Patent Publication Number: US-6703887-B2

Title: Long time-constant integrator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of a co-pending U.S. Provisional Patent Application entitled “Long Time-Constant Intergrator” Ser. No. 60/316,781 filed on Aug. 31, 2001, the disclosure of which is incorporated by reference herein in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to integrators, and more specifically to an integrator with a long time constant. 
     BACKGROUND OF THE INVENTION 
     The integrator is a key building block for analog signal processing. It attenuates high-frequency signals, shapes low-frequency signals, and tracks DC levels. 
     FIG. 1 shows a conventional operational amplifier integrator. The resistor (R) converts the input voltage v in  to a current that can be expressed as:          i     i                 n       =         v     i                 n       -     V     (   -   )         R                     
     where V (−)  is the voltage at the inverting terminal of the operational amplifier. The operational amplifier&#39;s large gain forces the voltages at its input terminals V (−)  and V (+)  to be equal, while its high input impedance directs the input current i in  to the capacitor. This causes a voltage to be developed across the capacitor (C) given by:            v   c     =       1   C          ∫       i     i                 n               t                                with   ;             v   out     =       -     v   C       =       1   RC          ∫       v     i                 n               t                             
     where RC is the time constant of the integrator. 
     A typical integrator uses large-valued resistors and capacitors to realize a long time constant, making it difficult to implement in monolithic form. As a result, off-chip capacitors are used or shorter time constants with some compromise are substituted. 
     The above integrator operates continuously as opposed to the switched-capacitor integrator shown in FIG.  2 . The switched-capacitor integrator is a discrete-time circuit that is capable of long time constants. During operation, when the switch is connected to the input, the input voltage v in  charges capacitor C 1 . The charge Q C1  stored by the capacitor C 1  is simply equal to: 
     
       
         Q C1 =C 1 v in   
       
     
     When the switch is connected to the inverting input of the operational amplifier, the capacitor C 1  is discharged to ground through capacitor C 2 . (Since the large gain of the operational amplifier drives its inverting terminal to be equal to its noninverting terminal.) That is, for each cycle of the switch, a packet of charge is transferred from the input to the integrating capacitor C 2 . The equivalent input current is therefore equal to: 
     
       
         I eq =Q C1 f s =C 1 f s v in   
       
     
     where f s  is the switching frequency. This results in an equivalent resistance (R eq ) of:          R   eq     =         v     i                 n         I   eq       =     1       C   1          f   s                           
     As with any discrete-time system, the switching frequency must be several times higher than the highest input frequency. However, by their nature, switched capacitor circuits generate switching noise that can affect other circuits and produce discrete-time outputs that typically require filtering to smooth the steps between discrete values. 
     It would therefore be advantageous to have a continuous-time integrator using nominal-valued components to achieve a long time constant. 
     SUMMARY OF THE INVENTION 
     The present invention includes a differential integrator circuit that uses a resistor array to reduce integrating current and thereby realize a long time constant. The performance of the circuit is based on resistor matching—a strong property in monolithic circuits—and nominal-valued components to achieve the long time constant. 
     In one embodiment of the invention, a long time-constant differential integrator is provided that comprises a differential operational amplifier having inverting and noninverting amplifier input terminals, and inverting and noninverting amplifier output terminals, and the output terminals form inverting and noninverting output terminals, respectively, of the differential integrator. The differential integrator also comprises a noninverting differential integrator input terminal and an inverting differential integrator input terminal. The differential integrator also comprises a resistor array that couples the noninverting differential integrator input terminal to the inverting and noninverting input terminals of the amplifier, and the resistor array also couples the inverting differential integrator input terminal to the inverting and noninverting input terminals of the amplifier. 
     In one embodiment, a long time-constant differential integrator is provided that comprises a differential operational amplifier having inverting and noninverting amplifier input terminals, and inverting and noninverting amplifier output terminals, and wherein the amplifier output terminals form inverting and noninverting output terminals, respectively, of the differential integrator. The differential integrator also comprises a noninverting differential integrator input terminal and an inverting differential integrator input terminal. The differential integrator also comprises a resistor array that couples the noninverting differential integrator input terminal to the inverting and noninverting input terminals of the amplifier, and the resistor array also couples the inverting differential integrator input terminal to the inverting and noninverting input terminals of the amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and the attendant advantages of this invention will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 shows a diagram of a continuous-time operational amplifier integrator; 
     FIG. 2 shows a diagram of a switched-capacitor, discrete-time integrator; 
     FIG. 3 shows a diagram of a differential integrator; 
     FIG. 4 shows a detailed schematic of one embodiment of a differential integrator and resistor array that realizes a long time constant in accordance with the present invention; and 
     FIG. 5 illustrates the performance of a conventional integrator and a long time constant integrator in accordance with the present invention using similar component values. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 shows a differential integrator that converts a differential input voltage to two charging currents i in+  and i in−  that are equal to:          i       i                 n     +       =             v       i                 n     +       -     V     c                 m           R   1                       i       i                 n     -         =         v       i                 n     -       -     V     c                 m           R   2                         
     where the common mode voltage V cm  is the mean of the two input voltages, lies equidistant between them, and can be expressed as:          V     c                 m       =       1   2          (       v       i                 n     +       +     v       i                 n     -         )                       
     The two input voltages can be rewritten as v in+ =V cm +Δv in  and v in− =V cm −Δv in  where Δv in  is the actual differential input voltage. This means that i in−  and i in−  are opposite polarities with;          i       i                 n     +       =           Δ                   v     i                 n           R   1                       i       i                 n     -         =     -       Δ                   v     i                 n           R   2                           
     FIG. 4 shows a detailed schematic of one embodiment of a differential integrator  400  that includes a resistor array  402  to realize a long time constant in accordance with the present invention. The actual integrating currents applied to capacitors C 1  and C 2  are reduced using the resistor array  402 . The integrating current associated with capacitor C 1  is given by:          i   C1     =         Δ                   v     i                 n           R   1       -       Δ                   v     i                 n           R   2                         
     where R 1  is slightly smaller than R 2 . If R 2  is set to R 2 =(1+ε) R 1  and ε&lt;&lt;1, then the above equations becomes:          i   C1     =         Δ                   v     i                 n           R   1       -       Δ                   v     i                 n             (     1   +   ɛ     )          R   1                           
     which can be simplified and expressed as:          i   C1     =       Δ                   v     i                 n             (       1   +   ɛ     ɛ     )          R   1                         
     The voltage developed across capacitor C 1  can therefore be expressed as:          v   C1     =       1       (       1   +   ɛ     ɛ     )          R   1          C   1              ∫     Δ                   v     i                 n               t                           
     As a result, the integrator time constant is increased by the factor.            1   +   ɛ     ɛ     .                   
     A similar analysis shows identical results for capacitor C 2  with regards to resistors R 3  and R 4 . For example, R 1  is matched with R 4  and R 2  is matched with R 3 , so that their ratios remain constant. 
     The novel long time-constant integrator of FIG. 4 utilizes the difference between two resistors to reduce the integrating current. As such, resistor matching is used. In monolithic circuits, resistor matching errors are typically less than 1%, allowing time constant increases better than 20 times. This benefit is illustrated in FIG.  5 . 
     FIG. 5 illustrates the performance of the conventional integrator and the long time-constant integrator using similar component values and setting ε=0.05. As a result, the unity gain frequency of the long time-constant integrator is shifted to a much lower frequency. The setting for E is small, however, its value is generally limited by the precision of the components used. 
     The long time-constant integrator uses an array of matched resistors—a strong suit of monolithic circuits—to significantly lower the integrating currents in a differential integrator and dramatically increase its effective time constant. Furthermore, the long time-constant integrator constructed in accordance with the present invention operates continuously, thereby avoiding switching effects and other problems common to switched-capacitor circuits. 
     The embodiments described herein are illustrative of the present invention and are not intended to limit the scope of the invention to the particular embodiments described. Accordingly, while one or more embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made to the embodiments without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.