Abstract:
The accuracy of output power in a digital-to-analog converter (DAC) is critical in certain applications. When bi-CMOS technology is used to implement a DAC, a number of factors affect the gain accuracy of the DAC. The present invention provides a circuit architecture to reduce the variation in these factors to ensure the accuracy of the output power of a DAC. The architecture comprises a bandgap portion, replica circuit and a DAC. The bandgap portion of the architecture provides a constant voltage, while the replica circuit provide a correct current to drive the DAC.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of priority to U.S. Provisional Patent Application No. 60/620,971, filed Oct. 22, 2004, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to digital-to-analog converters (DAC), and more particularly to automatic gain control in DACs. 
   2. Background Art 
   A digital-to-analog converter (DAC) is an electronic circuit that receives an n-bit digital word from an interface circuit and generates an analog output that is proportional to the received digital word. Digital codes are typically converted to analog voltages by assigning a voltage weight to each bit in the digital code and then summing the voltage weights of the entire code. 
   DACs can be designed for a wide range of applications, including general data acquisition applications and special applications, such as, but not limited to, video or graphic outputs, high definition video displays, ultra high-speed signal processing, and digital video recording. 
   The major factors that determine the performance quality of a DAC are resolution, speed, and linearity. Resolution refers to the smallest change in the output analog signal that is supported. The resolution determines the total number of digital codes, or quantization levels, that will be recognized by a converter. The speed of a DAC is determined by the time it takes to perform the conversion process. Measurement accuracy is specified by a DAC&#39;s linearity. Integral linearity, which is also referred to as relative accuracy, is a measure of the linearity over the entire conversion range. It is often defined as the deviation from a straight line drawn between the endpoints and through zero (or offset value) of the conversion range. Differential linearity is the linearity between code transition. 
   The accuracy of a DAC is critical in certain applications. When a DAC has a high degree of accuracy, the control range of a subsequent gain control loop and complexity of circuit stages that may follow a DAC can be reduced. As a result, system cost and power requirements can be minimized. 
   When Bi-CMOS technology is used to implement a DAC, at least three factors impact the accuracy of the DAC. First, resistive loads of resistors used in a DAC can vary depending on temperature and fabrication process. Second, the current through differential pairs of transistors within a DAC can vary based on fabrication process, temperature and supply voltage. Third, when bipolar devices are used there is some current flow into the base of the devices due to finite beta (β) values. 
   What are needed are systems and methods to reduce the impact of the above variations to improve DAC accuracy. 
   BRIEF SUMMARY OF THE INVENTION 
   The accuracy of output power in a digital-to-analog converter (DAC) is critical in certain applications. When bi-CMOS technology is used to implement a DAC, a number of factors affect the gain accuracy of the DAC. The present invention provides a digital-to-analog auto gain control system to reduce the variation in these factors to ensure the accuracy of the output power of a DAC. The architecture comprises a bandgap portion, replica circuit and a DAC. The bandgap portion of the architecture provides a constant voltage, while the replica circuit provides an appropriate current to drive the DAC. 
   Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention are described in detail below with reference to accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
       FIG. 1  is a diagram of DAC. 
       FIG. 2  is a diagram illustrating the relationship between base, collector and emitter currents. 
       FIG. 3  is a diagram of a DAC auto gain control system, according to an embodiment of the invention. 
       FIG. 4  is a chart showing DAC simulation results with and without beta current compensation, according to an embodiment of the invention. 
       FIG. 5  provides a method for auto gain control in a digital-to-analog converter, according to an embodiment of the invention. 
   

   The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
   DETAILED DESCRIPTION OF THE INVENTION 
   While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of utility. 
     FIG. 1  provides a diagram of DAC  100 . DAC  100  has resistor loads  102  and  104  sourcing or sinking current out from several differential pairs, as illustrated by differential pairs  110 ,  120 , and  130  with constant current source. Thus, the voltage drop across the resistor load is proportional to the digital codes, and output voltage swing is proportional to the product of the effective output resistance and the biasing current in the differential pair. The design and operation of DACs will be known by individuals skilled in the relevant arts. 
   As discussed above, when Bi-CMOS technology is used to implement a DAC, such as DAC  100 , at least three factors impact the accuracy of the DAC. First, performance of resistors used in a DAC can vary depending on temperature and fabrication process. Second, the current through differential pairs within a DAC can vary based on fabrication process, temperature and supply voltage. Third, when bipolar devices are used to implement the differential pairs there is some current flow into the base due to finite β values. β values refer to the ratio of the collector current versus base current. The sum of a collector current and base current must equal the emitter current, which is the biasing current in a differential pair. 
     FIG. 2  provides a diagram illustrating the relationship between base, collector and emitter currents to demonstrate the impact of beta variations on performance.  FIG. 2  shows differential pair  110 . Differential pair  110  includes transistor  210 , transistor  220 , and current source  260 . Transistor  210  includes collector  212 , base  214 , an emitter  216 . Similarly, transistor  220  includes collector  222 , base  224  and emitter  226 . Emitters  216  and  226  are coupled together and are coupled to current source  260 . Bipolar devices, such as transistors  210  and  220 , will have some current flow into their bases due to finite β values. The β value is the ratio of the collector current versus the base current. Thus, referring to transistor  220 , the beta value is the ratio of the current flowing through collector  222  versus the current flowing through base  224 . Furthermore, as illustrated by  FIG. 2 , the sum of the collector current and the base current must equal the emitter current, which is equal to the biasing current generated by current source  260  in the differential pair of transistors  210  and  220 . The collector current can also be referred to as I out , and the emitter current can be referred to as I cs , which is also the biasing current being generated by current source  260 . The relationship between I out  and I cs  is given as:
   I   out   =I   cs *β/(β+1)  (1) 
   Thus, I out , which is the current flowing through collector  222 , will be smaller when β is smaller. Because the current flowing through collector  222  is the current going to a resistor load, the output power of a DAC, such as DAC  100 , will be less. The operational challenge arises in that β is temperature and process dependent. 
   For example, typically a β value is around 100. But at a high temperature and a worst-case process corner, the β value could drop to 30. If β=30, the output current is about 97% of the current of the current source designed in a differential pair. This means that the output voltage will be 3% lower than what would be expected, and that the output power will by 9% lower that expected. 
     FIG. 3  is a diagram of DAC auto gain control system  300 , according to an embodiment of the invention. DAC auto gain control system  300  includes bandgap portion  310 , replica circuit  320  and DAC  330 . 
   DAC auto gain control system  300  provides superior DAC accuracy performance by compensating for resistor, current source, and bipolar accuracy impacts due to the process, temperature and supply voltage variations discussed above. Bandgap portion  210  provides a constant voltage, which is about 1.25 V when silicon technology is used. The invention is not, however, limited to silicon technology. Furthermore, an external voltage reference can be used to provide a constant voltage. Replica circuit  320  provides a correct current to drive DAC  330 . Replica circuit  320  provides a current such that the voltage drop across resistors  322  and  324  track with the bandgap voltage. This is achieved by using an operational amplifier, such as operational amplifier  340 , to force a current equal to the bandgap voltage divided by resistors, which have the same type of external resistors  334  and  322  and internal resistor  332  and  324  loads of DAC  330 . The same amount of current is mirrored to DAC  330 . Thus, the output voltage swing of DAC  330  tracks with the bandgap voltage generated by bandgap portion  310   
   Furthermore, to compensate for beta current loss in DAC  330 , replica circuit  320  also has a current bleeding path through the same type of bipolar devices as are used in DAC  330 , which is shown as beta current compensation module  350 . The differential pair within DAC  330  may have some cascaded bipolar devices at its output or in a current source. Similar bipolar devices can be inserted in the replica circuit to compensate the corresponding base current. The voltages across the bipolar devices in replica circuit  320  are biased so that they are matched to the ones in DAC  330 . 
     FIG. 4  provides chart  400  showing simulation results with and without beta current compensation. Curve  410  of  FIG. 4  shows that without beta current compensation, DAC  330  output voltage has a variation of 6 mV when temperature changes from 0 to 125° C. When a beta compensation circuit is used, the output voltage only has a 1 mV change when temperature changes from 0 to 125° C. 
     FIG. 5  provides a method  500  for auto gain control in a digital-to-analog converter, according to an embodiment of the invention. In step  510 , a constant current source is generated and provided to a digital-to-analog converter. For example, replica circuit  320  can provide a constant current source to DAC  330 . In step  520 , performance variations within DAC  330  are reduced. 
   These performance variations can be attributable to process variations in the manufacturing of components used within DAC  330 . Additionally, they can be the result of temperature changes, supply voltage changes or other real time variations. In an example, these variations are compensated for by forcing a correct current into DAC  330  from replica circuit  320 . In step  520 , for example, on-chip resistor variation and off-chip resister variation can be compensated for. 
   In step  530 , a digital code word is received. For example, DAC  330  can receive a digital code word. In one embodiment as depicted in  FIG. 5 , step  530  can occur in parallel with step  540 . In other embodiments, step  530  can occur at other times prior to the outputting of an analog signal. 
   In step  540  beta current loss in differential pairs within a digital-to-analog converter are compensated for. For example, beta current compensation module  350  can be used to compensate for beta current loss in differential pairs in DAC  330 . In other embodiments, beta current loss may not be compensated for. 
   In step  550  an analog signal is output from the digital-to-analog converter. The analog signal is based on the received digital code word. For example, DAC  330  can output an analog signal. In step  560 , method  500  ends. 
   CONCLUSION 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.