Abstract:
An input pad for a differential current input to an integrated circuit contains a switchable and tunable input impedance. The input pad also contains ESD protection and common mode rejection while maintaining low capacitance. The common mode rejection comprises two resistors that combine to remove the common mode current from the two input lines, produce a reference current, and drive two output transistors that cancel the common mode current from the outputs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    a. Field of the Invention  
           [0002]    The present invention pertains to integrated circuits and specifically to input pads used on integrated circuit dies.  
           [0003]    b. Description of the Background  
           [0004]    Standardized circuitry blocks are commonly used in today&#39;s very large scale integrated (VLSI) circuits. The circuitry blocks may have a specific functionality and may be combined with other blocks to create specialized integrated circuits for many types of applications. Examples of standardized circuitry includes memory modules, processor modules, and input/output (I/O) pads.  
           [0005]    I/O pad is a term used to define the bonding surface of an integrated circuit die. In many cases, an I/O pad may include some passive or active circuitry for various functions. The standardized I/O pads may include various functions through which an incoming or outgoing electrical signal may be conditioned or changed prior to entering or exiting the integrated circuit die. The I/O circuitry may reside underneath the actual bonding surface on the top layer of the integrated circuit.  
           [0006]    Many different circuits are commonly used in I/O pads. For example, such circuits may provide ESD protection, filter an incoming signal, or provide matching impedance for connecting circuitry. If the functions were not provided in the I/O circuitry, the same functionality would have to be provided somewhere else in the integrated circuit.  
           [0007]    As manufacturing techniques improve for integrated circuits, more circuitry can be placed in a specific area. For example, the amount of circuitry that may be placed beneath the bonding surface of an I/O pad is increasing as manufacturing techniques improve. Further, the need for many functions for signal preparation of incoming signals is likewise increasing.  
           [0008]    It would therefore be advantageous to provide an I/O pad that comprises ESD protection and adjustable impedance. It would be further advantageous if the I/O pad were suited for a differential input, provided a minimum of capacitance, and provided common mode rejection.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention overcomes the disadvantages and limitations of the prior art by providing a system and method for conditioning a differential input signal pair by providing ESD protection, common mode rejection, and switchable input impedance. The ESD protection may be accomplished with several diodes attached to the input lines. The common mode rejection may be accomplished by connecting the input signals through two resistors and driving a reference transistor that in turn drives two output transistors that remove any common mode signal from the outputs. The input impedance may have resistors that are switched in and out to change the input impedance.  
           [0010]    The present invention may therefore comprise an input pad for differential current signals on an integrated circuit comprising: a first input; a second input; an upper reference voltage; a lower reference voltage; a first primary ESD protection circuit connected to said first input; a second primary ESD protection circuit connected to said second input; a first amplifier connected to said first ESD protection circuit and having an output; a second amplifier connected to said second ESD protection circuit and having an output; a common mode rejection circuit connected between said first ESD protection circuit and said first amplifier and said second ESD protection circuit and said second amplifier, said common mode rejection circuit further having a first output and a second output; a first output connected to said first output of said common mode rejection circuit and said output of said first amplifier; and a second output connected to said second output of said common mode rejection circuit and said output of said second transistor.  
           [0011]    The advantages of the present invention are that a standard library input pad may be used for many applications for differential current signals. The input pad may provide ESD protection for an integrated circuit as well as conditioning the signals for further processing. In addition, the pad may be used in applications where different input impedances are required.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    In the drawings,  
         [0013]    [0013]FIG. 1 is an illustration of an embodiment of the present invention of an input pad for an integrated circuit.  
         [0014]    [0014]FIG. 2 is an illustration of an embodiment of the present invention of a common mode rejection circuit.  
         [0015]    [0015]FIG. 3 is an illustration of an embodiment of the present invention of a switchable and tunable input impedance circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 illustrates an embodiment  100  of the present invention of an input pad for an integrated circuit. The embodiment  100  is a differential input that has ESD protection  106 , common mode rejection circuitry  116 , and switchable input impedance  140  and  142 .  
         [0017]    The embodiment  100  may be connected directly to the bonding pad through which signals are transmitted onto an integrated circuit. The embodiment  100  may be a standard circuit that is used for conditioning differential input signals for an integrated circuit. Such circuits may be part of a library of standardized circuits that are used in designing very large scale integrated (VLSI) circuit chips. It is common to create libraries of blocks of circuitry such as input pads that may be used for many different applications.  
         [0018]    The primary ESD protection circuitry  106  comprises diodes  108  and  110  that connect the first input  102  to a high potential and low potential, respectively. Similarly, diodes  114  and  112  connect to a high potential and low potential, respectively. If the input voltage is above the breakdown voltage of the diodes, the current will be shunted to the power supply and the downstream circuitry will not see a high potential. The primary ESD circuitry thereby serves to protect the input pad and downstream circuitry from transient voltage spikes. The common mode rejection circuitry  116  comprises two resistors  118  and  120  that are a multiple of the resistance of resistors  122  and  124 . The resistors  122  and  124  are located in series with the signal path. The resistors  118  and  120  are coupled together and serve to draw off the common mode signal from the input lines  102  and  104 . The resistors  118  and  120  feed the reference transistor  126  that controls the output transistors  128  and  130 . The output transistors adjust the output signals  136  and  138  to compensate for the common mode signal. The ratio of the resistance of resistors  118  and  122  is related to the ratio of amplification of the output transistors  126  and  128  and the gate amplifiers  132  and  134 . The net effect is that the signals that are common to both input lines  102  and  104  are removed from the output signals  136  and  138  by the output transistors  128  and  130 .  
         [0019]    The switchable/tunable input impedance circuits  140  and  142  allow the input impedance to be switched from one nominal value to another. For example, some input signals may require 75 ohms of input impedance. Other signals may require 50 ohms of input impedance. In general, the input impedance requirements of a signal may be constant. However, the switchable input impedance allows the same block of circuitry for the input pad to be used with different input signals. This allows the same library circuit for an input pad to be used repeatedly without having to maintain separate library circuits for different input impedances.  
         [0020]    [0020]FIG. 2 illustrates an embodiment  200  of a common mode rejection circuit. The input lines  202  and  204  are connected to resistors  206  and  208  that are tied together and connected to a dummy amplifier  221 . The input lines  202  and  204  are also connected to resistors  210  and  212  in series with amplifiers  220  and  222 . The reference transistor  214  is connected to the dummy amplifier  221  through the gate and source. The drain of reference transistor  214  is connected to the lower reference voltage VDD. The reference transistor  214  drives output transistors  216  and  218  through the gates. The drains of output transistors  216  and  218  are connected to the lower reference voltage and the sources are connected to the output lines  224  and  226 .  
         [0021]    The reference transistor  214  and output transistors  216  and  218  operate as a current mirror with two outputs. In essence, any signal that is common to both input lines  202  and  204  is amplified through the current mirror and applied to both output lines  224  and  226  simultaneously, thereby canceling the effects of the common mode signal.  
         [0022]    As will be appreciated by those skilled in the arts, the ratio of the resistance between resistor  206  and  210  must be matched to the ratio of amplification of the dummy amplifier  221  and the amplifier  220 . When properly balanced, the input lines  202  and  204  may be amplified a desired amount as output signals  224  and  226  while removing unwanted common mode signals. In some embodiments, the overall amplification of the signals may be unity. In such embodiments, the differential output signals have an improved differential to common mode signal ratio. Further, the various transistors in the input pad circuitry, such as reference transistor  214  and the amplifiers  220  and  222  provide secondary ESD protection.  
         [0023]    [0023]FIG. 3 illustrates an embodiment  300  of a switchable and tunable input impedance circuit. Input lines  302  and  304  can have variable impedance set by a gross switch bit  306  and tuning bits  308  and  310 . VSS  312  is the lower reference voltage.  
         [0024]    Gross switch bit  306  controls transistors  314  and  316  that can switch resistors  318  and  320  between the input lines  302  and  304  and VSS  312 . Gross switch bit  306  also controls transistors  322 ,  324 ,  334 , and  336  that switch resistors  328 ,  332 ,  340 , and  344  into series with tuning resistors  326 ,  330 ,  338 , and  342 , respectively. When the gross switch bit  306  is high, the resistors  318  and  320  add impedance to the input lines  302  and  304 . In this manner, the impedance may be switched from 50 to 75 ohms. Similarly, when gross switch bit  306  is high, the effective resistance of the tuning resistors  326 ,  330 ,  338 , and  342  are switched into a range that may be more desirable for fine tuning within the second range of input impedance.  
         [0025]    The tuning bits  308  and  310  operate in the same manner as gross switch bit  306  to switch in and out various resistors that change the impedance of the input lines  302  and  304 . Tuning bit  308  operates transistors  346  and  348  that switch the resistors  326  and  330  into the circuit, respectively. Similarly, tuning bit  310  operates transistors  350  and  352  that switch resistors  338  and  342  into the circuit, respectively.  
         [0026]    The resistors switched by the gross switch bit  306  may be relatively large resistor values to change the range of impedance from a high range to a low range. The resistors switched by the tuning bits  308  and  310  may be relatively low values and may be used to compensate for slight adjustments of the input impedance to maximize performance. Those skilled in the arts will appreciate that various values of resistance may be used with the present embodiment while keeping within the sprit and intent of the present invention.  
         [0027]    Different numbers of switchable resistors may be used while keeping within the spirit and intent of the present invention. An embodiment may include only one pair of resistors that are switched into the circuit. Another embodiment may include 3 sets of resistors designed to switch between three different input impedances. Further embodiments may include many sets of resistors for fine tuning the input impedances.  
         [0028]    Secondary ESD protection is provided by resistances  318 ,  320 ,  326 ,  330 ,  338 , and  342 . It will be appreciated by those skilled in the art that correct physical and electrical sizing of these resistances will provide additional secondary protection of the down stream circuitry. This is especially true when the downstream connection is to the drain of a MOSFET of proper size when used either as a switch, as shown in FIG. 3 or when providing biasing current to the source of the common gate amplifiers in FIGS. 1 and 2. The biasing devices in these figures are omitted for clarity.  
         [0029]    The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.