Abstract:
A comparator includes a differential amplifier having first and second input terminals and first and second output terminals. An input stage is operable to receive first and second input signals. The input stage includes first and second capacitors coupled to the first and second input terminals, respectively. Circuitry is operable to selectively couple the first input signal to the first capacitor and the second input signal to the second capacitor, while coupling the first and second capacitors to the first and second output terminals, respectively, during an offset cancellation phase, and selectively couple the second input signal to the first capacitor and the first input signal to the second capacitor, while isolating the first and second capacitors from first and second output terminals during a comparison phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       BACKGROUND 
       [0003]    The disclosed subject matter relates generally to manufacturing and, more particularly, to electronic circuits having an analog comparator and also relates to integrated circuit devices and designs including an analog comparator circuit. 
         [0004]    In electronic designs and circuits, the amplitude of a signal level frequently has to be determined with a specified degree of accuracy. For this purpose, a plurality of techniques have been developed that include the comparison of a first signal level with a second signal level to decide whether the first signal level is higher or lower compared to the second signal level. Thus, a respective electronic circuit may provide a digital response to the question of which of two signals has a higher signal level. 
         [0005]    Corresponding electronic circuits may typically be referred to as comparators or analog comparators, when at least one of the two signal levels may vary continuously. Such analog comparator circuits may be used in situations in which a signal is to be compared with a reference signal, which may represent a substantially constant reference or a varying reference, so as to indicate by a digital response when the signal crosses the threshold defined by the reference signal. 
         [0006]    A comparator circuit typically comprises an appropriately designed input stage, including a pair of input transistors, which may receive the respective input signals. The comparator may generate a differential voltage that varies depending on the difference of the input signals. The differential voltage may be supplied to an output stage, which is typically designed to provide two predefined output signal levels depending on the voltage across the differential input stage. Consequently, for sophisticated applications, the characteristics of various elements of the comparator may have to be matched to each other to obtain a change of the output signal at a desired minimum value of the difference of the two input signals. Moreover, the response of the comparator circuit to the input signal should typically be as stable as possible for varying operational conditions, such as different temperatures, varying supply voltages, aging of the circuit components, and any other environmental influences, such as humidity, pressure and the like. Typical compensation techniques are complex and may require sophisticated and complex analog circuitry, which may contribute to overall design complexity and production costs. 
         [0007]    This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
       BRIEF SUMMARY 
       [0008]    The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
         [0009]    One aspect of the disclosed subject matter is seen in a comparator. The comparator includes a differential amplifier having first and second input terminals and first and second output terminals. An input stage is operable to receive first and second input signals. The input stage includes first and second capacitors coupled to the first and second input terminals, respectively. Circuitry is operable to selectively couple the first input signal to the first capacitor and the second input signal to the second capacitor, while coupling the first and second capacitors to the first and second output terminals, respectively, during an offset cancellation phase, and selectively couple the second input signal to the first capacitor and the first input signal to the second capacitor, while isolating the first and second capacitors from first and second output terminals during a comparison phase. 
         [0010]    Another aspect of the disclosed subject matter is seen in a method for comparing first and second input signals. The first input signal is coupled to a first capacitor and the second input signal is coupled to a second capacitor. The first and second capacitors are coupled to first and second input terminals of a differential amplifier, respectively. The differential amplifier is equalized to store a difference between a voltage of the first input signal and a threshold voltage of the differential amplifier on the first capacitor and store a difference between a voltage of the second input signal and the threshold voltage of the differential amplifier on the second capacitor. The first input signal is coupled to the second capacitor and the second input signal is coupled to the first capacitor after equalizing the differential amplifier. A difference between the first and second input signals is amplified in the differential amplifier. A first logical output is generated responsive to the amplified difference indicating the first input signal has a voltage higher than the second input signal and a second logical output is generated responsive to the amplified difference indicating the first input signal having a voltage lower than the second input signal. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    The disclosed subject matter will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0012]      FIG. 1A  is a circuit diagram of a comparator in accordance with one illustrative embodiment of the present subject matter, where the comparator is in a first logic state; 
           [0013]      FIG. 1B  is a circuit diagram of the comparator of  FIG. 1A  in a second logic state; 
           [0014]      FIG. 2  is a circuit diagram of a pass gate used in the comparator of  FIG. 1 ; and 
           [0015]      FIG. 3  is a timing diagram illustrating the operation of the comparator of  FIG. 1 . 
       
    
    
       [0016]    While the disclosed subject matter is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosed subject matter to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosed subject matter as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0017]    One or more specific embodiments of the disclosed subject matter will be described below. It is specifically intended that the disclosed subject matter not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the disclosed subject matter unless explicitly indicated as being “critical” or “essential.” 
         [0018]    The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the disclosed subject matter with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0019]    Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1A , the disclosed subject matter shall be described in the context of a comparator  10 . The comparator includes an input stage  15 , a first differential amplifier stage  20 , a second differential amplifier stage  30 , a third differential amplifier stage  40 , an output stage  50 , and a clock generator  60 . 
         [0020]    The input stage  15  includes pass gates  16 A,  16 B coupled to receive an input signal (VIN) and a reference signal (VREF), respectively, and selectively route either the input signal or the reference signal to an input stage capacitor  18 A. Pass gates  17 A,  17 B are coupled selectively route either the input signal or the reference signal to an input stage capacitor  18 B. 
         [0021]    The first differential amplifier stage  20  includes pass gates  22 A,  23 A coupled to the capacitors  18 A,  18 B, respectively, and to input terminals  21 A,  21 B, respectively. P-type pull-up transistors  24 A,  24 B are coupled to the pass gates  22 A,  23 A, respectively, and N-type pull-down transistors  25 A,  25 B are coupled to the capacitors  18 A,  18 B, respectively. The sources of the pull-up transistors  24 A,  24 B are coupled to a high reference voltage, VDD, and the sources of the pull-down transistors  25 A,  25 B are coupled through a pull-down resistor  26  to a low reference voltage, VSS. The output terminals  27 A,  27 B of the first differential amplifier stage  20  are provided to intermediate stage capacitors  80 A,  80 B, respectively. 
         [0022]    The intermediate stage capacitors  80 A,  80 B provide the inputs to the second differential amplifier stage  30 . The second differential amplifier stage  30  has essentially the same construction as the first differential amplifier  20 . The second differential amplifier stage  30  includes pass gates  32 A,  33 A coupled to the capacitors  80 A,  80 B, respectively, and to input terminals  31 A,  31 B, respectively. P-type pull-up transistors  34 A,  34 B are coupled to the pass gates  32 A,  33 A, respectively, and N-type pull-down transistors  35 A,  35 B are coupled to the capacitors  80 A,  80 B, respectively. The sources of the pull-up transistors  34 A,  34 B are coupled to VDD, and the sources of the pull-down transistors  35 A,  35 B are coupled through a pull-down resistor  36  to VSS. The output terminals  37 A,  37 B of the second differential amplifier stage  30  are provided to the input terminals  41 A,  41 B of the third differential amplifier stage  30 . 
         [0023]    The third differential amplifier stage  40  includes P-type pull-up transistors  44 A,  44 B and N-type pull-down transistors  45 A,  45 B. The N-type pull-down transistors  45 A,  45 B are coupled to the outputs of the second differential amplifier stage  30 . The sources of the pull-up transistors  44 A,  44 B are coupled to VDD, and the sources of the pull-down transistors  45 A,  45 B are coupled through a pull-down resistor  46  to VSS. An output terminal  47 A of the third differential amplifier stage  40  is provided to the output stage  50 . The other output terminal  47 B of the third differential amplifier stage  40  is left unconnected. 
         [0024]    The output stage  50  includes a sampling latch  52  connected to the output terminal  47 A. The output of the sampling latch  52  is passed through inverters  54 ,  56 , thereby provided a digital output signal, OUT, indicating whether the input signal is higher than the reference signal (logic “1”) or the input signal is lower than the reference signal (logic “0”). 
         [0025]    The clock generator  60  includes an AND gate  61  coupled to receive an enable signal, EN, and external clock signal, CLK. A data flip flop  62  is clocked by the output of the AND gate  61 , The clock signal is inverted by an inverter  65 , and the inverted clock signal is received as the clock input to a second data flip flop  63 . The output of the data flip flop  62  is provided to an inverter  64  and then fed back to the input of the data flip flop  62 , thereby causing the output of the data flip flop  62  to toggle on the falling edge of each clock cycle. Thus, the data flip flop  62  acts as a clock divider generating an output clock signal that is half the frequency of the input clock signal (CLK/2). 
         [0026]    The output of the second data flip flop  63  is also inverted by an inverter  66  and fed back to its input. Because the clock signal provided to the data flip flop  63  is inverted, it toggles on the rising edge of the CLK signal. Thus, the output of the inverter  66  defines a sample clock signal, CLKS, which represents the input clock signal divided by two (CLK/2) delayed by half a clock cycle. 
         [0027]    The output of the inverter  64  is used to generate clock signals, CLKI and CLKIB for controlling the pass gates  16 A,  16 B,  17 A,  17 B,  22 A,  23 A,  32 A,  33 A. The output of the inverter  64  is provided to a first network of inverters  67 ,  68 ,  69  to delay the clock signal and generate the CLKI signal. The output of the inverter  64  is also provided to a second network of inverters  70 ,  71 ,  72 ,  73  to delay the clock signal and generate the CLKIB signal. Cross-coupled inverters  74 ,  74  are provided to compensate the delay difference between the CLKI-path (2 inverters) and the CLKIB-path (3 inverters). The cross-coupled inverters  74 ,  75  have a relatively fast switching behavior due to the positive feedback during switching, thereby supporting the switching of the CLKIB-path. This arrangement provides a more symmetrical shape for the edges of CKLKI and the corresponding edges of CLKIB. 
         [0028]    Turning now to  FIG. 2 , a circuit diagram of exemplary pass gates  200 A,  200 B is provided. The pass gate  200 A includes an N-type transistor  210 A controlled by the CLKI signal and a P-type transistor  220 A controlled by the CLKIB signal. Hence, the pass gate  200 A is closed when the CLKI signal is high and the CLKIB signal is low. The pass gate  200 B includes an N-type transistor  210 B controlled by the CLKIB signal and a P-type transistor  220 B controlled by the CLKI signal. Hence, the pass gate  200 B is open when the CLKI signal is high and the CLKIB signal is low. Thus, the pass gates  200 A,  200 B operate at complimentary logic states. In the comparator  10  illustrated in  FIG. 1A , the pass gates  16 A,  17 A,  22 A,  23 A,  32 A,  33 A have the same logic orientation as the pass gate  200 A, and the pass gates  16 B,  17 B have the same logic orientation as the pass gate  200 B. 
         [0029]    Returning to  FIG. 1A , the pass gates  16 A,  16 B,  17 A,  17 B,  22 A,  23 A,  32 A,  33 A are illustrated as being in a logic state corresponding to CLKI=High and CLKIB=Low, which represents an offset cancellation phase of the comparator  10 . The “A” pass gates are closed, and the “B” pass gates are open. In this phase, the pass gates  22 A,  23 A,  32 A,  33 A keep the first and second differential amplifier stages  20 ,  30  at an operating point of VDD/2 by connecting the output terminals  27 A,  27 B of the differential amplifier stage  20  to the input terminals  21 A,  21 B. The pass gate  16 A routes the input signal, VIN, to the capacitor  18 A, and the pass gate  17 A routes the reference signal, VREF, to the capacitor  18 B. Hence, the input voltage difference (VIN-VREF) and the offset voltage of the first differential amplifier stage are stored on the capacitors  18 A,  18 B. The capacitor  18 A stores the difference between the input voltage and the threshold voltage of the first differential amplifier stage  20 , and the capacitor  18 B stores the difference between the reference voltage and the threshold voltage of the first differential amplifier stage  20 . The output voltage of the first differential amplifier stage  20  and the offset voltage of the second differential amplifier stage  30  are stored on the capacitors  80 A,  80 B. The second differential amplifier stage  30  operates in the same manner as the first differential amplifier stage  20 . 
         [0030]      FIG. 1B  illustrates the pass gates  16 A,  16 B,  17 A,  17 B,  22 A,  23 A,  32 A,  33 A as being in a second logic state corresponding to CLKI=Low and CLKIB=High, which represents a comparison phase of the comparator  10 . The “A” pass gates are open, and the “B” pass gates are closed. With the pass gates  22 A,  23 A,  32 A,  33 A open, the first and second differential amplifier stages  20 ,  30  operate as amplifiers. The pass gate  16 B routes the reference voltage, VREF, to the capacitor  18 A, and the pass gate  17 B routes the input voltage, VIN, to the capacitor  18 B, thereby reversing the polarity. Due to the polarity reversal, The inputs to the first differential amplifier stage  20  become: 
         [0000]      ( V IN− V REF)−( V REF− V IN)=2( V IN− V REF).
 
         [0031]    The input voltage difference stored on the capacitors  18 A,  18 B is amplified by all three differential amplifier voltage stages  20 ,  30 ,  40 . The sampling latch  52  latches the output voltage of the third differential amplifier stage  40 . 
         [0032]    In the illustrated embodiment, the third differential amplifier stage  40  is a simple difference amplifier without offset cancellation. Because the first and second differential amplifier stages  20 ,  30  provide a sufficiently amplified output signal, offset cancellation in the third differential amplifier stage  40  may be omitted. Although differential amplifier stages  20 ,  30 ,  40 , are illustrated, and only the first and second stages  20 ,  30  include offset cancellation, it is contemplated that the total number of stages may vary, as well as the number of stages with offset cancellation. 
         [0033]    The clock generator  60  defines the relative timings of the CLKI, CLKIB, and CLKS signals to control the phases of the comparator  10 .  FIG. 3  is a timing diagram  300  illustrating the operation of the comparator  10 . The sampling clock, CLKS, represents the input clock signal, CLK, divided by 2 and delayed by a half cycle. The clock signals, CLKI and CLKIB (not shown) are complimentary versions of the input clock signal divided by 2, CLK/2. The effects of the CLKI and CLKIB signals are evident in the signal received at the sampling latch  52 , as shown in the SL signal. In the offset cancellation phase illustrated in  FIG. 1A , where CLKI=High and CLKIB=LOW, the outputs of the first and second differential amplifier stages  20 ,  30  are equalized at VDD/2, as represented by point  310 . In the comparison phase illustrated in  FIG. 1B , where CLKI=LOW and CLKIB=HIGH, the output of the cascaded differential amplifier stages  20 ,  30 ,  40  is present at the input to the sampling latch  52 , represented by point  320 . The sampling latch  52  records the value of the SL signal on rising edges of the SCLK signal. 
         [0034]    Note that after the sampling at point  320 , the input signal, VIN, transitions from being above the reference voltage, VREF, to being below the reference voltage. The sampling latch  52  detects this change at point  330  during the next comparison phase. At a later time, the input signal transitions high again, and the sampling latch  52  detects this change at point  340 . 
         [0035]    The comparator  10  described herein exhibits increased measurement accuracy and is sensitive to voltage differences less than 1 mV. Due to the offset compensation, the measurement accuracy is independent of technology variations. The comparator  10  also exhibits good supply/ground noise immunity and robust operation over wide temperate and supply voltage ranges. 
         [0036]    The particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.