Abstract:
The invention is a method and apparatus for a digital DSL line driver that comprise a multi stage delta-sigma modulator with a high current 1-bit digital-to-analog converter (DAC) output stage. By adding the high current drive capability to the 1-bit DAC in a delta-sigma digital-to-analog converter the traditional DSL line driver used in digital subscriber lines applications is made less complex and it saves on the overall power consumption. The invention uses a transistor switch arrangement to save the overall power by alternately switching “ON” some the transistors while having other transistor turned “OFF”.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Priority is claimed based on U.S. Provisional Application No. 60/345,832 entitled “High-Drive-Current Digital-to-Analog Converter” filed Jan. 04, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a method and apparatus for reducing power and complexity in a digital subscriber line (DSL) driver with a multi stage delta-sigma modulator with a high current  1 -bit digital-to-analog converter (DAC) output stage.  
         BACKGROUND OF THE INVENTION  
         [0003]    With the advent of the internet and other high-bandwidth electronic communication systems and the consumer demand for information, interactive gaming and electronic entertainment such as video on-demand, there has been a substantial need for reliable and affordable high bandwidth mediums for facilitating data transmissions between service providers and their customers. Attempts to ensure that such mediums be affordable to consumers, and structurally attainable in a cost-effective manner for providing service to customers, involve using already existing copper wire telephone systems (plain old telephone system or POTS) infrastructure.  
           [0004]    There are many ways to bring home a high-speed link beyond the 56-kbps limit that POTS-based V.90 performance allows. The methods include integrated-services digital network (ISDN), satellite links, cable modems, hybrid fiber-coax, and wireless local loops. All these methods eventually encounter the virtually universal, metallic local loop that is the “last mile” between the phone company&#39;s central office (CO) and the end user in the public switched-telephone network (PSTN). This loop can be a significant barrier to installing yet another type of path.  
           [0005]    Digital subscriber line (xDSL) holds a lot of promise. It uses advanced signal-processing concepts that force mathematician Claude Shannon&#39;s 1948 theory of channel-information capacity to its ultimate level, pushing data onto the local loop at several-megabit-per-second rates. But making xDSL a reality takes more than sophisticated algorithms running on digital signal processors (DSPs). An analog-front-end (AFE) circuitry that sits between the telephone company loop and the DSP plays a key role in successful and practical implementation of any xDSL variation. This AFE must digitize or re-create analog signals with excellent dynamic-range performance, drive fast-slewing signals into lossy copper lines, and capture the weak incoming analog signals that far stronger outbound signals overshadow—all the while using little power at each end of the link. A number of DSL standards and protocols have been proposed such as VDSL, SHDSL, RADSL and ADSL.  
           [0006]    In existing systems, the CO receives digital information from a data source and sends information to an AFE that converts the data from a digital form into a continuous time analog signal that can be transmitted on the analog lines. The analog signal is delivered, via a DSL line driver, which is separate from the AFE, in accordance with the amount of power required to drive the amplified analog signal through the two-wire pair to the end-user. There is always a need to minimize the power required to transmit and receive data at a higher speed and because the existing DSL line drivers use a high-current, linear Class AB or Class G amplifier arrangements with a continuous time (analog) waveform, arrangements where the DSL line driver input is produced by filtering the input of a digital-to-analog converter. Such linear Class AB line drivers account for 50%-75% of the per-port power consumption and in the Class G line driver scenario, an incremental improvement in the power consumed is achieved at the cost of additional power supply rails. These and other drawbacks exist.  
         SUMMARY OF THE INVENTION  
         [0007]    In light of the foregoing, the invention is a method and apparatus for a high-drive current digital-to-analog converter that accommodates all DSL applications in a power efficient manner, by adding a high-current drive capability to the 1-bit digital-to-analog converter based on a delta-sigma modulator.  
           [0008]    In general, one embodiment of the invention provides a digital sequence x(n), clocked at an input sampling rate, and applied to an interpolator, where it is sampled at a higher rate. The sequence may then be applied into a multi-stage delta-sigma (ΔΣ) modulator where the sequence is once again sampled at a higher sampling rate of the input sampling rate. The multi-stage delta-sigma modulator produces a bi-level output of the sequence which includes output noise that has been spectrally shaped to insure that the noise power is small in the band of frequencies occupied by the desired signal.  
           [0009]    The invention has numerous advantageous features, a few examples of which are delineated hereafter. Embodiments of the invention, which are described herein, may possess one or more, but not necessarily all, of the features set out hereafter. One feature of the invention is that it provides a low cost solution to providing a line driver, which will function on all DSL applications.  
           [0010]    Another feature is that, while it provides a simpler line DSL driver with less components for use in all DSL applications, it reduces the amount of power needed for a conventional DSL line driver.  
           [0011]    Other objects, features, and advantages of the present invention will become apparent to one of ordinary skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional objects, features, and advantages be included herein within the scope of the present invention, as defined by the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention can be understood more completely by reading the following Detailed Description of the Invention, in conjunction with the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a block diagram of a high-current digital-to-analog-converter (DAC) according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is a drive wave-form diagram according to an embodiment of the present invention.  
         [0015]    [0015]FIG. 3 is a block diagram illustrating a DSL driver load according to an embodiment of the present invention.  
         [0016]    [0016]FIG. 4 is a drive wave-form diagram illustrating a return to zero in the digital domain according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 5 is a drive wave-form diagram illustrating delay of a rising edge in the analog domain according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The following description is intended to convey a thorough understanding of the invention by providing a number of specific embodiments and details involving an effective method and apparatus of removing inter-symbol interference from a received signal. It is understood, however, that the invention is not limited to these specific embodiments and details, which are exemplary only. It is further understood that one possessing ordinary skill in the art, in light of known systems and methods, would appreciate the use of the invention for its intended purposes and benefits in any number of alternative embodiments, depending upon specific design and other needs.  
         [0019]    In one embodiment and in reference to FIG. 1, a digital sequence x(n)  110 , clocked at an input sampling rate of FS1  120  is applied to an interpolator  115 , where it is sampled at a rate of FS2  125 , where the ratio of FS2 and FS1 is known as the oversampling rate (OSR)(i.e., FS2=FS1*OSR). The sequence x(n) is then fed into a multi-stage delta-sigma (ΔΣ) modulator  130  where the sequence is once again sampled at the higher sampling rate of FS2 in order to get a bi-level output of the sequence as well as spectrally shaping the noise signal. Such spectral shaping of the noise signal insures that the noise power is small in the signal band. The bi-level output of the multi-stage ΔΣ modulator  130 , which is a logic value, is applied to the a 1-bit digital-to-analog converter  135 , where the sampling is once again at the rate of FS2. The 1-bit digital-to-analog converter  135  converts the bi-level logic value to an analog waveform of one being high and another being low (i.e., a one or a zero). The output y(n)  140  of 1-bit DAC  135  is a two-level or binary sequence at clock rate FS2. Y(n) is applied to the a logic &amp; bias block  145 , clocked at a sampling rate of FS2, and is implemented as a synchronous state machine where the signal is converted into appropriate drive signals for the output transistors labeled  170 ,  175 ,  185  and  190  with input impedances  150 ,  155 ,  160  and  165 .  
         [0020]    In one embodiment of the present invention and in reference to FIG. 2, when the output sequence y(n)  140 , from the 1-bit digital-to-analog converter  135  is a high signal (e.g., a “1”), it drives pulse A in FIG. 2 high and pulse B low, which causes transistors  170  and  190  to be “ON” and transistors  175  and  185  to be “OFF”; similarly, when y(n) is a low signal, it drives pulse B in FIG. 2 high and pulse A low, which causes transistors  175  and  185  to be “ON” and transistors  170  and  190  to be “OFF.” 
         [0021]    In one embodiment of the present invention and in reference to FIG. 1, transistors  185  and  190 , may be eliminated altogether and the Z-load  180 , be driven single ended, so as to reduce circuit complexity by the reduction of the output transistors from four to two. FIG. 3 is a block diagram illustrating a Z-load  180  according to embodiments of the invention. As shown, Z-load  180  may further comprise a low-pass filter  310 , an isolation circuit  320 , and a load  330 . Other configurations may also be used.  
         [0022]    In one other embodiment of the present invention and in reference to FIG. 1, the bi-level sequence y(n)  140 , which can be only a high signal or a low signal becomes the input to the output transistors. Therefore, when y(n) is a high signal, the preceding pulse of the sequence y(n−1) is a low signal in which transistor  185  is turned “OFF” prior to transistor  190  being turned “ON” and transistor  175  is turned “OFF” prior to transistor  170  being turned “ON”, a technique that insures that transistors  185  and  190  on one instance and transistors  175  and  170  are not “ON” simultaneously on another instance, in order to avoid excessive current drain from the power supply.  
         [0023]    In another embodiment of the present invention and in reference to FIG. 1, the bi-level sequence y(n)  140 , which can be only a high signal or a low signal which is the input to the output transistors. Therefore, when y(n) is a low signal, the preceding pulse of the sequence y(n−1) is a high signal, transistor  190  is turned “OFF” prior to transistor  185  being turned “ON” and transistor  170  is turned “OFF” prior to transistor  175  being turned “ON”, a technique that insures that transistors  185  and  190  on one instance and transistors  175  and  170  are not “ON” simultaneously on another instance, in order to avoid excessive current drain from the power supply.  
         [0024]    In another embodiment of the present invention, a method of turning “OFF” transistor  185  prior to turning “ON” transistor  190  and turning “OFF” transistor  175  prior to turning “ON” transistor  170  on occasion and turning “OFF” transistor  190  prior to turning “ON” transistor  185  and turning “OFF” transistor  170  prior to turning “ON” transistor  175  on another occasion can be digitally achieved via the logic &amp; bias block  145 , which may be implemented as a synchronous state machine clocked at a higher rate than FS2  125 , in order to implement a return to zero drive waveform with a duty cycle of (m−1)/m or (m−2)/m, where m is integer value of the clock rate as shown in FIG. 4. The same implementation can be achieved in an analog fashion as shown in FIG. 5, where the rising edges of the waveform can be delayed in relation to the falling edges.  
         [0025]    In another embodiment of the present invention, the transistors  170  and  190  are conducting while transistors  175  and  185  are off, alternately,  175  and  185  conduct while  170  and  190  are off. The high-current digital-to-analog converter can be simplified by eliminating transistors  185  and  190  leaving transistors  170  and  175 , thereby, driving the load single-ended.  
         [0026]    While the invention has been described in conjunction with the preferred embodiments, it should be understood that modifications will become apparent to those of ordinary skill in the art and that such modifications are intended to be included within the scope of the invention and the following claims.