Abstract:
Two cost-effective low-voltage drop reference generation circuits for A/D converter of the present invention generates a plurality of reference voltages characterized by voltage increments between two fixed reference voltages, which are able to be close to positive rail and negative rail (or ground). These low-voltage drop generation circuits not only greatly increases the total reference voltage range (i.e., increases voltage difference between the most positive reference voltage and the most negative reference voltage, increases V REFT −V REFB , maximizes the most positive reference voltage and minimizes the most negative reference voltage), but also enables A/D converter to convert rail-to-rail analog input with maintaining good power supply rejection with respect to positive power and negative power (or ground).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of analog-to-digital converters and more particularly to low-voltage drop reference generation circuit for A/D converters basically utilizing a resistor string, two transistors, and two amplifiers. 
       BACKGROUND ART 
       [0002]    In interfacing between the analog and digital domain, the analog-to-digital (A/D) converter is a vitally important device. The A/D converter converts an analog signal such as a voltage or a current into a digital signal, which can be further processed, stored, and disseminated using digital processors. For example, A/D converters are used in communications, appliances, display, signal processing, computers, medical instrumentation, industries, and any other fields that require conversion of analog signals into digital forms. 
         [0003]    The A/D converter encodes an analog input signal into a digital output signal of a predetermined bit length. The A/D converter basically includes a resistor string which is comprised of a plurality of resistors. The resistors form a resistor string and are coupled in series between two reference voltages: the most positive reference voltage and the most negative reference voltage. These reference voltages are then fed into several blocks such as comparators, digital-to-analog (D/A) subsection, preamplifiers, and interstage amplifiers. 
         [0004]    Prior Art  FIG. 1  illustrates a circuit diagram of a conventional reference generation circuit for A/D converter  100 . The conventional reference generation circuit for A/D converter shown in Prior Art  FIG. 1  is comprised of a plurality of resistors and an amplifier  121 . The resistors form a resistor string and are coupled in series between two reference voltages: the most positive reference voltage, V REFT , and the most negative reference voltage, V REFB . It is noted that the negative input of the amplifier  121  is connected to its output node. Thus, it becomes voltage-follower configuration. This conventional circuit  100  generates a plurality of reference voltages characterized by voltage increments between the most positive reference voltage, V REFT , and the most negative reference voltage, V REFB . 
         [0005]    Unfortunately, the conventional reference generation circuit for A/D converter  100  is inefficient to implement in integrated circuit (IC) chip. First, the power supply rejection with respect to negative power supply (or ground) rather than positive power supply is significantly degraded at node  104 . In reality, switches and analog blocks are connected to the nodes between the serially coupled resistors in Prior Art  FIG. 1 . The regulation at the node  104  is much weaker than in the case of the node  101  whenever charge-injection error occurs at the node (i.e., whenever MOS switches (not shown), which connected to the nodes between the serially coupled resistors, turn off). Furthermore, voltage at the node  101  (i.e., V REFT ) becomes V REFTIN  for V DROPB &lt;V REFIN &lt;V DD −V DROPT . Thus, voltage drop between power supply and the most positive reference voltage (i.e., V DD −V REFT ) is usually lager than average MOS device threshold voltage (i.e., thick oxide MOS device threshold voltage). Thus, the conventional reference generation circuit for A/D converter  100  has a major limitation on rail-to-rail A/D converters and will miss codes for large-swing analog input signal, as shown in Prior Art  FIG. 1 . Thus, minimum performance of rail-to-rail A/D converters can not be achieved without low-voltage drop reference generation circuit for all types of A/D converters. 
         [0006]    Thus, what is needed is cost-effective low-voltage drop reference generation circuits for A/D converter that can be easily designed and efficiently implemented along with maximizing the total reference voltage range and power supply rejection with respect to both positive power supply and negative power supply (or ground). The present invention satisfies these needs by providing two low-voltage drop reference generation circuits for A/D converter basically utilizing a resistor string, two transistors, and two amplifiers. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides two cost-effective low-voltage drop reference generation circuits for A/D converter. The cost-effective low-voltage drop reference generation circuits for A/D converter of the present invention basically includes a resistor string, two transistors, two amplifiers (or operational amplifiers). The resistor string generates a plurality of reference voltages characterized by voltage increments between two fixed reference voltages. In this configuration, the two transistors are used as common-source amplifier and each amplifier receives a reference voltage at its negative input. The generated reference voltages are not only constant with respect to the fluctuations of positive power supply and negative power supply (or ground), but also greatly increases the total reference voltage range (i.e., increases voltage difference between the most positive reference voltage and the most negative reference voltage, increases V REFT −V REFB , maximizes the most positive reference voltage and minimizes the most negative reference voltage). The present invention achieves a drastic improvement in increasing the total reference voltage range with good power supply rejection with respect to positive power supply and negative power supply (or ground). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
           [0009]    Prior Art  FIG. 1  illustrates a circuit diagram of a conventional reference generation circuit for A/D converter. 
           [0010]      FIG. 2  illustrates a circuit diagram of a low-voltage drop reference generation circuit for A/D converter in accordance with the present invention. 
           [0011]      FIG. 3  illustrates a circuit diagram of a flexible low-voltage drop reference generation circuit for A/D converter according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    In the following detailed description of the present invention, two cost-effective low-voltage drop reference generation circuits for A/D converter, numerous specific details are set forth in order to provide a through understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
         [0013]      FIG. 2  illustrates a low-voltage drop reference generation circuit for A/D converter in accordance with the present invention. The low-voltage drop reference generation circuit for A/D converter  200  is comprised of n resistors, a PMOS transistor  231 , an NMOS transistor  232 , and two amplifiers (or operational amplifiers)  221  and  222 . The amplifiers  221  and  222  are termed upper amplifier and lower amplifier, respectively. The resistor string generates n−1 spaced reference voltages between two reference voltages such as V REFT  and V REFB . The dotted lines  213  represent m resistors where m is an integer greater than or equal to one. As shown in  FIG. 2 , the two transistors are used as common-source amplifier and each amplifier (or operational amplifier) receives a reference voltage at its negative input. It is noted that the positive input of the amplifier  221  is connected to the drain node of the PMOS transistor  231  and its output is connected to the gate node of the PMOS transistor  231 . The amplifier  221  and PMOS transistor  231  are in negative feedback configuration. Likewise, the amplifier  222  and NMOS transistor  232  are in negative feedback configuration. Even though V REFTIN  goes up close to positive power supply, voltage at a node  201  (i.e., V REFT ) still becomes equal to V REFTIN . Even though V REFBIN  goes down close to negative power supply (or ground), voltage at a node  204  (i.e., V REFB ) still becomes equal to V REFBIN . In addition, as voltages at both positive power supply and negative power supply (or ground) change, the voltage at the node  204  will be much more constant than the case of Prior Art  FIG. 1  (i.e., the voltage at the node  104 ). However, the regulation at all nodes  201  through  204  shown in  FIG. 2  is much stronger than in the case of Prior Art  FIG. 1  whenever charge-injection error occurs at the nodes (i.e., whenever MOS switches (not shown), which connected to the nodes between the serially coupled resistors, turn off). Thus, the low-voltage drop reference generation circuit for A/D converter  200  provides a strong basis for all types of rail-to-rail A/D converters, and can be efficiently implemented along with increasing the total reference voltage range and maintaining good power supply rejection with respect to both positive power supply and negative power supply (or ground). The present invention generates low-voltage drop reference voltages utilizing a resistor string, two amplifiers (or operational amplifiers), and two transistors. Amplifiers are well known circuits in the art and can be implemented using various well known devices such as transistors, capacitors, resistors, etc. In addition, the amplifiers (or operational amplifiers)  221  and  222  are differential-input single-ended output amplifiers and can have any number of gain stages with or without buffer stage (i.e., output stage). 
         [0014]      FIG. 3  illustrates a circuit diagram of a flexible low-voltage drop reference generation circuit for A/D converter  300  according to the present invention. The flexible low-voltage drop reference generation circuit for A/D converter  300  is comprised of n resistors, a PMOS transistor  331 , an NMOS transistor  332 , two amplifiers (or operational amplifiers)  321  and  322 . The amplifiers  321  and  322  are termed upper amplifier and lower amplifier, respectively. The dotted lines  313  represent m resistors where m is an integer greater than or equal to one. The resistor string consists of an upper part  311  and  312 , a middle part  313 , and a lower part  316  and  317 . However, the middle part of the resistor string is excluded in either feedback loop. 
         [0015]    In addition, it is also noted that the amplifier  321 , PMOS transistor  331 , and resistors  311  and  312  shown in  FIG. 3  are in negative feedback configuration in the same fashion as the amplifier  221  and PMOS transistor  231 . However, a difference between  FIG. 2  and  FIG. 3  is that upper part  311  and  312  and lower part  316  and  317  shown in  FIG. 3  are included in each feedback path. Thus, even though V REFUPIN  is not close to V DD  and V REFDNIN  is not close to −V SS  (or ground), voltage at node  301  (i.e., V REFT , the most positive reference voltage) and voltage at node  306  (i.e., V REFB , the most negative reference voltage) become a constant reference voltage closed to V DD  and a constant reference voltage closed to −V SS  (or ground), respectively. The flexible low-voltage drop reference generation circuit for A/D converter  300  is highly effective when V REFUPIN  is not high enough and V REFDNIN  is not low enough. 
         [0016]    In summary, the low-voltage drop reference generation circuit for A/D converter  200  and the flexible low-voltage drop reference generation circuit for A/D converter  300  can also be implemented using additional capacitors attached to the nodes  201  through  204  and the nodes  301  through  306 , respectively. In addition, the two low-voltage drop reference generation circuits of the present invention are highly efficient to implement in integrated circuit (IC) and system-on-chip (SOC). The low-voltage drop reference generation circuit for A/D converter  200  of the present invention achieves a drastic improvement in the total reference voltage range and converting larger analog signal swing and power supply rejection. In addition to the strengths mentioned above, the flexible low-voltage drop reference generation circuit for A/D converter  300  of the present invention provides the most positive reference voltage higher than V REFUPIN  and the most negative reference voltage lower V REFDNIN  when high V REFUPIN  and low V REFDNIN  are not available. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as being limited by such embodiments, but rather construed according to the claims below.