of signal F 1   a , via switch F 1   b  and resistance Rb during assertion of signal F 1   b , via switch F 1   c  and resistance Rc during assertion of signal F 1   c , and via switch F 1   d  and resistance Rd during assertion of signal F 1   d . As discussed above, these input resistances Ra, Rb, Rc, Rd are successively smaller in value (i.e., Ra&gt;Rb&gt;Rc&gt;Rd). The duty cycles of the signal assertions for the switch control signals F 1   a , F 1   b  can be adjusted for the desired performance. 
   Referring to  FIG. 4 , this switching technique for applying the discrete charge can be extended to a differential signal implementation as well. While similar to the single ended implementation of  FIG. 2A , this circuit receives its continuous current via respective input resistances R 1  as a differential signal VIP-VIN rather than as a single ended signal VIN, and also receives its discrete charge differentially via respective input resistances Ra, Rb and switches F 1   a , F 1   b  for each differential signal phase. The resulting integrated signal phases for the differential output signal VOP-VON appear across respective feedback capacitances Cf for the operational amplifier A 1 . Operation of this circuit for each differential signal phase is similar to that described above for the single ended circuit of  FIG. 2A . 
   As discussed above, by using multiple branches of switches operating in multiple steps, or phases, during the integrating phase, an otherwise large voltage transient appearing at the virtual ground of the operational amplifier is reduced, e.g., in correspondence to the number of phases when integrating the discrete charge. This allows improved linear performance for integrating the continuous current, as well as improved transfer of the discrete charge. Accordingly, overall signal integration is significantly more linear. 
   Various other modifications and alternations 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 the 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.
A signal integrator and method for integrating a continuous current and a discrete charge in which the discrete charge is provided for integration during multiple overlapping time intervals.