Abstract:
A system and method are provided for isolating an input without adding significant distortion and without significantly adversely affecting the bandwidth of input circuits. In one embodiment, a single ended signal is substantially cancelled by an arrangement including an input resistance path in parallel with a negative resistance path wherein both paths substantially match in resistance. In another embodiment, a differential signal is substantially cancelled by a pseudo differential arrangement including two independent input resistance paths each in parallel with a corresponding negative resistance path, wherein the resistance paths substantially match the input resistance paths. In yet another embodiment, a differential signal is substantially cancelled by a differential arrangement including two resistance paths wherein a first negative resistance path is coupled between the first differential input and the second differential output and the second negative resistance path is coupled between the second input and the first output. In yet another embodiment, a current controlled current source may provide the negative amplification for the negative resistance path.

Description:
COPYRIGHT AND LEGAL NOTICES 
       [0001]    A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyrights whatsoever. 
       BACKGROUND INFORMATION 
       [0002]    The present invention relates in general to input cancellation circuits, and more particularly to input cancellation in resistively coupled circuits. 
         [0003]    Low noise resistively coupled circuits may need a small input resistor to ensure that the noise contribution of the input resistor does not dominate the overall noise of the system. The system may need to be isolated, for example, for calibration purposes. To isolate the input, a series switch may be used. However, if the series switch resistance is allowed to dominate over the input resistor, it may result in significant distortion during normal operation due to non-linear junction capacitances. If the series switch resistance is made small with respect to the low input resistance, the large physical size of the switch can introduce significant parasitic capacitance and reduce the bandwidth of the input circuits. Furthermore, if the input is allowed to exceed the supply voltage by more than the gate oxide breakdown of the series switch, then when the series switch is isolated, the oxide of the switch may be exposed to damage. 
         [0004]    For example,  FIG. 1  shows an input cancellation circuit  100  in accordance with prior art. It comprises an input  110  for receiving an input signal from an external source, a series resistance  130  coupled to the input  110 , a switch  140  which may be an NFET transistor, coupled to the series resistance  130 , and an output  120  coupled to the output of the switch  140 . The NFET used as a switch  140  can add substantial distortion during normal operation due to its non-linear junction capacitance. For a given channel length for the NFET  140 , the “ON” resistance is a function of the width or size of the NFET  140 . Thus, to reduce the “ON” resistance, the width of the NFET  140  needs be increased. However, as the size of the NFET  140  increases, the parasitic capacitance of the device increases with it, thereby reducing the bandwidth of the input circuits. Furthermore, if the input  110  is allowed to exceed the supply voltage by more than the gate oxide breakdown of the NFET  140 , then when the NFET  140  is isolated, the oxide of the NFET may be exposed to damage. 
         [0005]    Thus, there is a need for a system and method for isolating an input without adding significant distortion and without significantly adversely affecting the bandwidth of input circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is illustrated in the figures of the accompanying drawings, which are meant to be exemplary and not limiting, and in which like references are intended to refer to like or corresponding parts. 
           [0007]      FIG. 1  shows an input isolation circuit as may be used in the prior art. 
           [0008]      FIG. 2  shows a block diagram of a single ended isolation method in accordance with an embodiment of the invention. 
           [0009]      FIG. 3  shows a single ended input isolation circuit with a voltage controlled current source in accordance with an embodiment of the invention. 
           [0010]      FIG. 4  shows a diagram of a single ended isolation method in accordance with an embodiment of the invention wherein the negative resistance path includes a resistance and a current controlled current source. 
           [0011]      FIG. 5  shows a pseudo-differential input isolation circuit in accordance with an embodiment of the invention. 
           [0012]      FIG. 6  shows a differential input isolation circuit in accordance with an embodiment of the invention. 
           [0013]      FIG. 7  shows a differential input isolation circuit with a current controlled current source in accordance with an embodiment of the invention. 
           [0014]      FIG. 8  shows a switch biasing means in accordance with an embodiment of the invention. 
           [0015]      FIG. 9  shows an embodiment of a current controlled current source. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A system and method is provided for effectively isolating an input from a circuit.  FIG. 2  shows a block diagram of a single ended isolation method in accordance with an embodiment of the invention. An input  210  is coupled to an input resistance  230  and a negative resistance  240 . The outputs of resistance  230  and negative resistance  240  are coupled to output  220 . By making the negative resistance path  240  substantially equal in magnitude to the resistance  230 , the signal at input  210  is effectively cancelled. For example, the current through resistance  230  Isig/2 and the current through the negative resistance  240  Isig/2 is equal but opposite in direction. Therefore, the current at the output  220  is 0, thereby effectively canceling the signal at input  210 . These relationships are summarized by the following equations: 
         [0000]    
       
         
           
             Iout 
             = 
             
               
                 
                   V 
                   Input 
                 
                 - 
                 
                   V 
                   Output 
                 
               
               
                 
                   R 
                    
                   
                      
                      
                   
                 
                 - 
                 R 
               
             
           
         
       
       
         
           
             
               
                 R 
                  
                 
                    
                    
                 
               
               - 
               R 
             
             = 
             
               
                 
                   - 
                   R 
                 
                 × 
                 R 
               
               
                 R 
                 + 
                 
                   ( 
                   
                     - 
                     R 
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               If 
                
               
                   
               
                
               
                  
                 R 
                  
               
             
             = 
             
                
               
                 - 
                 R 
               
                
             
           
         
       
       
         
           
             then 
             , 
             
               
                 
                   R 
                    
                   
                      
                      
                   
                 
                 - 
                 R 
               
               = 
               ∞ 
             
             , 
             
               
                 and 
                  
                 
                     
                 
                  
                 Iout 
               
               = 
               0 
             
           
         
       
     
         [0017]    As illustrated in  FIG. 4 , the input resistance  230  may comprise a resistor  320 . The negative resistance path  240  may comprise a current controlled current source (CCCS)  325  with a gain substantially equaling −1 and a resistor  420 . In yet another embodiment, negative resistance path  240  may further comprise a switch  330 . It is understood that the switch  330  may comprise bipolar or FET transistors. For example, in one embodiment the switch may be an NFET or a PFET. The switch  330  may be turned “OFF” during normal operation, thereby allowing signals to travel from the input  210  through the resistance  230  to the output  220 . When the switch  330  is “ON,” the resistance  420  should match resistance  320  and the gain of the CCCS should be substantially equal to −1. The closer the resistance of the resistance  420  matches input resistance  320  and the gain of the CCCS equals −1, the less cancellation current inaccuracies arise. In another embodiment, the gain of the CCCS  325  may be non-unity if the total resistance of the negative resistance path  240  is scaled up or down such that the current through  230  is equal in magnitude and opposite in direction to the current through the output terminal of  240 , leading to output  220 . For example, if the total resistance of resistor  420  is 2 times the resistance of resistor  320 , then K may be scaled to −0.5 to compensate. The CCCS  325  may have a control  332  to isolate or enable the negative resistance path. Other terminals of the CCCS  325  include a first terminal leading to input  210 , a second terminal leading to output  220 , a third terminal connected to ground, and a fourth terminal connected to a control voltage  334  such that when the negative resistance  240  is enabled the voltage on output  220  is substantially equal to said control voltage  334 . Control  332  may be used instead of switch  330  or in addition to switch  330 . 
         [0018]    In an alternative embodiment, a voltage controlled current source (VCCS) may be used instead of a CCCS. For example,  FIG. 3 . shows a single ended input isolation circuit with a VCCS in accordance with an embodiment of the invention. The voltage across the input resistor R  320  is sensed by the VCCS  380 . If Gm of the VCCS is 1/R, then the current at the node  220  will be 0. Driver  370  represents the load of the negative resistance circuit. The following relationships summarize the operation illustrated in  FIG. 3 : 
         [0000]    
       
         
           
             Iout 
             = 
             
               
                 
                   Vin 
                   - 
                   Vout 
                 
                 R 
               
               + 
               
                 Gm 
                  
                 
                   ( 
                   
                     Vout 
                     - 
                     Vin 
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               if 
                
               
                   
               
                
               Gm 
             
             = 
             
               
                 
                   1 
                   / 
                   R 
                 
                  
                 
                     
                 
                  
                 then 
                  
                 
                     
                 
                  
                 Iout 
               
               = 
               
                 0 
                  
                 
                     
                 
                  
                 as 
                  
                 
                     
                 
                  
                 desired 
               
             
           
         
       
     
         [0019]      FIG. 5  shows a pseudo-differential input isolation circuit in accordance with an embodiment of the invention. It may comprise two systems  200 ,  300 , or  400 . For example, in the pseudo differential circuit  500 , the top and bottom paths each comprise system  400 . The positive input  510  is coupled to the input of the top system  400 . Positive output  520  is coupled to the output of the top system  400 . Similarly, negative input  530  is coupled to the input of the bottom system  400  and negative output  540  is coupled to the output of the bottom system  400 . The arrangement in  500  accommodates differential signals wherein each end of the differential signal is cancelled single-endedly. The positive input of the differential signal may be applied to input  510  and the negative input to input  530 . During normal operation, switches  330 , if present, may be turned “OFF,” thereby allowing the differential signal to travel from inputs  510  and  530  to outputs  520  and  540  accordingly. When the switches  330  are turned “ON,” as explained, for example, in the description of system  400  of  FIG. 4 , each end of the differential signal is cancelled, thereby canceling the differential signal. Instead of the switches  330  or in addition to, control signal  332  may isolate or enable the negative resistance paths. 
         [0020]      FIG. 6  shows a differential input isolation circuit in accordance with a preferred embodiment of the present invention. Differential inputs, wherein the positive input is applied to input  610  and the negative input is applied to input  650 , are isolated by effectively canceling the differential input signal. Resistance  630  is coupled between the positive input  610  and the positive output  620 . This may represent the first input resistance path. Resistance  660  is coupled between negative input  650  and negative output  670 . This represents the second input resistance path. The path between the positive input  610  and the negative output  670  represents the first negative resistance path. Similarly, the path between the negative input  650  and the positive output  620  represents the second negative resistance path. The total resistance of the first negative resistance path is configured to substantially match the resistance of the first input resistance path. Similarly, the total resistance of the second negative resistance path is configured to substantially match the resistance of the second input resistance path. The closer the negative resistance path matches the input resistance path, the less cancellation current inaccuracies arise, leaving just common mode currents at the positive output  620  and negative output  670  accordingly. The first negative resistance path may comprise different components. For example, it may comprise a resistor  615 . Further, the first negative resistance path may comprise a switch  665  instead of resistor  615  or in addition to resistance  615 . Regardless of the number or type of components in the first negative resistance path, the magnitude of the resistance of the first negative resistance path should substantially match the first input resistance path. 
         [0021]    The aforementioned description of the relationship between the first negative resistance path and the first input resistance path also applies to the relationship between the second negative resistance path and the second input resistance path. Thus, the second negative resistance path may comprise a resistor  655 . Further, the second negative resistance path may comprise a switch  625  instead of resistor  655  or in addition to resistor  655 . As in the first negative resistance path, the magnitude of the resistance of the second negative resistance path should substantially match the second input resistance path. 
         [0022]    In a configuration where switches are included, as illustrated in the exemplary embodiment of  FIG. 6 , switches  625  and  665  may be turned “OFF” during normal operation, thereby allowing signals to travel from the differential inputs  610  and  650  to the outputs  620  and  670  accordingly. Thus, the first side of the differential signal may travel from the input  610  through input resistance  630  to output  620 . Similarly, the second side of the differential signal may travel from the input  650  through input resistance  660  to output  670 . In one embodiment, switches  625  and  665  may be NFETs or PFETs. 
         [0023]    As explained in the discussion above, the closer the negative resistance path matches the input resistance path, the better signal cancellation is achieved. When the negative resistance path includes a switch, for example an NFET or PFET, it must be configured such that the total of the “ON” resistance of the switch in addition to any other elements in the negative resistance path substantially match that of the input resistance path. 
         [0024]      FIG. 8  illustrates an exemplary circuit which allows the resistance of the negative resistance path to substantially match that of the input resistance path. Input  860  coupled to resistor  870  which is connected in series to PFET  875  coupled to output  880 , represents a negative resistance path. The components within the dotted rectangle  802  are part of a dummy circuit which biases the gate of PFET  875  such that the negative resistance path substantially matches the input resistance path. Amplifier  825  senses the voltage at the output of resistor  820 , which represents an input resistance. Input resistance  820  is biased by current source  840 . The combination of the resistor  830  in series with the PFET  835  represents the negative resistance path. Since the voltage at the two inputs of an amplifier  825  are substantially similar, the output of the PFET  835  is forced to the same voltage as the output of input resistance  820 . Thus, the feedback loop forces the gate voltage of the PFET  835  to a level which effectively forces the source to drain resistance of PFET  835  to be such that the total resistance of resistor  830  and the source to drain resistance of the PFET  835  substantially match the resistance of the input resistor  820 . Now moving outside of the dummy cells of  802 , since the same gate voltage that is applied to PFET  835  is applied to PFET  875  as well, the total resistance of input resistor  870  and the drain to source resistance of PFET  875  will substantially match the input resistance  820 . The drain of PFET  875  and the drain of PFET  835  are substantially at the same voltage. Further, the Vdd  810  is the common mode voltage that is seen at input  860 . This approach may further benefit from a common mode correction circuit which senses the output  880  and forces it to a common mode level by either sourcing or sinking current through the input resistor  870 . 
         [0025]      FIG. 7  shows a differential input isolation circuit in accordance with a preferred embodiment of the invention. The system  700  may comprise elements similar to system  600  of  FIG. 6  except that in system  700  the switches  625  and  665  are removed and system  700  further comprises a CCCS  750  and CCCS  751 . One of the benefits of using CCCS  750  and CCCS  751  is that it may prevent potential damage to switches  625  and  665  as compared to the embodiment illustrated in  FIG. 6 . In one embodiment, switches  625  and  665  may be NFETs or, alternatively PFETs. For example, referring to  FIG. 8  to illustrate, when PFET  875  is turned “OFF,” the source of the PFET  875  tracks the input  860  which may go above the supply, thereby turning on the inherent parasitic source to bulk diode of the PFET  875 , which is undesirable. If PFET  875  is replaced with an NFET and the NFET is turned “OFF,” the drain to gate junction of the NFET may be exposed to the signal from input  860  which, depending on the sensitivity of the NFET and the magnitude of the input signal, may damage the junction. The use of CCCS  750  and  751 , as illustrated in  FIG. 7 , does not need to rely on, for example, NFETs or PFETs to be used as switches, thereby further improving the reliability of the input cancellation circuit. There may be a control  780  to isolate or enable the cancellation of the signals at input  610  and input  650 . 
         [0026]    There are different ways that one skilled in the art may implement a CCCS.  FIG. 9  shows an example of a CCCS as may be used in an embodiment of the input cancellation circuit. The CCCS circuit may comprise PFETs  910 ,  915 ,  920 ,  925 ,  930 ,  940   945 , and  935 , NFETs  950 ,  955 ,  960 , and  965 , and Op-Amp  970 . The system  900  conveys substantially the same current to output  990  that it senses at input  980  when it is configured with unity gain. Similarly, the same current at output  995  is supplied that is sensed at input  985 . The output currents at  990  and  995  are cross coupled to provide a differential sign inversion and hence a CCCS gain substantially equal to −1 and may also provide common mode correction. The CCCS  900  may, for example, replace the CCCS  750  and  751  of  FIG. 7 . 
         [0027]    Although the present invention has been described with reference to particular examples and embodiments, it is understood that the present invention is not limited to those examples and embodiments. For example, ones skilled in the art may use bipolar devices instead of FETs. The present invention as claimed, therefore, includes variations from the specific examples and embodiments described herein, as will be apparent to one of skill in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.