Abstract:
An on-chip test bus circuit for testing a plurality of circuits and an associated method. The test bus circuit consists of a test bus and a plurality of switching circuits which selectably provide electrical connections between the respective circuits and the test bus. The plurality of switching circuits are configured to transfer an electrical charge between a node disposed within each switching circuit not selected to provide an electrical connection and a respective charge source or sink. The charge source or sink may consist of a low-impedance, substantially noise-free DC voltage or signal source. The associated method of the present invention consists of the following steps: (1) providing a test bus; (2) providing a plurality of switching circuits for selectively providing electrical connections between the respective circuits and the test bus; (3) providing one or more charge sources or sinks coupled to the respective switching circuits; (4) setting the respective switching circuit associated with a selected one of the circuits to a conducting state; (5) setting the one or more respective switching circuits associated with the one or more unselected circuits to a non-conducting state; (6) transmitting a test signal between the selected circuit and the test bus via the switching circuit in the conducting state; and (7) transferring an electrical charge between a node disposed within each of the one or more switching circuits in the non-conducting state and the respective charge source or sink. The test bus circuit and associated method are especially suitable for use with high-speed analog or mixed-signal integrated circuits.

Description:
TECHNICAL FIELD 
     This invention relates generally to the testing of analog or mixed-signal integrated circuits and, in particular, to a test bus circuit suitable for testing multiple high-speed analog circuits disposed on a common substrate. 
     BACKGROUND OF THE INVENTION 
     The observation and control of signals in integrated circuits is necessary for the manufacturing testing and diagnosis of the integrated circuits. However, modern digital, analog and mixed-signal integrated circuits, which comprise of hundreds of thousands or even millions of interconnected circuit elements disposed on a common semiconductor substrate, are often difficult to test because the signals inside the integrated circuits are not directly observable and controllable. For digital circuits, many techniques have been developed that address this problem including scan-path design and the IEEE 1149.1 (commonly known as JTAG) standard. These techniques typically employ scan registers or other dedicated logic such that the storage elements in a digital circuit can be used as direct observation and control points, independent of their proximity to the functional interface of the circuit. 
     Digital circuits benefit from the fact that digital signals can be observed, controlled and brought in proximity to each other through multiplexing or other combinatorial operations with little or no effect on their normal intended function. This is because coupling mechanisms (principally capacitive and conductive) that are common to all proximate signals on a chip are of such a magnitude that they have little deleterious effect on digital signals. As a result, digital circuits can be accurately tested using these techniques without affecting the normal operation of the circuit in any appreciable way. 
     For analog or mixed-signal integrated circuits, there is also a strong need to observe and control signals for testing and diagnosis. Since the analog signals generated by these integrated circuits include both voltage and frequency (or time-dependent) information, however, it is necessary to stimulate or analyze the complete full-bandwidth properties of the signals. A parametric analysis of the properties and integrity of the analog signals is often critical to understanding the functioning of the analog or mixed-signal integrated circuit. This is very different from digital signal analysis, which usually consists of a simple static analysis of logical signal values. 
     Analog signals, especially in communications and other high-speed applications, do not have the same immunity from degradation as digital signals because they are typically subject to very stringent signal integrity requirements. Consequently, analog signals are often specially routed and shielded in circuit design and construction to avoid coupling mechanisms to other nearby analog or digital circuits or signals, or to signals of such a nature as to cause destructive interference regardless of their proximity. 
     For the above-mentioned reasons, conventional scan techniques that are used to test digital circuits cannot be used to test analog or mixed-signal integrated circuits. For example, if simple multiplexing transistors and scan circuits are used to transmit multiple analog signals between external observation or control points and analog circuits under test, an unacceptable level of degradation in one or more critical signal properties would typically occur. The signal degradation is primarily caused by the capacitive coupling between the multiplexing transistors and between the analog signals that are brought to the multiplexing point. In earlier integrated circuit designs, such degradation was often acceptable. However, with the recent advent of integrated circuits capable of very-high rate analog signal processing, made possible by the development of deep sub-micron circuit technologies, this level of degradation cannot be tolerated. 
     Conventional scan techniques are unsuitable for testing analog or mixed-signal integrated circuits also because they require signals to be stored for observation or read-out at a later time. However, analog signals are usually not static, so they cannot be readily or inexpensively stored. Furthermore, the storage of a signal implies that a non-real-time analysis of the properties or logical functioning of the signal is useful. Since most important properties of analog signals in modem analog or mixed-signal integrated circuits must be observed in real-time, however, conventional scan techniques cannot be used. 
     To preserve their integrity, analog signals may be observed or controlled directly through the pins of the integrated circuit package. To implement directly observable and controllable points in an integrated circuit, it is generally required that multiple test pins be added to the package of the integrated circuit. However, the additional pins significantly increase the cost of the integrated circuit because the cost of the integrated circuit package, which is typically greater than the cost of the silicon chip itself, is principally determined by the pin count. 
     The integrity of analog signals can also be preserved by placing a shield around the conductors on the integrated circuit. A shield electrically isolates a signal transmitted on a conductor from interfering electric fields. Shields are commonly used in the manufacture of non-integrated electronics and coaxial cables. Shielding signals on modern integrated circuits, however, is problematic because the shield must be placed very close to the conductor being shielded and thus often presents an unacceptably large capacitive load on the conductor. 
     The useful observation and control of analog signals for the purpose of manufacturing testing and diagnosis sometimes requires that other analog circuits on the integrated circuit function during testing. As mentioned earlier, however, conventional scan techniques typically suffer from crosstalk such that the testing of an analog circuit may interfere with the normal operation of nearby or related circuits. 
     The above-described limitations on the observation and control of analog signals exist in stark contrast to the scan methodology used in the digital world, where a circuit often can be transformed from a functional mode to a test mode at will to facilitate the use of the scan path registers. 
     In view of the shortcomings of the earlier approaches to testing analog or mixed-signal integrated circuits, it is an object of the present invention to maintain a high level of signal integrity and observation or stimulation bandwidth when observing or controlling signals in analog or mixed-signal integrated circuits. 
     Another object of the invention is to observe or control signals in analog or mixed-signal integrated circuits without interfering with the normal operation of nearby or related circuits. 
     A further object of the invention is to minimize the number of pins necessary to observe or control signals in analog or mixed-signal integrated circuits. 
     SUMMARY OF THE INVENTION 
     The present invention consists of an on-chip test bus circuit for testing a plurality of circuits and an associated method. The test bus circuit comprises a test bus and a plurality of switching circuits which selectably provide electrical connections between the respective circuits and the test bus. The plurality of switching circuits are configured to transfer an electrical charge between a node disposed within each switching circuit not selected to provide an electrical connection and a respective charge source or sink. The charge source or sink may consist of a low-impedance DC voltage or signal source. 
     The associated method of the present invention consists of the following steps: (1) providing a test bus; (2) providing a plurality of switching circuits for selectively providing electrical connections between the respective circuits and the test bus; (3) providing one or more charge sources or sinks coupled to the respective switching circuits; (4) setting the respective switching circuit associated with a selected one of the circuits to the conducting state; (5) setting the one or more respective switching circuits associated with the one or more unselected circuits to the non-conducting state; (6) transmitting a test signal between the selected circuit and the test bus via the switching circuit in the conducting state; and (7) transferring an electrical charge between a node disposed within each of the one or more switching circuits in the non-conducting state and the respective charge source or sink. 
     Some advantages of the test bus circuit and the associated method over earlier testing approaches are as follows. First, the test bus circuit provides a high level of signal integrity and bandwidth for signals to be observed or controlled on an integrated circuit due to its relatively simple and short circuit path. Second, the test bus circuit provides a high level of electrical isolation due to its combined use of multiple, serially-coupled selection devices and the charge-transferring circuit. As a result, signals can be observed and controlled without interfering with the normal operation of nearby or related circuits on the integrated circuit. Third, the test bus circuit minimizes the number of pins necessary for the integrated circuit package to observe or control the signals. Fourth, the test bus circuit is relatively simple to implement and does not significantly increase the size or complexity of the integrated circuit. Fifth, if the test bus circuit includes an optional driven shield, the test bus circuit provides a reduced level of interference with the operation of the circuitry being tested. For these reasons, the test bus circuit is especially suitable for use with high-speed analog or mixed-signal integrated circuits. 
     These and other features and advantages of the invention will be better appreciated from the following detailed description of the invention together with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a test bus circuit in accordance with a first preferred embodiment of the present invention including a driven shield. 
     FIG. 2 is a circuit diagram of a test bus circuit in accordance with a second preferred embodiment of the present invention having improved electrical isolation. 
     FIG. 3 is a circuit diagram of a test bus circuit in accordance with a third preferred embodiment of the present invention for use with fully differential analog circuits. 
     FIG. 4 is a circuit diagram of a test bus circuit in accordance with a fourth preferred embodiment of the present invention including a driven shield that extends to cover an internal portion of the switching circuits. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention comprises a novel test bus circuit for use in highly integrated electronic systems such as analog or mixed-signal integrated circuits. The test bus circuit is especially suitable for the careful examination or stimulation of internal analog signals that are not otherwise directly observable or controllable from the functional interface of the system. The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, in the following description, numerous details are set forth for purpose of explanation. However, one of ordinary skill in the art would realize that the invention may be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     FIG. 1 is a circuit diagram of a test bus circuit  130  in accordance with a first preferred embodiment of the present invention. The test bus circuit  130  selectively effects electrical connections between multiple analog circuits  100 ,  110  and  120  and a test bus observe/control point  180  so that analog signals in the circuits can be observed or controlled. The test bus observe/control point  180  is used to transmit analog test signals to and from the analog circuits  100 ,  110  and  120 . The analog circuits  100 ,  110  and  120 , the test bus circuit  130  and the test bus observe/control point  180  are all disposed on a common substrate. In a preferred embodiment, the test bus circuit  130  is implemented on an analog or mixed-signal integrated circuit, where the test bus observe/control point  180  is an integrated circuit package pin and the substrate is composed of a semiconducting material such as silicon. 
     The test bus circuit  130  comprises a test bus  170  and one or more switching circuits  140 ,  150  and  160 . The test bus  170  connects the switching circuits  140 ,  150  and  160  to the test bus observe/control point  180 . The test bus  170  comprises a test bus conductor  171  and an optional shield  172 , which will be described in more detail below. The switching circuits  140 ,  150  and  160  selectively connect the analog circuits  100 ,  110  and  120 , respectively, to the test bus  170  so that a signal in one of the analog circuits can be observed or controlled. The switching circuits  140 ,  150  and  160  are operatively connected to the respective analog signal lines  107 ,  117  and  127  in the analog circuits  100 ,  110  and  120 . The analog signal lines  107 ,  117  and  127  may be internal to the analog circuits or alternatively, may connect to another functional block (e.g., block  105  ). In the embodiment of the invention shown, the analog signal lines  107 ,  117  and  127  transmit a single, digital (i.e., non-differential) signal. In other embodiments described later, the analog signal lines  107 ,  117  and  127  transmit fully differential signals. 
     The switching circuits  140 ,  150  and  160  are identical in structure and operation and thus will be described using the switching circuit  140  as an example. The switching circuit  140  comprises two or more serially-coupled selection devices  142  and  144  and a charge-transferring circuit  147 . The selection devices  142  and  144  are serially coupled in that their drain or source terminals are operatively connected at a node  143 . A first portion or end of the serially-coupled selection devices  142  and  144  is operatively connected to the test bus  170  while a second portion or end is operatively connected to the signal  107  of the analog circuit  100 . In a preferred embodiment, the selection devices  142  and  144  are each composed of a p-type and an n-type transistor operatively connected in parallel (i.e., a CMOS pass device). The selection devices  142  and  144  are sized to the minimum size consistent with the impedance of the signal  107  to be observed or controlled in the analog circuit  100 . For the observation function, it is usually desirable for the test bus  170  to have a high impedance and thus the selection devices  142  and  144  should be small in size. For the control function, however, it is typically preferred that the test bus  170  have a low impedance and the selection devices be larger in size. Techniques for determining the optimum size for the selection devices  142  and  144  are well-known in the art and will not be discussed further here. 
     The charge-transferring circuit  147  is operatively connected at one end (e.g., the drain terminal) to the node  143  located between the serially-coupled selection devices  142  and  144 . The other end (e.g., the source terminal) of the charge-transferring circuit  147  is operatively connected to a low-impedance signal line  109  originating from the analog circuit  100  or an external circuit (not shown). The signal transmitted by the signal line  109  is substantially noise-free and is generated by a charge source or sink (not shown). The charge source or sink may be a low-impedance DC voltage source (e.g., ground) or reference signal source disposed in the analog circuit  100  or the external circuit. The impedance of the signal line  109  should be relatively low as compared to the impedance of the analog signal line  107 . The charge-transferring circuit  147  transfers electrical charge between the node  143  and the DC voltage or reference signal source, thereby discharging any electrical charge that may accumulate at the node  143 . Consequently, the charge-transferring circuit  147  improves the electrical isolation provided by the switching circuit  140  when it is disabled between the analog signal line  107  and the test bus  170 . The charge-transferring circuit  147  may comprise a p- or n-type transistor, a CMOS pass device or a plurality of serially-coupled transistors. The charge-transferring circuit  147  is sized to the minimum size consistent with a low-impedance connection to the node  143 . 
     The gate terminals of the transistors in the selection devices  142  and  144  and the charge-transferring circuit  147  receive the control signals EN 1 /{overscore (EN 1 )} as shown in FIG.  1 . Similarly, the gate terminals of the transistors in the selection devices and charge-transferring circuits of the switching circuits  150  and  160  receive the control signals EN 2 /{overscore (EN 2 )} and EN 3 /{overscore (EN 3 )}, respectively. The control signals EN 1 /{overscore (EN 1 )}, EN 2 /{overscore (EN 2 )} and EN 3 /{overscore (EN 3 )} are generated by a control circuit (not shown) in the test bus circuit  130 . The control signals are used to set the corresponding selection devices and the charge-transferring circuits to a conducting or non-conducting state so as to selectively effect an electrical connection between the analog circuits  100 ,  110  and  120  and the test bus  170 . 
     The test bus circuit  130  operates as follows. When the observation or control of the signal line  107  in the analog circuit  100  is desired, the selection devices  142  and  144  are set to an on or conducting state and the charge-transferring circuit  147  is set to an off or non-conducting state to create an electrical connection between the analog circuit  100  and the test bus  170 . At about the same time, the selection devices  152 ,  154 ,  162  and  164  are set to the non-conducting state and the charge-transferring circuits  157  and  167  are set to the conducting state to electrically isolate the analog circuits  110  and  120  from the test bus  170 . The selection devices and charge-transferring circuits in the test bus circuit  130  are set to the conducting or non-conducting state using the control signals EN 1 /{overscore (EN 1 )}, EN 2 /{overscore (EN 2 )} and EN 3 /{overscore (EN 3 )}. Similarly, when the observation or control of the signal line  117  in the analog circuit  110  is desired, the selection devices  152  and  154  are set to the conducting state, the charge-transferring circuit  157  is set to the non-conducting state, the selection devices  142 ,  144 ,  162  and  164  are set to the non-conducting state and the charge-transferring circuits  147  and  167  are set to the conducting state. Likewise, when the observation or control of the signal line  127  in the analog circuit  120  is desired, the selection devices  162  and  164  are set to the conducting state, the charge-transferring circuit  167  is set to the non-conducting state, the selection devices  142 ,  144 ,  152  and  154  are set to the non-conducting state and the charge-transferring circuits  147  and  157  are set to the conducting state. 
     Table 1 shows the logic values required for the control signals EN 1 /{overscore (EN 1 )}, EN 2 /{overscore (EN 2 )} and EN 3 /{overscore (EN 3 )} to observe or control the signal lines  107 ,  117  and  127 . 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Function of test 
                 Control Signal Logic Values 
               
             
          
           
               
                   
                 bus circuit 130 
                 EN1/{overscore (EN1)} 
                 EN2/{overscore (EN2)} 
                 EN3/{overscore (EN3)} 
               
               
                   
                   
               
               
                   
                 Observe/control 
                 1/0 
                 0/1 
                 0/1 
               
               
                   
                 signal line 107 
               
               
                   
                 Observe/control 
                 0/1 
                 1/0 
                 0/1 
               
               
                   
                 signal line 117 
               
               
                   
                 Observe/control 
                 0/1 
                 0/1 
                 1/0 
               
               
                   
                 signal line 127 
               
               
                   
                   
               
             
          
         
       
     
     When disabled, the switching circuits  140 ,  150  and  160  provide a high level of electrical isolation between the analog signal lines  107 ,  117  and  127  and the test bus  170 . This level of isolation is achieved, using the switching circuit  140  as an example, with the two selection devices  142  and  144  operatively connected in series in combination with the charge-transferring circuit  147 . When in the non-conducting state, the selection devices  142  and  144  create a very high DC and low-frequency AC isolation between the analog signal line  107  and the test bus  170 . However, the selection devices  142  and  144  are insufficient by themselves to provide good isolation from high-frequency AC signals and noise due to the substantial coupling effects caused by the gate-to-source and gate-to-drain overlap capacitances of the selection devices in the non-conducting state. To reduce these coupling effects, the charge-transferring circuit  147  is set to the conducting state, thereby shunting the high-impedance coupling created between the selection devices  142  and  144 . In other words, the charge-transferring circuit  147  discharges the charge accumulated in the gate-to-source and gate-to-drain overlap capacitances of the selection devices  142  and  144 . Because it is only required to shunt overlap capacitances, the charge-transferring circuit  147  can be made very small. As a result, the amount of additional loading on the signal path between the analog signal line  107  and the test bus  170  produced by the charge-transferring circuit  147  when the switching circuit  140  is enabled is negligible. 
     The design of the test bus circuit  130  permits the circuit to be physically laid out on an integrated circuit in a variety of configurations. For example, in the switching circuit  140 , the selection device  142  and the charge-transferring circuit  147  can be located near the analog circuit  100  while the selection device  144  can be placed near the test bus  170 . As a result, the test bus circuit  130  can be used even with designs where the analog circuit  100  is located a significant distance from the test bus  170 . The test bus circuit  130  thus provides significant flexibility in the layout and routing of signals, as is often required in integrated circuit designs. 
     FIG. 2 shows a test bus circuit  130 ′ in accordance with a second preferred embodiment of the present invention. The test bus circuit  130 ′ provides an even greater level of electrical isolation between the analog signal line  107  and the test bus  170  than the test bus circuit  130 . In this embodiment, one end of the charge-transferring circuit  147  is operatively connected to the output of an inverting buffer  148  rather than to the low-impedance signal line  109 . A negative input (−) of the inverting buffer  148  is operatively connected to the test bus conductor  171  of the test bus  170  and a positive input (+) is operatively connected to the low-impedance signal line  109 . As mentioned earlier, the signal transmitted by the signal line  109  is generated by a charge source or sink (not shown), such as a low-impedance DC voltage source (e.g., ground) or reference signal source. Thus connected, the buffer  148  inverts the signal on the test bus  170 . 
     The charge-transferring circuit  147  in this embodiment is configured to be always in the off or non-conducting state. In a preferred embodiment, the charge-transferring circuit  147  is a CMOS pass device (as shown in the figure) with the gate terminals of the p- and n-type transistors operatively connected to V DD  and GND, respectively. The charge-transferring circuit  147  attenuates the output of the inverting buffer  148  by an amount about equal to that of the selection device  144  when it is off (in fact, the charge-transferring circuit  147  can be a copy of the selection device  144  ). The switching circuit  140 ′ in effect combines a signal with its complement at the node  143  to cancel any undesired signal transmitted from the test bus  170  through the selection device  144 . As a result, the switching circuit  140 ′, when disabled, provides nearly complete electrical isolation of the analog signal line  107  from the test bus  170 . 
     FIG. 3 shows a test bus circuit  130 ″ in accordance with a third preferred embodiment of the present invention (showing two switching circuits  140 ″ and  150 ″). The test bus circuit  130 ″ is intended for use with analog circuits having fully differential signals, which are commonly found in modern analog or mixed-signal integrated circuits. The analog circuits  100 ″ and  110 ″ have two analog signal lines each,  107 ( a )/ 107 ( b ) and  117 ( a )/ 117 ( b ), respectively, for transmitting the two complementary components of the fully differential signals. 
     The test bus circuit  130 ″ is similar in most respects to the test bus circuits  130  and/or  130 ′ described earlier except for modifications to accommodate the fully differential signals. The test bus circuit  130 ″ comprises a test bus  170 ′ and one or more switching circuits  140 ″ and  150 ″. The test bus  170 ′ includes a pair of test bus conductors  171 ( a ) and  171 ( b ) that are used to transmit the complementary differential signals TEST+ and TEST−, respectively. The test bus conductors  171 ( a ) and  171 ( b ) are operatively connected to two test bus observe/control points  180 ( a ) and  180 ( b ), which may comprise a pair of integrated circuit package pins. 
     The switching circuits  140 ″ and  150 ″ are identical in structure and operation and thus will be described using the switching circuit  140 ″ as an example. The switching circuit  140 ″ includes two sets of two or more serially-coupled selection devices  142 ( a )/ 144 ( a ) and  142 ( b )/ 144 ( b ). The two sets of serially-coupled selection devices  142 ( a )/ 144 ( a ) and  142 ( b )/ 144 ( b ) selectively connect the respective analog signal lines  107 ( a ) and  107 ( b ) of the analog circuit  100 ′ to the test bus  170 ′. In a preferred embodiment, the selection devices  142 ( a ),  144 ( a ),  142 ( b ) and  144 ( b ) each comprise a CMOS pass device. 
     The switching circuit  140 ″ also includes charge-transferring circuits  147 ( a ) and  147 ( b ). Like the charge-transferring circuit  147  of the switching circuit  140 ′ (see FIG.  2 ), the charge-transferring circuits  147 ( a ) and  147 ( b ) in effect combine a signal with its complement to cancel any noise transmitted from the test bus  170 ′ through the selection devices  144 ( a ) and  144 ( b ). The switching circuit  140 ″, when disabled, thus provides nearly complete electrical isolation of the analog signal lines  107 ( a ) and  107 ( b ) from the test bus  170 ′. Unlike the switching circuit  140 ′, however, the switching circuit  140 ″ takes advantage of the fact that both the signal TEST+ and its differential complement TEST− are available on the test bus  170 ′ and thus does not need the inverting buffer  148 . The charge-transferring circuits  147 ( a ) and  147 ( b ) are operatively connected at one end to the respective nodes  143 ( a ) and  143 ( b ) located between the respective serially-coupled selection devices  142 ( a )/ 144 ( a ) and  142 ( b )/ 144 ( b ). The other end of the charge-transferring circuits  147 ( a ) and  147 ( b ) is operatively connected to the respective test bus conductors  171 ( b ) and  171 ( a ). 
     In a preferred embodiment, the charge-transferring circuits  147 ( a ) and  147 ( b ) each comprise two serially-coupled CMOS pass devices  147 ( a )( 1 )/ 147 ( a )( 2 ) and  147 ( b )( 1 )/ 147 ( b )( 2 ), respectively, as shown in FIG.  3 . Two pass devices rather than one are used to transfer charge to/from the nodes  143 ( a ) and  143 ( b ) to provide increased electrical isolation of the nodes  143 ( a ) and  143 ( b ) from the test bus conductors  171 ( b ) and  171 ( a ), respectively, when the switching circuit  140 ″ is enabled. The gate terminals of the p- and n-type transistors of the CMOS pass devices  147 ( a )( 1 ) and  147 ( b )( 1 ) are operatively connected to V DD  and GND, respectively, so that the pass devices are always in the off or non-conducting state. 
     The gate terminals of the p- and n-type transistors in the CMOS pass devices  147 ( a )( 2 )/ 147 ( b )( 2 ) and  157 ( a )( 2 )/ 157 ( b )( 2 ) are operatively connected to the control signals EN 1 /{overscore (EN 1 )} and EN 2 /{overscore (EN 2 )}, respectively. The control signals EN 1 /{overscore (EN 1 )} and EN 2 /{overscore (EN 2 )} are the same as those used in the test bus circuit  130  of FIG.  1 . When the switching circuit  140 ″ is enabled, the CMOS pass devices  147 ( a )( 2 ) and  147 ( b )( 2 ) are placed in the non-conducting state to provide increased electrical isolation. When the switching circuit  140 ′ is disabled, the CMOS pass devices  147 ( a )( 2 ) and  147 ( b )( 2 ) are placed in the conducting state. In this case, since the charge-transferring circuit  147 ( a ) and the selection device  144 ( a ) both have one CMOS pass device in the non-conducting state, the signals TEST+ and TEST− are attenuated by about the same amount at the node  143 ( a ) so that they cancel each other. Similarly, since the charge-transferring circuit  147 ( b ) and the selection device  144 ( b ) both have one CMOS pass device in the non-conducting state, the signals TEST+ and TEST− are attenuated by about the same amount at the node  143 ( b ) so that they cancel each other. The switching circuit  140 ′ in effect combines a signal with its complement at the nodes  143 ( a ) and  143 ( b ) to cancel any undesired signals transmitted from the test bus  170  through the selection devices  144 ( a ) and  144 ( b ). As a result, the switching circuit  140 ′, when disabled, provides nearly complete electrical isolation of the analog signal lines  107 ( a ) and  107 ( b ) from the test bus  170 ′. 
     Referring back to FIG. 1, in a preferred embodiment the test bus  170  includes the optional shield  172 . As is known in the art, the shield  172  forms a Faraday barrier around the test bus conductor  171 , thereby electrically isolating the signal transmitted on the test bus conductor  171  from interfering electric fields. The shield  172  at least partially surrounds the test bus conductor  171  between the switching circuits  140 ,  150  and  160  and the test bus observe/control point  180 . The shield  172  permits the test bus  170  to be routed among other potentially interfering signals without degrading the integrity of the signals present on the test bus. A shielded test bus is especially appropriate for integrated circuits that are densely laid out or generate significant electrical noise. 
     In a preferred embodiment, the shield  172  is implemented as a driven shield in which the shield  172  is driven by a shield amplifier  174 . A driven shield  172 , in addition to electrically isolating the test bus  170 , reduces the capacitive load presented by the test bus to the analog circuits  100 ,  110  and  120 . This is especially advantageous in modern integrated circuits, where the shield  172  usually must be placed very close to the test bus conductor  171  to provide adequate shielding and thus increases the test bus capacitance to unacceptably large levels. A large capacitive load on the test bus  170  is undesirable because it increases the size of the drivers in the analog circuits  100 ,  110  and  120  required to drive signals onto the test bus  170 , thereby increasing the power dissipated, noise generated and area occupied by the analog circuits. 
     The shield amplifier  174  maintains the shield  172  at a voltage that follows the instantaneous (or alternatively, time-averaged) voltage of the test bus conductor  171  so that the voltage between the test bus conductor  171  and the shield  172  is at all times approximately zero. Because the charge Q of the capacitance C formed between the test bus conductor  171  and the shield  172  is given by the well-known equation Q=C * V, where the voltage V is about zero, the charge Q and thus the current required to drive the test bus  170  is negligible. As a result, the drivers in the analog circuits  100 ,  110  and  120  do not need to supply any current to drive the test bus  170 . In other words, the test bus  170  with the driven shield  172  presents a negligible capacitive load to the analog circuits  100 ,  110  and  120 . 
     Although the use of a driven shield is not essential to the practice of the invention, there are several advantages to implementing the test bus  170  with the driven shield  172 . First, the design of the analog circuits  100 ,  110  and  120  do not need to be modified for use with the test bus circuit  130  because of the negligible capacitive load presented by the test bus  170 . The analog circuits  100 ,  110  and  120  may use the same low-power circuitry used if the analog circuits were not connected to the test bus circuit. Moreover, the test bus circuit  130  does not increase the power dissipation of the integrated circuit during normal operation because the shield amplifier  174  is turned on only during testing. Second, the propagation of signals through the test bus  170  for observation or control is improved. For the observation function, this is important because in many integrated circuit designs it is either not possible or desirable for the analog circuits to provide a sufficiently low impedance to drive the test bus  170 . For the control function, it is important to effectively propagate signals with the test bus  170  because the signals input to the analog circuits  100 ,  110  or  120  are often of very high-frequency. 
     The shield amplifier  174  is preferably implemented on the same integrated circuit as the rest of the test bus circuit  130 . This arrangement minimizes the number of pins required for the integrated circuit package. Alternatively, the shield amplifier  174  may be placed external to the integrated circuit package with the shield amplifier output signal driven through a shield point  190 , which may be a package pin. This arrangement allows the driven shield  172  to extend all the way from test equipment external to the integrated circuit (not shown), through the pin of the integrated circuit package and to the test bus  170 . A disadvantage of this embodiment is that an additional pin is required for the shield point  190 . 
     FIG. 4 shows a fourth preferred embodiment of the test bus circuit  130 ″′ in which the shield  172 ′ extends beyond the test bus  170 ′ to cover an internal portion of the switching circuit  140 . The shield  172 ′, which in a preferred embodiment is driven (as shown), includes a shield portion  172 ( a )′ that at least partially surrounds the conductor at the node  143  connecting the serially-coupled selection devices  142  and  144 . The shield portion  172 ( a )′ preferably extends from the selection device  142  all the way to the selection device  144 . A shield conductor  173  provides an electrical connection from the main body of the shield  172 ′ to the shield portion  172 ( a )′. The shield  172 ′ provides an improved level of shielding for the test bus circuit  130 ″′ by protecting the conductor at the node  143  from electrical interference. If driven, the shield  172 ′ also reduces the capacitive load presented by the conductor at the node  143  to the analog circuit  100  in the manner described earlier. Consequently, the shield  172 ′ permits the test bus  170 ″ to be placed a significant distance from the analog circuits  100 ,  110  and  120  without appreciably increasing the capacitive load presented to the analog circuits, thereby providing increased flexibility in the layout and routing of an integrated circuit. 
     In summary, the test bus circuit of the present invention provides several advantages. The test bus circuit maintains a high level of signal integrity and bandwidth for signals to be observed or controlled on an analog or mixed-signal integrated circuit. In addition, the test bus circuit minimizes interference with the normal operation of nearby or related circuits on the integrated circuit. As a result, circuits can be individually designed without concern for interference caused by the other circuits or activities occurring on the test bus. The test bus circuit also minimizes the number of pins required, needing only one or two dedicated pins (which may be otherwise unused pins) to directly observe or control signals in multiple circuits. Finally, the test bus circuit is simple and inexpensive to add to a design and does not significantly increase the size or complexity of the circuitry. 
     While specific embodiments of the invention have been described and illustrated, it will be appreciated that modifications can be made to these embodiments without departing from the spirit of the invention. For example, two or more instances of the test bus circuit  130  can be employed where multiple signals are to be simultaneously observed or controlled in an integrated circuit. Therefore, it is intended that the scope of the invention be defined by the following claims and their equivalents.