Abstract:
Methods and apparatus to control current steering digital to analog converters are described herein. In one example, a digital to analog converter includes a first unit cell including a positive output and a negative output, wherein the positive output of the first unit cell and the negative output of the first unit cell comprise substantially equal magnitudes and wherein the positive and negative outputs of the first unit cell are substantially one hundred eighty degrees out of phase; and a second unit cell including a positive output and a negative output, wherein the positive output of the second unit cell is substantially zero when the negative output of the second unit cell is non-zero.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. Ser. No. 11/848,181, filed on Aug. 30, 2007, which claims the benefit of U.S. Provisional Application No. 60/823,977, filed Aug. 30, 2006. The contents of U.S. Ser. Nos. 11/848,181 and 60/823,977 are hereby incorporated by reference in their entirety. 

   TECHNICAL FIELD 
   The present disclosure is generally directed to digital to analog converters (DACs) and, more specifically, to methods and apparatus to control current steering digital to analog converters. 
   BACKGROUND 
   Current steering DACs receive digital input signals and produce positive and negative analog output currents based thereon. The positive and negative output currents are supplied from connections that are typically referred to as IOUT+ and IOUT−, and a differential voltage corresponding to the digital input signal is produced therebetween. 
   To facilitate current generation and steering functionality, current steering DACs include several parallel-connected unit cells, each of which includes a current source (typically including a transistor), cascode transistors, an amplifier, and switches. Each unit cell is controlled by a decoder that receives digital input signals that are to be converted to analog signals and produces drive signals that control the switches to produce the proper analog output current from the unit cell based on the digital input signals. The sum of the currents produced by the unit cells is the resulting overall analog current corresponding to the digital input signal. 
   An example, bias circuit  102  and unit cell  104  are shown in  FIG. 1 . A bias voltage, VBIAS, is provided to a buffer  106  that is coupled to the base of a transistor  108 , which acts as a current source for the unit cell  104 . The transistor  108  is coupled between a ground node  110  and a node  112  to which switches  114  and  116  are coupled. Cascode transistors  118 ,  120  are respectively coupled to the switches  114 ,  116 . Currents are provided from the unit cell  104  by the cascode transistors  118 ,  120  at nodes IOUT+ and IOUT−. As shown in  FIG. 1 , various parasitic capacitances result between the inputs to the switches  114 ,  116  and the node  112 . As a result, digital signals provided to the switch inputs are coupled to the node  112  via the parasitic capacitances. As such, when one or both of the inputs to the switches  114 ,  116  transitions from a logical one to a logical zero, this transient is conveyed to the node  112 , which discharges the node  112  to a zero voltage and couples glitches to the gate of the transistor  108 . 
   The unit cells in a DAC either all operate in a class A mode or all operate in a class B mode, based on the drive signals that control the switches in the unit cells. In the class A mode of operation, the drive signals controlling the switches are always complementary to each other, so that the current from the current source always goes to either IOUT+ or IOUT−. Thus, in the class A mode of operation, a constant current is drawn from the power supply, regardless of whether the output differential voltage is at zero or at its peak. For example, as shown in  FIG. 2 , digital signals provided to the DAC indicate that the DAC is to output a sinusoidal differential current signal  200 , which can easily be converted to a differential voltage using a resistor. To that end, the decoder produces drive signals that are fed to switches in the unit cells. These switches controlled by the drive signals result in the output of complementary sinusoidal current signals on IOUT+  202  and IOUT−  204  that range in amplitude between 0 amperes (A) and, for example, 0.04 A and that intersect at one-half amplitude (i.e., in  FIG. 2  the signals  202  and  204  cross one another at 0.02 A and are 180 degrees out of phase with one another). Thus, the sum of the current signals  202  and  204  is constant at 0.04 A, as shown by the waveform at reference numeral  206 . However, as shown by the current signal at reference numeral  200 , the differential current created by the signals  202  and  204  is sinusoidal. To produce the currents shown in  FIG. 2 , the current sources and the cascode transistors of each unit cell are always ON and operate in correct bias voltage conditions. However, because the class A operating mode always consumes energy, even when a zero current differential is desired, the class A operating mode has the attribute of high current consumption. 
   An illustration of the class B operating mode is shown in  FIG. 3 . In general, the class B operating mode is one in which during the production of a positive output current the switches controlling IOUTP+ are turned ON proportionally to input signal amplitude, while the switches controlling IOUT− are all turned OFF. Conversely, for the production of a negative current, the switches controlling IOUT+ are turned OFF, while the switches controlling IOUT− are turned ON proportionally to the input signal. As shown in  FIG. 3 , a differential output signal  200  is produced by a differential between only positive half-cycle currents IOUT+  302  and IOUT−  304 . As described above, when the current IOUT+ is positive, the current IOUT− is 0; conversely, where the current IOUT− is positive, the current IOUT+ is 0. Thus, in the class B mode of operation, no switches are turned ON during the production of a zero differential output current. Accordingly, the current that is drawn from the current supply is proportional to the amplitude of the input signal, as shown at reference numeral  306 . In the class B operating mode, the DAC has the lowest operating power for a given input signal. 
   While class B mode is efficient, in class B mode, when the current sources and cascodes are turned OFF completely and then turned ON, the transient glitches couple to sensitive bias nodes in the unit cells. As these bias nodes are usually driven by analog circuits having finite output impedance and parasitic or intentional capacitors, the disturbance in the bias voltages are carried forward to clock periods other than the one in which the glitches occurred. This leads to severe distortion in the output of the line driver/DAC. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
       FIG. 1  is a diagram illustrative of an example unit cell. 
       FIG. 2  is a graph showing Class A operation. 
       FIG. 3  is a graph showing Class B operation. 
       FIG. 4  is a diagram illustrative of Class AB Operation (AB Factor=0.5). 
       FIG. 5  is a graph showing AB_Factor vs. No. of Units cells in Class B mode in a 8 bit DAC. 
       FIG. 6  is a diagram illustrating an example low impedance bias scheme with charge injection cancellation. 
       FIG. 7  is a diagram illustrative of a charge injection cancellation scheme, which is repeated for every unit cell in the LD/DAC. 
   

   DETAILED DESCRIPTION 
   As noted above, each of class A and class B modes of unit cell operation has operational issues. A method is described hereinafter to operate a DAC in varying proportions of class A and class B modes simultaneously (i.e., class AB), thereby offering different linearity/power tradeoffs. As further described below, the operation of the class AB system enables the numbers of DAC unit cells that operate in class A and class B modes to be adjusted. 
   Examples illustrating aspects of the invention are described more fully hereinafter with reference to the accompanying drawings. However, it should be noted that such examples should not be construed as limiting and, in fact, aspects of the invention may be implemented in various different forms. Rather, the examples herein are provided so that this description will be thorough and complete so that one having ordinary skill in the art will be able to make and use the various examples described herein without undue experimentation. 
   As described above in conjunction with  FIG. 1 , in class A mode, for a sine wave input, the IOUT+ and IOUT− output currents  102 ,  104  intersect exactly at Ip EAK /2. As additionally described above in conjunction with  FIG. 2 , in class B mode, for a sine wave input, the IOUT+ and IOUT− output currents  202 ,  204  intersect at 0. As described below, in class AB mode, the IOUT+ and IOUT− output currents intersect point at a location between 0 and Ip EAK /2, by selectively choosing the number of unit cells that operate in class A and class B modes. For example, if this intersection point is placed at IpEAK/4, then 50% of the DAC unit cells operate in Class B mode while the balance operated in class A mode. 
   As shown in  FIG. 4 , to produce a differential current signal  400 , the signal IOUT+  402  ranges between 0.01 A and 0.04 A during positive half cycles of the differential signal  404 , and between 0.01 A and 0 A during the negative half cycles of the differential signal  404 . Conversely, the signal IOUT−  406  ranges between 0.01 A and 0.04 A during negative half cycles of the differential signal  404 , and between 0.01 A and 0 A during the positive half cycles of the differential signal  404 . The resulting total current provided by the supply varies as shown at reference numeral  406 . Operating in this manner ensures that the number of current sources in which both IOUT+ and IOUT− switches are turned OFF simultaneously can be reduced by about half. This, in turn, proportionally reduces the amount of transient glitches and corresponding non-linearity associated with class B mode switching. However, as shown at the signal  406 , the power consumption is proportionately increased over that of pure class B operation. 
   The ratio the current intersection point in AB mode to Ip EAK /2, is defined as the AB_Factor. Thus, an AB_Factor of 1 is class A mode operation as shown in  FIG. 1 . Conversely, an AB_Factor of 0 is pure class B mode operation. As the AB_Factor increases from 0 (i.e., from pure class B mode operation toward pure class A mode of operation), the no. of DAC or line driver (LD) unit cells operating in class B mode progressively decreases. For example, considering an 8 bit LD/DAC used in gigabit Ethernet system, having 4 thermometric weighted bits and 4 binary weighted bits, for a sine wave input, as the AB_Factor increases from 0.1 to 0.9, the number of unit cells operating in Class B mode progressively decreases as shown in  FIG. 5 . 
   This class AB implementation of placing an intersection point for IOUT+ and IOUT− between 0 and Ip EAK /2 is achieved by controlling the drive signals coupled to the switches of the unit cells. In other words, the class AB implementation may be carried out in a purely digital fashion in hardware, software, firmware, or any suitable combination of the same. For that matter, any form of logic, such as decoders used to drive the DAC cell switches, may be used to implement the class AB solution described herein. Due to the fact that the class AB solution is digital, the intersection point between the IOUT+ and the IOUT− signals (i.e., the AB_Factor) may be programmable using three or four digital control bits. Advantageously, the class AB solution, which may be implemented in DAC decoders, does not require any additional analog or digital hardware to convert a conventional DAC to a DAC capable of operating in class AB mode. As opposed to using analog methods to reduce the glitches associated with class B mode of operation, the class AB solution reduces overall non-linearity by limiting the number of current sources in the DAC that operate in class B mode, based on the programmable AB_Factor that may be set using digital bits. 
   The foregoing describes a class AB system and solution in which some DAC unit cells operate in class A mode and some DAC unit cells operate in class B mode. Due to the fact that some of the unit cells still operate in class B mode, there may still be glitches that are generated as a result of the class B operating DAC unit cells. The glitches from such unit cells may be coupled to sensitive bias nodes within one or more DAC unit cells. To address the glitches resulting from the class B mode of operation, as well as other operational deficiencies, several approaches may be taken, either separately or in combination. 
   A solution to reduce the impact of glitches includes drastically reducing the output impedance of the bias node so that the bias node can quickly recover after a glitch from a class B operating cell is passed thereto. In one approach, this may be accomplished through the use of a source follower based bias generator. 
   Another solution to address glitches due to the class B operating mode unit cells is to include charge injection cancellation capacitors on the bias nodes. Such capacitors function to decouple or bypass to ground the glitches imparted on the bias node. The number of charge injection capacitors that are present in the circuit at a given time may be changed or switched out based on the number of class B operating unit cells. For example, a low AB_Factor requires means that a significant percentage of the unit cells are operating in class B mode, which means that the number glitches or the magnitude of a glitch may be significant. To address the increased glitching created by the unit cells operating in class B mode, a significant number of charge injection cancellation capacitors may be coupled to the bias node. Conversely, when the AB_Factor is closer to 1, fewer DAC unit cells are operating in class B mode. Accordingly, fewer numbers of charge injection cancellation capacitors are required, thus some charge injection cancellation capacitors may be switched out of the circuit and, thereby, disconnected from the bias node. 
   In some instances, bias generators suffer from a direct current (DC) error that is caused predominantly by process variations. To address this DC error, a tuning circuit may be used to fine-tune the bias voltage to the proper level. 
     FIG. 6  shows an example implementation of a circuit  600  including a bias portion  602  and a unit cell  604 . Although only a single unit cell  604  is shown in  FIG. 6 , those having ordinary skill in the art will readily recognize that more unit cells are provided in DACs. Digital signals to be converted to analog signals are coupled to a decoder  606  that drives the switches  608 ,  610  of the unit cell  604 . To reduce the output impedance of the bias portion  602 , a source follower configuration is used to implement the bias portion. Furthermore, to reduce the affect of glitches on the bias signal, a charge injection cancellation unit  612  is provided, further detail of one example of which is provided in conjunction with  FIG. 7 . 
   As described above, a DAC may operate in a class AB mode of operation in which some of the unit cells (e.g., the unit cell  604 ) operate in the class A mode and some of the unit cells operate in the class B mode. The logic to facilitate this operation, which consists of timing signals provided to the switches of unit cells (e.g., the switches  608 ,  610  of the unit cell  604 ), lies in the decoder  606  that programmed to operate the unit cells in class A mode or class B mode in response to the AB_Factor that is provided to the decoder  606 . 
   In some examples, the AB_Factor is also provided to the charge injection cancellation unit  612  so that the charge injection cancellation unit  612  can adjust the capacitance applied to the bias node. The lower the AB_Factor, the more capacitance is used to protect the bias node. Alternatively, a different technique may be used to switch the capacitance of the charge injection cancellation unit  612 . 
   One example implementation of a charge injection cancellation unit is shown in  FIG. 7  at reference numeral  700 . The example charge injection cancellation unit  700  includes a capacitor  702 , such as a MOS capacitor that may be implemented using a drain to source connected transistor, the gate of which is connected to the bias node (VBIAS). A switch  704  selectively couples the capacitor  702  between a supply voltage (VDD) and a ground voltage (gnd), to change the capacitance of the capacitor  702  as experiences at the bias node. 
   The switch  704  is controlled by logic  706 . In the example logic  706  of  FIG. 7 , includes a NOR gate  710 , the inputs to which are coupled the signals provided to the unit cell switches (e.g., the switches  608 ,  610 ). As will be readily appreciated by those having ordinary skill in the art, when both the inputs to the NOR gate  710  are logical zero, the output of the NOR gate  710  is a logical one, but is a logical zero otherwise. The signal output from the NOR gate  710  controls the switch  704  to selectively couple the capacitor  702  to the positive supply. In particular, the NOR gate  710  output is only a logical one when both positive and negative current switches are off (e.g., during a class B operating mode). 
   The output from the NOR gate  710  is also coupled to an inverter  712 , the output of which controls the switch  704  to selectively couple the capacitor  702  to ground. Because the signal controlling the selective coupling of the capacitor  702  to ground and positive supply are separated by the inverter  712 , the capacitor  702  is not simultaneously coupled to the positive supply and ground. 
   Of course, other modifications may be made to the charge injection cancellation unit  700  of  FIG. 7 . Additionally, such a unit may be implemented in every unit cell in a DAC so that the bias line may be protected during class B operation. 
   Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions, and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Method to trade linearity for power in current steering Digital to Analog converter based line drivers using variable class mode operation.