Abstract:
An AC coupling network has (a) a first pair of capacitances C 1  connected between the input nodes and the output nodes and (b) a second pair of capacitances C 2  cross-connected between the input nodes and the output nodes. The capacitances C 1  and C 2  are formed by sets of switched capacitors that can be configured to provide the network with different levels of attenuation while maintaining a constant AC coupling pole frequency. In particular, the sets of switched capacitors can be configured to ensure that C 1+ C 2  remains constant, while C 1− C 2  varies. The present invention enables AC coupling to be implemented without using active devices such as operational amplifiers and/or buffers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to electrical signal processing, and, in particular, to AC coupling networks.  
         [0003]     2. Description of the Related Art  
         [0004]     An AC coupling network is used to connect two sets of electrical circuitry, where the AC coupling network substantially blocks any DC component in the output signal from the first set of electrical circuitry from passing to the second set of electrical circuitry. In many electrical signal processing applications, it is also desirable for the AC coupling network to apply a variable amount of attenuation to the output signal received from the first set of electrical circuitry before it is passed to the second set, while maintaining a constant pole frequency for the network. Prior-art techniques for creating an AC coupling network with variable attenuation and constant pole frequency employ an active operational amplifier or buffer stage.  
         [0005]      FIG. 1  shows a schematic circuit diagram of a prior-art AC coupling network  100  that employs an active operational amplifier Al having variable capacitors C 1  connected at its inputs. In network  100 , attenuation is controlled by adjusting the ratio of C 2  to C 1 , while the AC coupling pole frequency ω APCL  is given by Equation (1) as follows:  
               ω   ACPL     =     1     R1   *   C2               (   1   )               
 By varying C 1  and keeping C 2  constant, the attenuation (or gain) of network  100  can be adjusted without affecting the AC coupling pole frequency. 
 
         [0006]      FIG. 2  shows a schematic circuit diagram of a prior-art AC coupling network  200  that employs active buffer stages A 2 . In network  200 , attenuation is controlled by adjusting the ratio of R 4  to R 3 , while the AC coupling pole frequency ω APCL  is given by Equation (2) as follows:  
               ω   ACPL     =     1     R2   *   C3               (   2   )               
 By varying R 4  and keeping R 3  constant, the attenuation of network  200  can be adjusted without affecting the AC coupling pole frequency. 
 
         [0007]     Both of these prior-art circuits can provide independent adjustment of attenuation and the AC coupling pole frequency. However, both circuits require active devices in the form of an operational amplifier or a buffer.  
       SUMMARY OF THE INVENTION  
       [0008]     Problems in the prior art are addressed in accordance with the principles of the present invention by an AC coupling network that does not require the use of an active device such as an operational amplifier or a buffer.  
         [0009]     In one embodiment, the present invention is circuitry having an AC coupling network. The AC coupling network comprises two input nodes (e.g.,  302   a  and  302   b  of  FIG. 3 ) and two output nodes (e.g.,  304   a  and  304   b ). A first resistor (e.g.,  310   a ) is connected between a first output node (e.g.,  304   a ) and a first common-mode voltage node (e.g., Vcm 1 ). A second resistor (e.g.,  310   b ) is connected between a second output node (e.g.,  304   b ) and a second common-mode voltage node (e.g., Vcm 2 ). A first capacitance (e.g.,  306   a ) is connected between a first input node (e.g.,  302   a ) and the first output node. A second capacitance (e.g.,  308   a ) is connected between the first input node and the second output node. A third capacitance (e.g.,  308   b ) is connected between a second input node and the first output node. And a fourth capacitance (e.g.,  306   b ) is connected between the second input node and the second output node. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements.  
         [0011]      FIG. 1  shows a schematic circuit diagram of a prior-art AC coupling network that employs an active operational amplifier;  
         [0012]      FIG. 2  shows a schematic circuit diagram of a prior-art AC coupling network that employs active buffer stages;  
         [0013]      FIG. 3  shows a schematic circuit diagram of an AC coupling network, according to one embodiment of the present invention; and  
         [0014]      FIG. 4  shows a schematic circuit diagram of one possible implementation of the network of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0015]     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments.  
         [0016]      FIG. 3  shows a schematic circuit diagram of an AC coupling network  300 , according to one embodiment of the present invention. As shown in  FIG. 3 , network  300  has two input nodes  302   a  and  302   b  and two output nodes  304   a  and  304   b,  where the input signal Vin is applied between nodes  302   a  and  302   b  and the output signal Vout appears at nodes  304   a  and  304   b.  A pair of equivalent capacitors  306   a  and  306   b  having substantially identical capacitance C 1  are connected between nodes  302   a  and  304   a  and between nodes  302   b  and  304   b.  Similarly, a pair of equivalent capacitors  308   a  and  308   b  having substantially identical capacitance C 2  are cross-connected between nodes  302   a  and  304   b  and between nodes  302   b  and  304   a.  In addition, a pair of equivalent resistors  310   a  and  310   b  having substantially identical resistance R 1  are connected between node  304   a  and a first common-mode voltage Vcm 1  and between node  304   b  and a second common-mode voltage Vcm 2 . (Depending on the application, Vcm 1  and Vcm 2  could be the same voltage level or they could be different, e.g., in order to inject an offset between the two output nodes to compensate for a corresponding offset in the second set of circuitry connected to output nodes  304   a  and  304   b. )  
         [0017]     The gain of network  300  is given by Equation (3) as follows:  
               Gain   =       C1   -   C2       C1   +   C2         ,           (   3   )             
 
 while the voltage attenuation (in dB) is given by Equation (4) as follows:  
               Attenuation   dB     =     20   *   log   ⁢         C1   +   C2       C1   -   C2       .               (   4   )             
 
 According to Equation (4), attenuation is 0 dB for C 2 =0. As C 2  increases, attenuation increases, approaching infinity when C 2 =C 1 . 
 
         [0018]     The AC coupling pole frequency ω APCL  for network  300  is given by Equation (5) as follows:  
               ω   ACPL     =       1     R1   *     (     C1   +   C2     )         .             (   5   )             
 
         [0019]     As indicated by Equations (3)-(5), network  300  can provide variable attenuation, while maintaining a constant AC coupling pole frequency by allowing C 1 -C 2  to vary, while keeping C 1 +C 2  constant. In particular, attenuations from 0 dB to approaching infinity can be achieved by varying C 2  from 0 to C 1 , while keeping C 1 +C 2  constant. This means that every increase in the value of C 2  is accompanied by a corresponding decrease in the value of C 1 .  
         [0020]      FIG. 4  shows a schematic circuit diagram of one possible implementation of network  300  of  FIG. 3 . According to this implementation, a set of switched capacitors  402   a - c  having capacitances {Ca, Cb, Cc}, respectively, is connected between input node  302   a  and output nodes  304   a  and  304   b,  and an equivalent set of switched capacitors  404   a - c  having capacitances {Ca, Cb, Cc}, respectively, is connected between input node  302   b  and output nodes  304   a  and  304   b.    
         [0021]     By closing switches S 1  and S 3 , while keeping switches S 2  and S 4  open, C 1  (i.e., the capacitances connected between nodes  302   a  and  304   a  and between nodes  302   b  and  304   b ) will be (Ca+Cb+Cc), while C 2  (i.e., the capacitances cross-connected between nodes  302   a  and  304   b  and between nodes  302   b  and  304   a ) will be 0. According to Equation (3), the gain for this configuration of network  300  is given by Equation (6) as follows:  
             Gain   =         Ca   +   Cb   +   Cc       Ca   +   Cb   +   Cc       =   1.             (   6   )             
 
         [0022]     Similarly, by closing switches S 1  and S 4 , while keeping switches S 2  and S 3  open, C 1  will be Ca+Cb, while C 2  will be Cc. In this case, the gain of network  300  is given by Equation (7) as follows:  
               Gain   =       Ca   +   Cb   -   Cc       Ca   +   Cb   +   Cc         ,           (   7   )             
 
 which is less than the gain of Equation (6). 
 
         [0023]     Similarly, by closing switches S 2  and S 3 , while keeping switches S 1  and S 4  open, C 1  will be Ca+Cc, while C 2  will be Cb. In this case, the gain of network  300  is given by Equation (8) as follows:  
               Gain   =       Ca   -   Cb   +   Cc       Ca   +   Cb   +   Cc         ,           (   8   )             
 
 which will be less than the gain of Equation (7) for Cb&gt;Cc. 
 
         [0024]     Lastly, by closing switches S 2  and S 4 , while keeping switches S 1  and S 3  open, C 1  will be Ca, while C 2  will be Cb+Cc. In this case, the network gain is given by Equation (9) as follows:  
               Gain   =       Ca   -   Cb   -   Cc       Ca   +   Cb   +   Cc         ,           (   9   )             
 
 which is even less than the gain of Equation (8). 
 
         [0025]     For all four of these switch combinations, the AC coupling pole frequency ω APCL  for network  300  is given by Equation (10) as follows:  
               ω   ACPL     =       1     R1   *     (     Ca   +   Cb   +   Cc     )         .             (   10   )             
 
 Thus, the implementation of network  300  shown in  FIG. 4  is capable of providing four different levels of attenuation, all of which have the same AC coupling pole frequency. 
 
         [0026]     For example, if the ratio of Ca:Cb:Cc were 3:2:1, then the gain of the configuration corresponding to Equation (6) would be 1, the gain of the configuration corresponding to Equation (7) would be 2/3 , the gain of the configuration corresponding to Equation (8) would be 1/3, and the gain of the configuration corresponding to Equation (9) would be 0, where all four configurations would have the same AC coupling pole frequency.  
         [0027]     The pole frequency for network  300  can be determined by applying a differential pulse between Vcm 1  and Vcm 2  and measuring the time constant at Vout. If the capacitors are selected such that Ca=Cb+Cc, then closing switches S 2  and S 4  (while keeping switches S 1  and S 3  open) will set C 1 =C 2 , which, according to Equation (4), corresponds to a very large attenuation of signal applied at Vin (limited only by capacitor matching), while maintaining the AC coupling pole frequency of Equation (10). This would allow the pole frequency to be determined even while a large signal is present at Vin.  
         [0028]     Although the present invention has been described in the context of the implementation shown in  FIG. 4  having particular sets of switched capacitors that provide four different attenuation levels, those skilled in the art will understand that the present invention can be implemented using sets of switched capacitors that can, in theory, provide an arbitrary number of different attenuation levels, by designing networks having a different number of switched-capacitor paths with capacitors having different relative capacitances.  
         [0029]     The present invention may be implemented as circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack.  
         [0030]     The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.  
         [0031]     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.