Abstract:
Systems and methods are disclosed for performing data conversion by matching current sources using a thin oxide device; and minimizing voltage stress on the thin oxide device during operation or power down.

Description:
BACKGROUND 
     Current steering digital-to-analog converter (DAC) is widely used in modern analog and mixed-signal circuit system especially for high sampling rate applications. It consists of an array of current sources, the number of array depends on DAC&#39;s decoding scheme either thermometer, binary or segmented. The DAC linearity is mainly determined by the matching of the current sources. 
     When designing a high speed DAC many tradeoffs have to be taken into account. These include the type of the DAC (current steering or a voltage output DAC), the number of bits and required speed. For high speed DACs usually a current steering converter is selected because of the speed advantages of this converter type with respect to the voltage output DAC. In most current steering DAC designs a combination of binary and thermometer code weighting is used. The thermometer code is used for the Most Significant Bits (MSBs), while binary code for the Least Significant Bits (LSBs) is used. This is called segmentation: 0% segmentation means a fully binary converter and 100% segmentation means a full thermometer code converter. The DAC designer has to choose the optimum amount of segmentation taking into account the physical problems associated with the segmentation. Current steering DACs with a low amount of segmentation have the advantage that they are simple. They only need a few current sources and switches. The disadvantage of low segmentation is the possible larger DNL compared to the converter converters with larger segmentation. Converters with larger segmentation result in a lower DNL, but the problems associated with the output conductance, capacitance and charge feed-through increase and also the area and power that is needed increases. In addition, due to the larger area required and the larger difference in output impedance between the lowest output code and the highest output code, the timing errors become more dominant. 
     The basic principle of current steering DACs is the summation of currents according to the input. In a binary current steering converter, the current sources are connected parallel to each other. These current sources are connected to switches; the switches connect the current source to the output node. The switches are controlled by the input code of the DAC. The output current of the DAC is therefore proportional to the input code word. The output node of the DAC is connected to a resistor. This resistor converts the output current of the DAC into a voltage. 
       FIG. 1  shows a conventional PFET current source and NFET current source. In the PFET current source, a thick oxide transistor or device  10  is connected to the voltage rail. A positive thick oxide device  12  switches the output of thick oxide device  10  (which is connected to VDD supply rail) to generate a positive current output and a negative thick oxide device  14  switches the output of the thick oxide device  10  to generate a negative current output. Similarly, for an NFET current source, a positive thick oxide device  22  switches the output of thick oxide device  20  to generate a positive current output and a negative thick oxide device  14  switches the output of thick oxide  20  (which is connected to ground) to generate a negative current output. 
     The technology process from chip manufacturers usually provides two kinds of transistor devices: thick-oxide and thin-oxide devices. Faster thin-oxide devices sometimes are called core devices and they are used mainly for logic circuitry. Thick-oxide device can operate at higher supply voltage and they are used in IO, interface and analog circuitry. For advanced process such as 65 nm or below, the operation supply voltage for core device is around 1.0 Volt and the supply voltage for thick-oxide device is either 2.5V or 3.3V. Operating thin-oxide devices on higher supply results in voltage stress, device breakdown and causes device reliability issue. The mismatch of devices is mainly due to the threshold voltage of devices. Since the thin-oxide has smaller threshold voltage, its matching characteristic is superior to thick-oxide devices. For IO supply powered DAC such as 3.3V, conventionally thick-oxide devices are required in the current source to prevent the voltage stress on the devices. 
     SUMMARY 
     In one aspect, systems and methods are disclosed for performing data conversion by matching current sources using a thin oxide device; and minimizing voltage stress on the thin oxide device during operation or power down. 
     In another aspect, a current source for a data converter includes a thin oxide device coupled to a current source to receive power; a thick oxide device coupled to the current source thin oxide device, wherein the thick oxide device and the current source thin oxide device form a cascoded device for increased current source output impedance; and one or more thick oxide switches coupled to the thick oxide device. 
     In a third aspect, a data converter includes a plurality of thin oxide devices coupled to a current source to receive power, wherein the thin oxide devices matches current sources; a plurality of thick oxide devices coupled to the thin oxide devices to minimize voltage stress on the thin oxide devices during operation or power down; and a controller coupled to the thick oxide devices to generate a voltage output for the data converter. 
     Implementations of the above aspects can include one or more of the following. The current source thin oxide device matches current sources. The current source thin oxide device is a thin oxide transistor. The thin oxide transistor minimizes mismatch voltage 
               Δ   ⁢           ⁢     V   TH       =       A   vt         W   ×   L               
where Avt parameter is provided by a foundry and Area is a transistor width multiplied by a transistor length. A circuit is provided for turning off the thin oxide device during power down. The circuit also limits a power voltage to below the thin oxide device supply voltage. The circuit can also bias the thin oxide device supply voltage. The biasing circuit includes a second thin oxide transistor in the biasing circuit. The circuit turns off the thin oxide device and the second thin oxide device during power down. The switches can include a first thick oxide switch connected to the thick oxide device to generate a positive current output and a second thick oxide switch connected to the thick oxide device to generate a negative current output.
 
     The preferred embodiments achieves better current source matching than conventional current sources. This is achieved with a very small silicon area without suffering device stress and breakdown. The current source using thin-oxide device architecture achieves better current cell matching which results in better DAC linearity. With proper bias design, the thin-oxide current source DAC achieves a much smaller silicon area without suffering any device voltage stress, device voltage breakdown or other reliability issue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An appreciation of the features and benefits of the present invention and a more complete understanding of its structure may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 
         FIG. 1  shows conventional current sources. 
         FIG. 2  shows an exemplary current source with a thin oxide device. 
         FIG. 3  shows a biasing circuit with the current source of  FIG. 2 . 
         FIG. 4  shows an exemplary data converter using the current sources of  FIG. 2   
     
    
    
     DESCRIPTION 
     The term “NFET” refers to an N-type metal oxide semiconductor field effect transistor. Likewise, when used herein, “PFET” refers to a P-type metal oxide semiconductor field effect transistor. It should be appreciated that whenever the terms, “NFET”, “PFET”, and “transistor,” are used, unless otherwise expressly indicated or dictated by the nature of their use, they are being used in an exemplary manner. They encompass the different varieties of MOS devices including devices that don&#39;t literally use “metal”, i.e., using another suitable material in place of metal such as polysilicon, and devices with different VTs and oxide thicknesses to mention just a few. Moreover, unless specifically referred to as MOS or the like, the term transistor can include other suitable transistor types, e.g., junction-field-effect transistors, bipolar-junction transistors, and various types of multi-gated transistors, known today or not yet developed. 
       FIG. 2  shows an exemplary current source with a thin oxide device. The current source for a data converter includes a thin oxide device M 1   111  coupled to a current source to receive power and controlled by bias voltage BP 1   101 . A thick oxide device M 2   112  is connected to the current source thin oxide device M 1   111  and controlled by bias voltage BP 2   102 . The thick oxide device  112  and the current source thin oxide device  111  form a cascoded device for increased current source output impedance. The system also includes first and second thick oxide switches M 3  and M 4   113 - 114  connected to the thick oxide device to provide positive and negative current outputs, respectively. The matching of current sources is determined by M 1 . M 2  protects M 1  from voltage stress and also acts as a cascoded device to increase the current source output impedance. M 3  and M 4  are switches to determine the current goes to either OUTP or OUTN. Thus, the current sources use a thin oxide transistor as the current source, a thick oxide transistor as the cascoded device and two oxide devices for switches. 
     The current source thin oxide device  111  improves matched current sources as it is a thin oxide transistor. The matching between the current sources is the main factor of DAC&#39;s linearity characteristic. And the linearity characteristic directly affects DAC&#39;s static performance Integral Nonlinearity (INL), Differential Nonlinearity (DNL) and dynamic performance Spurious Free Dynamic Range (SFDR) etc. The main mismatch between current sources is due to threshold voltage (Vt) mismatch. The use of thin oxide transistor minimizes mismatch voltage 
               Δ   ⁢           ⁢     V   TH       =         A   vt         W   ×   L         ⁢           ⁢   or   ⁢           ⁢   alternatively                 Vmismatch   =       A   vt       Area             
where Avt parameter is provided by a foundry and Area is a transistor width W multiplied by a transistor length L.
 
     Due to the fact that thin-oxide core devices have smaller threshold voltage (Vt) and result in better matching than thick-oxide devices, it is advantageous to use thin oxide in the current sources if the voltage stress issue can be prevented. The benefit is a much smaller silicon area for a given matching requirement. 
     Although the system of  FIG. 2  is a PFET thin oxide current source, it is contemplated that thin oxide NFET current sink and associated switches can be used. 
       FIG. 3  shows a biasing circuit with the current source of  FIG. 2 . The circuit allows for turning off the thin oxide device during power down. A circuit is provided for limiting a power voltage to below the thin oxide device supply voltage. 
     Turning now to  FIG. 3 , a circuit is provided for biasing a thin oxide device supply voltage. The biasing circuit includes a second thin oxide transistor in the biasing circuit. The circuits includes thick oxide transistors M 9   210  connected to thick oxide transistor M 8   212  and M 6   216  which generates the IBIA S 1  and S 2  signals, respectively. Transistor M 8   212  in turn generates signal BP 1   101 . The circuit turns off the thin oxide device and the second thin oxide device during power down. The switches can include a first thick oxide switch connected to the thick oxide device to generate a positive current output and a second thick oxide switch connected to the thick oxide device to generate a negative current output. 
     During the normal operation, M 3  and M 4  switches are driven by decoder and one of them is turned on and the other is turned off. Assuming switch M 3  is on while M 4  is off, the drain voltage of M 1  V D  (M 1 ) is set by BP 2  voltage with the following equation while M 2  is in the saturation region: 
     
       
         
           
             
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     It is critical to make sure that voltage of VDDA-V D (M 1 ) does not exceed manufacture&#39;s suggested value, usually around 20% over the core supply. By using thick oxide devices on M 2 , M 3  and M 4  and similarly M 6 , M 7 , thin-oxide devices M 1  and M 5  can be prevented from voltage stress. 
     During the power down, both BP 1  and BP 2  are pulled to VDDA by device M 8  and M 9  so that thin oxide devices M 1  and M 5  are turned off and they are not stressed on voltage neither. 
     With a predetermined bias voltage BP 1  and BP 2 , the current source structure thin oxide device M 1  is not voltage stressed to exceed the foundry suggested voltage either in normal operation or power down mode. 
       FIG. 4  shows an exemplary data converter using the current sources  310  as detailed in  FIG. 2 . The current sources  310  are connected to a current generator  304 . A controller  300  controls the switches to generate the desired output voltage. The data converter is a current steering digital to analog converter (DAC) which consists of an array of current sources shown in  FIG. 4 . The currents are summed at the output and delivered to the output resistor load  320 , which is predetermined for a 6-bit DAC in this example. The number of current sources depends on the architecture of thermometer, binary or segmented decoding schemes. Take an N-bit DAC as an example, thermometer DACs have 2^N equal current amount current sources. In this example, the controller  300  is a binary to thermometer coder for 6-bits of resolution. Other DAC architecture can be used. For example, binary DACs have N current sources with binary weighted current amount and segmented DACs have the number of current source between 2^N to N depending on the ratio of thermometer and binary current sources. For the applications that DAC&#39;s output voltage swing is greater than the core voltage, the supply voltage is higher than the core voltage and the thick oxide devices are used in the current source to prevent device voltage stress or breakdown. 
     It should be noted that the depicted system could be implemented in different forms. That is, it could be implemented in a single chip module, a circuit board, or a chassis having multiple circuit boards. Similarly, it could constitute one or more complete computers or alternatively, it could constitute a component useful within a computing system. 
     The invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. For example, it should be appreciated that the present invention is applicable for use with all types of semiconductor integrated circuit (“IC”) chips. Examples of these IC chips include but are not limited to processors, controllers, chip set components, programmable logic arrays (PLA), memory chips, network chips, and the like. 
     Moreover, it should be appreciated that example sizes/models/values/ranges may have been given, although the present invention is not limited to the same. As manufacturing techniques (e.g., photolithography) mature over time, it is expected that devices of smaller size could be manufactured. In addition, well known power/ground connections to IC chips and other components may or may not be shown within the figures for simplicity of illustration and discussion, and so as not to obscure the invention. Further, arrangements may be shown in block diagram form in order to avoid obscuring the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present invention is to be implemented, i.e., such specifics should be well within purview of one skilled in the art. Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting. 
     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiment is not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are therefore intended to be embraced by the appended claims.