Abstract:
A low bandwidth signal path is added to copy internal node DC signal to output node. Therefore, for a DC or low frequency signal, the output signal is controlled by this loop. On the other hand, a high frequency signal is not affected because of the low-bandwidth of added loop. Thus, both DC and AC coupling modes are realized for components such as low-voltage video drivers.

Description:
BACKGROUND 
       [0001]    Video drivers, or video amplifiers, supply or deliver video signals carried over an AC (alternating current) signal. A video driver should be able to deliver the video signal with high linearity to a consuming or receiving component and to match an inherent or characteristic impedance of a connecting cable to the receiving component. The receiving component may be a video receiver or what is commonly termed a TV (i.e., television). Video receivers or TVs may be part of various systems and devices that present or display video. Systems and devices that may have video drivers and video include wireless communication devices such as mobile or cellular telephones. 
         [0002]    In many instances, it may be desirable or necessary for video drivers to consume a very small amount of power, while operating on a limited voltage supply. For example, devices such as those described above, may only provide or use a relatively small or limited power supply. Because of the limited power supply constraints of such devices, video drivers of such devices typically are required to consume a small amount of power. In other words, the video driver may not be able to draw too much power from a limited or small power supply. 
         [0003]    Such low-voltage or low-power video drivers may typically couple a DC (direct current) signal to ground, or alternatively couple an AC signal to ground. In particular, the low-power video driver may either be DC coupled or AC coupled, depending on a cable that connects the video driver to the video receiver. In other words, the connecting cable may be either DC coupled or AC coupled. It may be typical that different applications or markets dictate the use of a DC coupled or AC coupled cable. In situations where the low-power video driver supports a DC coupling application, such a low-power video driver has no control of the DC level if it is driving an AC coupling cable. This results in a video signal that is unacceptably distorted. 
       SUMMARY 
       [0004]    In an embodiment, a component, such as video driver, provides a video signal that includes a DC component and AC component. The video signal is provided to a receiving component, such as a video receiver. For DC coupling mode, the video driver relies on a receiving component to establish the DC component. For AC coupling mode, where a cable that connects component to the receiving component, a capacitor prevents the DC component from traveling to the receiving component. Without correction of the DC component, the AC component in video signal is distorted. An internal signal path in the video driver is provided to establish the DC component at video driver such that AC component of video signal can pass through without distortion. 
         [0005]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE CONTENTS 
         [0006]      FIG. 1  illustrates a high level diagram of an exemplary system that implements AC coupling and DC coupling. 
           [0007]      FIG. 2  illustrates an exemplary circuit diagram that provides AC coupling and DC coupling. 
           [0008]      FIG. 3  is a flow diagram that describes steps in a method that provides AC coupling and DC coupling. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  is an exemplary high level diagram of device or system  100  that supports AC coupling and DC coupling. In this example, the AC coupling and DC coupling support a video driver  102 ; however, it is contemplated that the AC coupling and DC coupling can support other components and applications. The system  100  represents an embodiment of various systems and devices, including but not limited to wireless communication devices such as mobile or cellular telephones; personal digital devices; audio/video entertainment devices; and generally devices that provide a video output displayed to a user. 
         [0010]    The system  100  includes one or more controllers or processor(s)  104 . Furthermore, the system  100  may include various memory components as represented by memory  106 . Memory  106  may store machine or computer readable instructions and is accessed or controlled by processor(s)  104 . Memory  106  may include read only memory (ROM), random access memory (RAM), flash memory, and various media (e.g., compact disk, digital versatile disk, etc.). 
         [0011]    Various analog and/or digital data inputs and outputs are represented by input/output  108  which may be coupled to or controlled by processor(s)  104 . In particular, input/output  108 , includes an output to a video display or screen (not shown) which the video driver  102  supports. 
         [0012]    A digital to analog converter (DAC)  110  may be included in system  100 . The DAC  110  converts a digital signal into an output current signal. The output current (i.e. DAC output) may then be converted into voltage by an I-V (current to voltage) converter (not shown), where the voltage is fed or input into video driver  102 . DAC  110  may include a current steering DAC to support an exemplary input bandwidth of 6 MHz received by system  100 . A current steering DAC provides an advantage of speed and good performance in a relatively small form factor (i.e., less area). 
         [0013]    The video driver  102  is a relatively low-power video driver working in a low voltage supply. An example power supply value of video driver  102  is approximately 1.8 volts. An example of video driver is described in co-pending patent application incorporated by reference herein: U.S. patent application Ser. No. 11/642,133, entitled Integrated Analog Video System, filed on Dec. 20, 2006, and assigned to Texas Instruments Incorporated. The video driver  102  is connected by a connecting cable  112  to a receiving component as represented by video receiver or TV  114  that consumes video data provided in a video signal from the video driver  102 . The video receiver or TV  114  supports one of the outputs of input/output  108  for video. The connecting cable  112  may be either an AC coupled cable or a DC coupled cable. 
         [0014]    A low-pass circuit  116  provides an additional signal path to copy a DC signal to output (i.e., input to video receiver or TV  114 ) so that DC level at the output (i.e., input to video receiver or TV  114 ) is decisively defined. Although the example describes the use of a video driver (i.e., video driver  102 ), other components and applications may make use of the low-pass circuit  116  and AC coupling. Examples of AC coupling include video equipment and devices that AC couple the output of a video signal, since it is a safer way to cross voltage domains. While DC coupling is accepted as being a more power efficient solution, there is a potential risk as two different power sources are not connected through a load. Therefore, examples of AC coupling are also applicable to systems that involve crossing across voltage domains. 
         [0015]      FIG. 2  is an exemplary circuit  200  that provides AC and DC coupling for a component or circuit, such as a low-power video driver (e.g., video driver  102 ) discussed above. The video driver may include an amplifier  202  which receives a voltage input from a voltage source V cm    204 . The digital to analog converter  110  described above, provides a current i in    206 , with a current output value of I O    208  that is received by resistor R f    212  of the video driver. The video driver includes a pair of transistors M 1    214  and M 2    216 . From M 1    214  a summation of current (or current source) I O    208 , I 1    218  and I 2    216  is provided. The video driver further includes a resistor R m    220 . The video driver has an output voltage value of V out    222 . 
         [0016]    The receiving component (i.e., video receiver or TV  114 ) discussed above is represented by a resistor R L    224 . The connecting cable  112  discussed above is represented by cable  226 . In AC coupling mode, the cable  226  has a capacitance represented by C cable    228 , where C cable    228  may be inherent to cable  226 . In DC coupling mode, C cable    228  is not present. 
         [0017]    The low-pass circuit  116  of  FIG. 1  is presented by an amplifier  230 , a transistor M 3    232  and capacitor  234 . The low-pass circuit along with the video driver may be provided as a separate module exclusive of DAC  110 , cable  226  and receiving component or R L    224 . 
         [0018]    In DC coupling mode, the amplifier  202  and transistor M 1    212  realize or provide an I-V (current to voltage) conversion. A video signal which includes DC and AC information is established at the drain of transistor M 1    212 . The transistor M 2    214  copies the current of M 1    212  and sends it to cable  226 . The resistor R m    220  matches the characteristic impedance of the receiver cable  226 . If the impedance is matched, no current flows through R m    220 . 
         [0019]    In AC coupling mode, capacitor C cable    228  is between cable  226  and output voltage V out    222 . Therefore, no path exists for a DC signal establishment at V out . This may lead to a DC level at V out    222  that is not well defined. Furthermore, transistor M 2    214  may not be able to work at normal condition. This leads to a video signal going to cable  226  that may be totally distorted. The additional low-pass circuitry as described above is added to form a DC signal path and establish the DC condition at V out    222 . The DC level at V out    222  is well controlled and the video signal can be transmitted to cable  226  with high integrity. In particular, the amplifier  230  provides a low bandwidth signal path for the DC signal to flow, where the DC signal is represented by I DC    236 . I DC    236  is a controlled amount of DC current, as determined by input values provided by the video driver (i.e., current i in    206 ). Since amplifier  230  uses relatively low bandwidth, any additional power consumption of the DC coupling circuit is relatively negligible. Amplifier  230 , capacitor  234 , and transistor M 3    232  form a closed loop to decisively control DC level at output equal to the DC component in video signal. 
         [0020]      FIG. 3  shows a process  300  that provides for AC coupling and DC coupling for a video driver, other component or circuit, or application. In particular, in AC coupling mode, an internal low-pass path is provided to establish the DC component or DC signal in a video signal. The process  300  is illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware such as described above, software, firmware, or a combination thereof. Although described as a flowchart, it is contemplated that certain blocks may take place concurrently or in a different order. 
         [0021]    At block  302 , a determination is made if a cable connecting the video driver or other component, has a capacitance that necessitates AC coupling mode as describe above. If DC coupling mode is implemented, the video signal received at the cable may be allowed to simply pass through. As discussed above, if the video signal includes a DC component or DC signal that cannot pass through the capacitor of the cable, the DC signal is established at output. 
         [0022]    At block  304 , for AC coupling mode, a determination is made as to a current input value received by the video driver or component. The current input value determines the DC signal value to be passed or coupled in AC coupling mode from the cable. 
         [0023]    At block  306 , a low bandwidth path is provided for the DC signal. This low bandwidth signal path is a relatively low power consuming circuit relative to the video driver or component. An AC component or AC signal that carries actual video information is allowed to pass to the cable and is consumed by a receiving component such as a video receiver or TV. 
         [0024]    At bock  308 , the DC signal is established through the low bandwidth signal path. 
       CONCLUSION 
       [0025]    The above-described systems and methods describe supporting AC coupling mode and DC coupling mode for a video driver or other component that provides a video signal. 
         [0026]    Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claimed invention.