Patent Publication Number: US-2023163777-A1

Title: Comparator and analog to digital converter

Description:
TECHNICAL FIELD 
     The present disclosure relates to a comparator and an analog to digital converter. 
     BACKGROUND ART 
     A double-tail latch-type comparator has been proposed (see Non-Patent Document 1). In the double-tail latch-type comparator, a transistor that is switched on or off in accordance with logic of a clock signal is provided on a source side of a pair of transistors to which a differential input signal pair are input, a latch circuit is connected to a drain side of the pair of transistors described above, and another transistor that switches whether or not to perform a latch operation in accordance with logic of the clock signal is provided on one end side of the latch circuit. 
     CITATION LIST 
     Non-Patent Document 
     
         
         Non-Patent Document 1: “A Double-Tail Latch-Type Voltage Sense Amplifier with 18 ps Setup+Hold Time” (ISSCC 2007, 17.7) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In a case where the double-tail latch-type comparator is used in a successive approximation type analog to digital converter (hereinafter, ADC), the transistor on the source side of the pair of transistors in a comparator is off while the differential input signal pair are sampled. Accordingly, a potential of a source side path becomes an unstable state, so that the sampled differential input signal pair are affected by a parasitic capacitance of the path through which the sampled differential input node pair are input, thereby an input voltage dependent error occurs in the differential input node pair of the comparator. This error adversely affects the comparator when the comparator performs a comparison operation thereafter, and becomes a factor that reduces accuracy of the ADC. 
     Therefore, the present disclosure provides a comparator and an analog to digital converter that prevent occurrence of an input voltage dependent error due to an input parasitic capacitance. 
     Solutions to Problems 
     In order to solve the problems described above, according to the present disclosure, there is provided a comparator including: a first transistor and a second transistor that include two sources connected to each other, two gates to which a differential input signal pair are input, and two drains that output a differential output signal pair corresponding to a difference signal of the differential input signal pair; 
     a third transistor that is connected between both the sources of the first transistor and the second transistor and a first reference voltage node, the third transistor being switched on or off in accordance with logic of a first signal; and 
     a fourth transistor that is connected between both the sources of the first transistor and the second transistor and a second reference voltage node, the fourth transistor being switched on or off in accordance with logic of a second signal having logic different from the logic of the first signal. 
     The fourth transistor may be turned on in a period in which the first transistor and the second transistor do not perform a comparison operation of the difference signal of the differential input signal pair. 
     The third transistor may be intermittently turned on during a period in which the fourth transistor is off. 
     A latch circuit that holds the differential output signal pair may be included. 
     A waveform shaping circuit that performs waveform shaping of the differential output signal pair output from both the drains of the first transistor and the second transistor may be included, and 
     a signal after waveform shaping is performed by the waveform shaping circuit may be input to the latch circuit. 
     The waveform shaping circuit may include two inverters that invert logic of the differential output signal pair. 
     A fifth transistor that switches whether or not to perform a holding operation by the latch circuit in accordance with the logic of the first signal may be included, and 
     the fifth transistor may be intermittently turned on during a period in which the fourth transistor is off. 
     The fourth transistor may have a conductivity type different from a conductivity type of the third transistor. 
     The first transistor, the second transistor, and the third transistor may include N-type MOS transistors, and 
     the fourth transistor may include a P-type MOS transistor. 
     The first transistor, the second transistor, and the third transistor may include P-type MOS transistors, and 
     the fourth transistor may include an N-type MOS transistor. 
     There may be provided the comparator further including: 
     a first input terminal and a second input terminal to which a first differential input signal pair are input; 
     a third input terminal and a fourth input terminal to which a second differential input signal pair are input; and 
     a comparison circuit that outputs a signal corresponding to a difference signal of the first differential input signal pair input to the first input terminal and the second input terminal and a difference signal of the second differential input signal pair input to the third input terminal and the fourth input terminal, 
     in which the comparison circuit includes: 
     a first comparison device that includes sixth to ninth transistors having a same circuit configuration as a circuit configuration of the first to fourth transistors; and 
     a second comparison device that includes tenth to thirteenth transistors having the same circuit configuration as the circuit configuration of the first to fourth transistors. 
     According to the present disclosure, there is provided an analog to digital converter including: a first sampling switch that switches whether or not to sample one signal of a differential input signal pair; 
     a first digital to analog converter that converts the one signal sampled into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to an unconverted bit; 
     a second sampling switch that switches whether or not to sample another signal of the differential input signal pair; 
     a second digital to analog converter that converts the another signal sampled into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to an unconverted bit; 
     a comparator that outputs a signal corresponding to a difference signal of a first differential input signal pair in which an output signal of the first digital to analog converter and an output signal of the second digital to analog converter form a pair; and 
     a control circuit that controls the first digital to analog converter and the second digital to analog converter on the basis of an output signal of the comparator, 
     in which the comparator includes: 
     a first transistor and a second transistor that include two sources connected to each other, two gates to which the first differential input signal pair are input, and two drains that output a differential output signal pair corresponding to the difference signal of the first differential input signal pair; 
     a third transistor that is connected between both the sources of the first transistor and the second transistor and a first reference voltage node, the third transistor being switched on or off in accordance with logic of a first signal; and 
     a fourth transistor that is connected between both the sources of the first transistor and the second transistor and a second reference voltage node, the fourth transistor being switched on or off in accordance with logic of a second signal having logic different from the logic of the first signal. 
     A filter circuit that samples and outputs the output signal of the first digital to analog converter and the output signal of the second digital to analog converter may be included, and 
     the comparator may output a signal corresponding to the difference signal of the first differential input signal pair in which the output signal of the first digital to analog converter and the output signal of the second digital to analog converter form a pair and a difference signal of a second differential input signal pair output from the filter circuit, 
     the comparator may include: 
     a first input terminal and a second input terminal to which the first differential input signal pair are input; 
     a third input terminal and a fourth input terminal to which the second differential input signal pair are input; and 
     a comparison circuit that outputs a signal corresponding to the difference signal of the first differential input signal pair input to the first input terminal and the second input terminal and the difference signal of the second differential input signal pair input to the third input terminal and the fourth input terminal, and 
     the comparison circuit may include the first to fourth transistors. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a circuit diagram of a comparator according to a first embodiment. 
         FIG.  2    is a circuit diagram of a comparator according to a comparative example. 
         FIG.  3    is a graph illustrating input voltage dependence of input parasitic capacitances of the comparators in  FIGS.  1  and  2   . 
         FIG.  4    is a circuit diagram of a successive approximation type ADC  4  including the comparator in  FIG.  1   . 
         FIG.  5    is a waveform diagram of a control signal, an enable signal, and a clock signal of the comparator in  FIG.  4   . 
         FIG.  6    is a circuit diagram of a successive approximation type ADC, according to a comparative example, including the comparator in  FIG.  2   . 
         FIG.  7    is a circuit diagram of a comparator according to a second embodiment. 
         FIG.  8    is a circuit diagram of a comparator according to a third embodiment. 
         FIG.  9    is a circuit diagram of a comparator according to a fourth embodiment. 
         FIG.  10    is a circuit diagram of an ADC including a filter circuit in addition to a capacitive DAC. 
         FIG.  11    is a circuit diagram illustrating an example of an internal configuration of a comparator used in the ADC in  FIG.  10   . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of a comparator and an analog to digital converter will be described with reference to the drawings. Hereinafter, the description will be focused on the main components of the comparator and the analog to digital converter, but there may exist components and functions that are not illustrated or described in the comparator and the analog to digital converter. The following description does not exclude the components and the functions that are not illustrated or described. 
     First Embodiment 
       FIG.  1    is a circuit diagram of a comparator  1  according to a first embodiment. The comparator  1  in  FIG.  1    is used in, for example, a successive approximation type ADC as described later, but the application of the comparator  1  in  FIG.  1    is not necessarily limited to the ADC. A differential input signal pair Vin_p, Vin_n are input to the comparator  1  in  FIG.  1   . The comparator  1  outputs a differential output signal pair Vout_p, Vout_n corresponding to a difference signal of the differential input signal pair Vin_p, Vin_n. The comparator  1  in  FIG.  1    is a kind of double-tail latch-type comparator. 
     The comparator  1  in  FIG.  1    includes transistors (first to fourth transistors) Q 1  to Q 4 . With respect to the transistor Q 1  and the transistor Q 2 , sources thereof are connected to each other, the differential input signal pair Vin_p, Vin_n are input to gates thereof, and a differential output signal pair Vgm_p, Vgm_n are output from drains thereof. 
     The transistor Q 3  is connected between both the sources of the transistor Q 1  and the transistor Q 2  and a first reference voltage node (for example, a ground node), and is switched on or off in accordance with logic of a first signal. The first signal is, for example, a clock signal Clk having a predetermined frequency. When the clock signal Clk is at a high level, the transistor Q 43  is turned on, and the comparator  1  in  FIG.  1    performs a comparison operation. The comparator  1  in  FIG.  1    is in a stopped state without performing the comparison operation during a period in which the clock signal Clk is at a low level. 
     The transistor Q 4  is connected between both the sources of the transistor Q 1  and the transistor Q 2  and a second reference voltage node (for example, a power supply voltage node), and is switched on or off in accordance with logic of a second signal having logic different from that of the first signal. The second signal is, for example, an enable signal En. As described later, the enable signal En is turned on in a period in which the comparator  1  in  FIG.  1    does not perform the comparison operation. When the transistor Q 4  is turned on, a path (hereinafter, it may be referred to as a tail node) connecting both the sources of the transistor Q 1  and the transistor Q 2  is set to a power supply potential. That is, the tail node connecting both the sources of the transistor Q 1  and the transistor Q 2  is set to the power supply potential, not to floating during a period in which the comparator  1  in  FIG.  1    does not perform the comparison operation. Therefore, the voltage level of the tail node connecting both the sources of the transistor Q 1  and the transistor Q 2  is fixed to the power supply potential during a period in which the comparator  1  does not perform the comparison operation, and the voltage dependence of input parasitic capacitances Cin_p and Cin_n of a differential input node pair of the comparator  1  can be sufficiently reduced. 
     In the comparator  1  in  FIG.  1   , the transistors Q 1  to Q 3  are N-type MOS transistors, and the transistor Q 4  is a P-type MOS transistor. As described later, the conductivity types of the transistors Q 1  to Q 4  can be reversed from those in  FIG.  1   . 
     The comparator  1  in  FIG.  1    includes a pull-up circuit  2  and a latch circuit  3  in addition to the transistors Q 1  to Q 4  described above. 
     The pull-up circuit  2  includes a transistor (fifth transistor) Q 5  and a transistor (sixth transistor) Q 6 . The transistor Q 5  and the transistor Q 6  are, for example, P-type MOS transistors. A clock signal Clk is input to both gates of the transistor Q 5  and the transistor Q 6 . Power supply voltage nodes are connected to both sources of the transistor Q 5  and the transistor Q 6 . The differential output signal pair Vout_p, Vout_n are output from respective drains of the transistor Q 5  and the transistor Q 6 . 
     The latch circuit  3  includes transistors (seventh to thirteenth transistors) Q 7  to Q 13 . The transistors Q 7  to Q 10  are, for example, N-type MOS transistors, and the transistors Q 11  to Q 13  are, for example, P-type MOS transistors. The differential output signal pair Vgm_p, Vgm_n are input to respective gates of the transistor Q 7  and the transistor Q 9 . A first output node n 1  is connected to each gate of the transistor Q 10  and the transistor Q 12 , and each drain of the transistor Q 7 , the transistor Q 8 , and the transistor Q 11 , and the Vout_p is output. A second output node n 2  is connected to each gate of the transistor Q 8  and the transistor Q 11 , and each drain of the transistor Q 9 , the transistor Q 10 , and the transistor Q 12 , and the Vout_n is output. A power supply voltage node is connected to a source of the transistor Q 13 , and each source of the transistor Q 11  and the transistor Q 12  is connected to a drain of the transistor Q 13 . An inversion signal xClk of the clock signal Clk is input to a gate of the transistor Q 13 . 
     The comparator  1  in  FIG.  1    starts a comparison operation when the clock signal Clk transitions from a ground level (low level) to a power supply voltage level (high level). Before the comparator  1  in  FIG.  1    starts the comparison operation, the clock signal Clk is at a low level, and the differential output signal pair Vgm_p, Vgm_n are pulled up to the power supply level. 
     When the clock signal Clk transitions to the power supply level, potentials of the differential output signal pair Vgm_p and Vgm_n decrease due to discharging by the transistors Q 1 , Q 2 . When “Vin_p&gt;Vin_n”, “discharge rate of Vgm_p&gt;discharge rate of Vgm_n” is obtained. On the other hand, when “Vin_p&lt;Vin_n”, “discharge rate of Vgm_p&lt;discharge rate of Vgm_n” is obtained. In this manner, the difference signal (Vin_p−Vin_n) of the differential input signal pair Vin_p, Vin_n of the comparator  1  causes a difference in discharge rate between Vgm_p and Vgm_n. 
     Furthermore, logic of the latch circuit  3  is determined according to the difference in discharge rate. If “discharge rate of Vgm_p&gt;discharge rate of Vgm_n”, Vout_p=high/Vout_n=low is obtained, and if “discharge rate of Vgm_p&lt;discharge rate of Vgm_n”, Vout_p=low/Vout_n=high is obtained. 
       FIG.  2    is a circuit diagram of a comparator  1   a  according to a comparative example. In  FIG.  2   , components common to those in  FIG.  1    are denoted by the same reference numerals, and the description will be focused on differences hereinafter. 
     In the comparator  1   a  in  FIG.  2   , the transistor Q 4  in  FIG.  1    is omitted. Other circuit configurations are similar to those in  FIG.  1   . In the comparator  1   a  in  FIG.  2   , source potentials of the transistors Q 1 , Q 2  are in a floating state during a period in which a comparison operation is not performed. Therefore, an input voltage dependent error occurs in the voltage level of the differential input node pair due to input parasitic capacitances of input nodes of the differential input signal pair Vin_p, Vin_n of the comparator  1   a.    
       FIG.  3    is a graph illustrating the input voltage dependence of input parasitic capacitances of the comparators  1 ,  1   a  in  FIGS.  1  and  2   .  FIG.  3    illustrates simulation results. In FIG.  3 , the horizontal axis represents the input voltage [V], and the vertical axis represents the input parasitic capacitance [fF]. In  FIG.  3   , a broken line waveform w 1  indicates the input voltage dependence of the comparator  1  in  FIG.  1   , and a solid line waveform w 2  indicates the input voltage dependence of the comparator  1   a  in  FIG.  2   . The broken line waveform w 1  in  FIG.  3    is a result of simulation performed in a state in which the enable signal En is set to a low level, and the tail node connecting the sources of the transistors Q 1 , Q 2  to each other is pulled up to the power supply voltage in the comparator  1  in  FIG.  1   . In the comparator  1   a  in  FIG.  2   , the input parasitic capacitance greatly fluctuates according to the input voltage, but in the comparator  1  in  FIG.  1   , a fluctuation amount of the input parasitic capacitance is greatly suppressed even if the input voltage changes. 
       FIG.  4    is a circuit diagram of a successive approximation type ADC  4  including the comparator  1  in  FIG.  1   . The successive approximation type ADC  4  in  FIG.  4    illustrates an example in which the differential input signal pair Vin_p, Vin_n are converted into a 5-bit digital signal. Note that the bit depth of the successive approximation type ADC  4  is arbitrary. Furthermore, the circuit configuration of the successive approximation type ADC  4  is not limited to that in  FIG.  4   . 
     The successive approximation type ADC  4  in  FIG.  4    includes a first sampling switch  5 , a second sampling switch  6 , a first digital to analog converter (hereinafter, first DAC)  7 , a second digital to analog converter (second DAC)  8 , the comparator  1 , and a control circuit (SAR logic)  10 . In the present specification, the first DAC  7  and the second DAC  8  are collectively referred to as a capacitive DAC  20 . 
     The first sampling switch  5  switches whether or not to sample one signal Vin_p of the differential input signal pair Vin_p, Vin_n. The second sampling switch  6  switches whether or not to sample the other signal Vin_n of the differential input signal pair Vin_p, Vin_n. 
     The first DAC  7  converts the one sampled signal Vin_p into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to unconverted bits. 
     The first DAC  7  includes five capacitors C 1  to C 5  having different capacitances by powers of two, and three switches (first to third switches) SW 1  to SW 3  connected to each of the capacitors C 1  to C 5 . The first switches SW 1  switch whether or not to set one ends of the corresponding capacitors C 1  to C 5  to 0 V. The second switches SW 2  switch whether or not to set one ends of the corresponding capacitors C 1  to C 5  to a common voltage Vcom. The third switches SW 3  switch whether or not to set one ends of the corresponding capacitors C 1  to C 5  to a reference voltage Vref. The common voltage Vcom is, for example, a voltage level of ½ of the reference voltage Vref. 
     The first to third switches SW 1  to SW 3  are switched on or off on the basis of a control signal from the control circuit  10 . The control circuit  10  turns on the second switch SW 2  at the start of a comparison operation. Thereafter, the control circuit  10  turns on the first switch SW 1  in a case where it is desired to lower the output node voltage Vin_p of the first DAC  7 , and turns on the third switch SW 3  in a case where it is desired to increase the output node voltage Vin_p of the first DAC  7 . 
     The second DAC  8  converts the other sampled signal into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to unconverted bits. The second DAC  8  is configured similarly to the first DAC  7 , and switches the first to third switches SW 1  to SW 3  on the basis of the control signal from the control circuit  10  similarly to the first DAC  7 . 
     The comparator  1  has the configuration illustrated in  FIG.  1   . The differential input signal pair Vin_p, Vin_n in which an output signal of the first DAC  7  and an output signal of the second DAC  8  form a pair are input to the comparator  1 . The comparator  1  outputs signals corresponding to the difference signal of the differential input signal pair Vin_p, Vin_n. 
     The control circuit  10  performs switching control of the first to third switches SW 1  to SW 3  in the first DAC  7  and the second DAC  8  on the basis of the output signals of the comparator  1 . 
     A control signal Clk_adc is input to the ADC  4 . The control signal Clk_adc is inverted by an inverter  9   a  to generate an enable signal En. The enable signal En is input to the gate of the transistor Q 4  in the comparator  1  in  FIG.  1   . When the control signal Clk_adc is at a high level, the enable signal En becomes a low level, and the transistor Q 4  pulls up the tail node connecting the sources of the transistors Q 1 , Q 2  to each other. 
     Furthermore, an inversion signal of the control signal Clk_adc by an inverter  9   b  is input to an AND gate  11 . When the control signal Clk_adc becomes a low level from a high level, the clock signal Clk becomes a high level. Accordingly, the comparator  1  performs a comparison operation. The differential output signal pair Vout_p, Vout_n output from the comparator  1  are input to the control circuit  10  and input to a NOR gate  12 . When one signal of the differential output signal pair Vout_p, Vout_n becomes a high level, the output of the NOR gate  12  becomes a low level. In this case, the output of an AND gate  13  becomes a low level, and the clock signal Clk is fixed at a low level. Accordingly, the comparator  1  is reset. 
     The AND gate  11  calculates a logical product of the signal obtained by inverting the control signal Clk_adc by the inverter  9   b  and the output signal of the NOR gate  12 . The output of the AND gate  11  becomes a high level in a case where the control signal Clk_adc is at a low level and both the differential output signal pair Vout_p, Vout_n of the comparator  1  are at a low level. 
     The AND gate  13  calculates a logical product of the output signal of the AND gate  11  and the signal obtained by inverting a flag signal comp_end of the control circuit  10  by the inverter  14 . The output of the AND gate  13  becomes a high level in a case where the flag signal comp_end of the control circuit  10  is at a low level, and in a case where the output of the AND gate  11  is at a high level. When all control in the capacitive DAC  20  is completed, the control circuit  10  sets the flag signal comp_end to a high level. 
     In the ADC  4  in  FIG.  4   , first, the first sampling switch  5  and the second sampling switch  6  are turned on, and the differential input signal pair Vin_p, Vin_n are sampled to the capacitive DAC  20 . At this time, logic of the control signal Clk_adc is at a high level, and the clock signal Clk input to the comparator  1  is at a low level. 
     Thereafter, when the control signal Clk_adc transitions from the high level to the low level, sampling is completed, and the clock signal Clk transitions from the low level to the high level, and the comparator  1  starts a comparison operation. The control circuit  10  turns on any of the first to third switches SW 1  to SW 3  connected to the capacitor of the most significant bit in the capacitive DAC  20  on the basis of the comparison result by the comparator  1 , and controls the output voltage (Vin_p−Vin_n) of the capacitive DAC  20 . The control circuit  10  controls switching on or off of the first to third switches SW 1  to SW 3  connected to the capacitors C 1  to C 5  bit by bit in order from the high-order side bit in the capacitive DAC  20 . Therefore, the output voltage (Vin_p−Vin_n) of the capacitive DA is gradually approaches zero. 
     In the control circuit  10 , the output of the NOR gate  12  transitions to a low level each time each comparison operation ends. Therefore, the clock signal Clk, which is the output of the AND gate  13 , becomes a low level, and the transistor Q 3  in the comparator  1  is turned off. Therefore, the comparator  1  is reset each time each comparison operation ends. Thereafter, when the output of the AND gate  13  transitions to a high level, and the clock signal Clk becomes a high level, switching on or off of the first to third switches SW 1  to SW 3  connected to the capacitor C 2 , which is a second bit from the most significant bit in the capacitive DAC  20 , is performed. 
     By repeating the above control, the output voltage of the capacitive DAC  20  gradually approaches zero. The capacitive DAC  20  in  FIG.  4    has 5 bits, so that charging and discharging control of the capacitors C 1  to c 5  in the capacitive DAC  20  is performed five times. When the charging and discharging control is completed, the control circuit  10  outputs the flag signal comp_end, the Clk becomes low, and the comparator  1  becomes a reset state. Thereafter, when the control signal Clk_adc transitions from low to high, the differential input signal pair Vin_p, Vin_n are sampled again. 
     Input parasitic capacitances Cin_p, Cin_n exist in a differential input node pair n 1 , n 2  of the comparator  1  in  FIG.  4   . In the embodiment, the tail node connecting the sources of the transistors Q 1 , Q 2  to each other is pulled up to the power supply voltage by the transistor Q 4  during a period in which the comparator  1  does not perform a comparison operation. Therefore, even if there are input parasitic capacitances in the differential input node pair n 1 , n 2 , the input voltage dependence of the parasitic capacitances can be suppressed. 
     Accordingly, the control amount when controlling the capacitive DAC  20  is not affected by an input voltage dependent error of the differential input node pair n 1 , n 2 , so that deterioration of various characteristics such as distortion of the ADC  4  can be suppressed. 
       FIG.  5    is a waveform diagram of the control signal Clk_adc, the enable signal En, and the clock signal Clk in the comparator  1  in  FIG.  4   . As illustrated, the logic of the control signal Clk_adc is opposite to that of the enable signal En. The clock signal Clk intermittently becomes a high level a plurality of times during a period in which the enable signal En is at a high level. The comparator  1  performs a comparison operation during a period in which the clock signal Clk is at a high level. In a case where the capacitive DAC  20  includes five capacitors C 1  to C 5 , the comparator  1  performs the comparison operation five times, and turns on any of the first to third switches SW 1  to SW 3  connected to one ends of the capacitors in order from the capacitor on the high-order side to control charging and discharging of the capacitor. 
       FIG.  6    is a circuit diagram of a successive approximation type ADC  4   a , according to a comparative example, including the comparator  1   a  in  FIG.  2   . The ADC  4   a  in  FIG.  6    has a configuration in which the inverter  9   b  is omitted from the ADC  4  in  FIG.  4   . The enable signal En is not input to the comparator  1   a  in the ADC  4   a  in  FIG.  6   . 
     In the ADC  4   a  in  FIG.  6   , during a sampling period of the differential input signal pair Vin_p, Vin_n, the logic of the control signal Clk_adc is at a high level, and the clock signal Clk input to the comparator  1   a  is at a low level. That is, during the sampling period, the tail node connecting the sources of the transistors Q 1 , Q 2  in the comparator  1  in  FIG.  2    to each other is floating, and the potential is undefined. 
     The input parasitic capacitances Cin_p, Cin_n of the comparator  1  exist in the output signal paths n 1 , n 2  of the capacitive DAC  20  in the ADC  4   a  in  FIG.  6   . In the state where the tail node connecting the sources of the transistors Q 1 , Q 2  in the comparator  1  to each other is in a floating state, the input parasitic capacitances Cin_p, Cin_n have input voltage dependence. More specifically, when the first sampling switch  5  and the second sampling switch  6  are turned on to sample the differential input signal pair Vin_p, Vin_n, the differential input signal pair Vin_p, Vin_n are sampled also in the input parasitic capacitances Cin_p, Cin n. Charges sampled in the input parasitic capacitances Cin_p, Cin_n are determined according to capacitance values of the input parasitic capacitances Cin_p and Cin_n. However, when the input parasitic capacitances Cin_p and Cin_n have input voltage dependence, charges stored in the input parasitic capacitances Cin_p and Cin_n also have input voltage dependence. This dependence of the charges gives an input voltage dependent error to the control amount when controlling the capacitive DAC  20 , and also gives an input voltage dependent error to the conversion result of the ADC  4   a . As a result, there is a possibility that deterioration of various characteristics such as distortion of the ADC  4   a  is caused. 
     On the other hand, in the comparator  1  in  FIG.  1   , the tail node connecting the sources of the transistors Q 1 , Q 2  in the comparator  1  to each other is pulled up to the power supply voltage by the transistor Q 4  at the time of sampling the differential input signal pair Vin_p, Vin_n. Accordingly, there is no possibility that the input parasitic capacitances Cin_p and Cin_n have input voltage dependence, so that the capacitive DAC  20  can be controlled with high accuracy. 
     In this manner, in the comparator  1  according to the first embodiment, the tail node connecting the sources of the transistors Q 1 , Q 2 , which generate signals corresponding to the difference signal of the differential input signal pair Vin_p, Vin_n, to each other is pulled up to the power supply voltage level before the comparator  1  starts the comparison operation. Therefore, the input parasitic capacitances Cin_p, Cin_n of the comparator  1  do not have input voltage dependence, so that the comparison operation by the comparator  1  can be performed with high accuracy. Accordingly, AD conversion accuracy of the ADC  4  incorporating the comparator  1  can also be improved. 
     Second Embodiment 
     A comparator  1   b  according to a second embodiment is obtained by reversing the conductivity type of each transistor in the comparator  1   b  from that in the comparator  1  in  FIG.  1   . 
       FIG.  7    is a circuit diagram of the comparator  1   b  according to the second embodiment. The comparator  1   b  in  FIG.  7    includes transistors Q 21  to Q 24 . The transistors Q 21  to Q 23  are P-type MOS transistors, and the transistor Q 24  is an N-type MOS transistor. An inversion signal xEn of the enable signal En in  FIG.  1    is input to a gate of the transistor Q 24 . An inversion signal of the clock signal Clk in  FIG.  1    is input to a gate of the transistor Q 23 . 
     In addition, the comparator  1   b  in  FIG.  7    includes a pull-down circuit  15  and a latch circuit  3   a . The pull-down circuit  15  includes a transistor Q 45  and a transistor Q 46  connected between respective drains of the first and the transistor Q 2  and the ground terminals. The inversion signal xClk of the clock signal Clk is input to each gate of the transistor Q 45  and the transistor Q 46 . The transistor Q 45  and the transistor Q 46  are N-type MOS transistors. 
     The latch circuit  3   a  includes transistors Q 27  to Q 33 . Among them, the transistors Q 27  to Q 30  are P-type MOS transistors, and the transistors Q 31  to Q 33  are N-type MOS transistors. 
     In the comparator  1   b  in  FIG.  7   , the conductivity type of each transistor is opposite to that in the comparator  1  in  FIG.  1   , and connection order of the transistors connected between the power supply voltage node and the ground node is opposite to that in the comparator  1  in  FIG.  1   , but the comparison operation itself is the same. 
     When the xClk transitions to a low level, potentials of the differential output voltage pair Vgm_p, Vgm_n increase due to charging by the transistors Q 1 , Q 2 . When “Vin_p&gt;Vin_n”, “charge speed of Vgm_p&lt;charge speed of Vgm_n” is obtained. On the other hand, when “Vin_p&lt;Vin_n”, “charge speed of Vgm_p&gt;charge speed of Vgm_n” is obtained. In this manner, the difference signal (Vin_p−Vin_n) of the differential input voltage pair of the comparator  1   b  causes a difference in charge speed between Vgm_p and Vgm_n. 
     Moreover, logic of the latch circuit  3   a  in the subsequent stage is determined according to the difference in charge speed. If “charge speed of Vgm_p&lt;charge speed of Vgm_n”, Vout_p=High/Vout_n=Low is obtained, and if “charge speed of Vgm_p&gt;charge speed of Vgm_n”, Vout_p=Low/Vout_n=High is obtained. 
     The transistor Q 24  is provided at the tail node connecting sources of the transistors Q 1 , Q 2  in the comparator  1   b  in  FIG.  7    to each other. The transistor Q 24  is controlled by the inversion signal xEn of the enable signal En. The transistor Q 24  is turned on to be pulled down to the ground level while the ADC  4  performs sampling. In this state, the voltage dependence of the input parasitic capacitances Cin_p and Cin_n of the comparator  1   b  can be sufficiently reduced. Therefore, deterioration of various characteristics such as distortion of the ADC  4  can be prevented. 
     Third Embodiment 
     In a third embodiment, configurations of a pull-up circuit  2  and a latch circuit  3  are different from those in  FIG.  1   . 
       FIG.  8    is a circuit diagram of a comparator  1   c  according to the third embodiment. The comparator  1   c  in  FIG.  8    includes N-type MOS transistors Q 1  to Q 4 , Q 14  to Q 15 , and P-type MOS transistors Q 16  to Q 19 . 
     The transistors Q 16 , Q 17  constitute a pull-up circuit  2   a . In  FIG.  1   , the pull-up circuit  2  is connected to the drains of the transistors Q 1 , Q 2 , but in  FIG.  8   , the pull-up circuit  2   a  is connected to a first output node and a second output node Vout_p, Vout_n. 
     A latch circuit  3   b  in  FIG.  8    includes the transistors Q 14 , Q 15 , Q 18 , and Q 19 . Each gate of the transistors Q 14 , Q 18  and each drain of the transistors Q 15 , Q 19  are connected to the first output node Vout_p. Each gate of the transistors Q 15 , Q 19  and each drain of the transistors Q 14 , Q 18  are connected to the second output node Vout_n. 
     Also in the comparator  1   c  in  FIG.  8   , each drain of the transistors Q 3 , Q 4  is connected to the tail node connecting the respective sources of the transistors Q 1 , Q 2 , and when the enable signal En is at a low level, the transistor Q 4  is turned on to pull up the tail node to the power supply voltage level. Therefore, the voltage dependence of the input parasitic capacitances Cin_p and Cin_n of the comparator  1   c  can be sufficiently reduced, so that deterioration of various characteristics such as distortion of the ADC  4  can be prevented. 
     Fourth Embodiment 
     In a fourth embodiment, a differential signal output pair output from the respective drains of the transistors Q 1 , Q 2  are waveform-shaped and then input to a latch circuit  3 . 
       FIG.  9    is a circuit diagram of a comparator  1   d  according to the fourth embodiment. The comparator  1   d  in  FIG.  9    includes inverters  16 ,  17  connected to the drains of the transistors Q 1 , Q 2 . Output signals of the inverters  16 ,  17  are input to a latch circuit  3   c . The latch circuit  3   c  includes N-type MOS transistors Q 14  to Q 17 , Q 34 , and Q 35 , and P-type MOS transistors Q 18 , Q 19 . 
     The latch circuit  3   c  in  FIG.  9    has a configuration in which the transistors Q 34 , Q 35  are added to the latch circuit  3   b  in  FIG.  8   . The transistor Q 34  is connected between the respective drains of the transistors Q 14 , Q 18 . The transistor Q 35  is connected between the respective drains of the transistors Q 15 , Q 19 . 
     Output signals of the inverters  16 ,  17  are input to the respective gates of the transistors Q 34 , Q 35 . 
     The inverters  16 ,  17  perform waveform shaping to steepen waveforms of the differential output signal pair Vout_p, Vout_n output from the respective drains of the transistors Q 1 , Q 2 . By inputting the differential output signal pair Vout_p, Vout_n to the latch circuit  3   c  via the inverters  16 ,  17 , a latch operation of the latch circuit  3   c  can be speeded up. 
     Also in the comparator  1   d  in  FIG.  9   , each drain of the transistors Q 3 , Q 4  is connected to the tail node connecting the respective sources of the transistors Q 1 , Q 2 , and when the enable signal En is at a low level, the transistor Q 4  is turned on to pull up the tail node to the power supply voltage level. Therefore, the voltage dependence of the input parasitic capacitances Cin_p and Cin_n of the comparator  1  can be sufficiently reduced, so that deterioration of various characteristics such as distortion of the ADC  4  can be prevented. 
     Fifth Embodiment 
     The comparators  1  to  1   d  in  FIGS.  1  and  7  to  9    described above can be used in the ADC  4  in  FIG.  4   , but can also be applied to other types of ADCs, and there may be a case where internal configurations of the comparators  1  to  1   d  need to be partially changed in accordance with the configuration of the ADC. 
       FIG.  10    is a circuit diagram of an ADC  4   b  including a filter circuit  21  in addition to the capacitive DAC  20 . The filter circuit  21  in  FIG.  10    samples a differential output signal pair Vin_p, Vin_n output from the capacitive DAC  20 . The first differential output signal pair Vin_p, Vin_n output from the capacitive DAC  20  and a second differential output signal pair Vns_p, Vns_n output from the filter circuit  21  are input to a comparator  1   e  in the ADC  4   b  in  FIG.  10   . In this manner, the comparator  1   e  in  FIG.  10    includes terminals to which two differential input signal pairs (Vin_p, Vin_n), (Vns_p, Vns_n) are input, and outputs signals corresponding to the difference signals of the respective differential input signal pairs (Vin_p, Vin_n), (Vns_p, Vns_n). 
     The ADC  4   b  in  FIG.  10    includes the filter circuit  21 . Accordingly, the differential output signals, of the capacitive DAC  20 , that have not become zero are sampled by the filter circuit  21 , and are input to the comparator  1   e . Therefore, accuracy of analog to digital conversion can be further improved. 
       FIG.  11    is a circuit diagram illustrating an example of an internal configuration of the comparator  1   e  used in the ADC  4   b  in  FIG.  10   . The comparator  1   e  in  FIG.  11    includes a first input terminal TL 1  and a second input terminal TL 2  to which the first differential input signal pair Vin_p, Vin_n are input, a third input terminal TL 3  and a fourth input terminal TL 4  to which the second differential input signal pair Vns_p, Vns_n are input, and a comparison circuit  22 . 
     The comparison circuit  22  outputs signals corresponding to a difference signal of the first differential input signal pair Vin_p, Vin_n input to the first input terminal TL 1  and the second input terminal TL 2  and a difference signal of the second differential input signal pair Vns_p, Vns_n input to the third input terminal TL 3  and the fourth input terminal TL 4 . 
     The comparison circuit  22  outputs, to the first output node n 1  and the second output node n 2 , a first differential output signal pair, corresponding to the difference signal of the first differential input signal pair Vin_p, Vin_n, generated by connecting the first input terminal TL 1  to a positive side and connecting the second input terminal TL 2  to a negative side. The voltage fluctuation amount of the third input terminal TL 3  generated according to the voltage fluctuation of the first output node n 1  is equal to the voltage fluctuation amount of the fourth input terminal TL 4  generated according to the voltage fluctuation of the second output node n 2 . 
     The comparison circuit  22  includes a first comparison device  24  and a second comparison device  25 . 
     The first comparison device  24  outputs, to the first output node n 1  and the second output node n 2 , the first differential output signal pair, corresponding to the difference signal of the first differential input signal pair Vin_p, Vin_n, generated by connecting the first input terminal TL 1  to the positive side and connecting the second input terminal TL 2  to the negative side. 
     The second comparison device  25  outputs, from the first output node n 1  and the first output node n 2 , the second differential output signal pair, corresponding to the difference signal of the second differential input signal pair, generated by connecting the third input terminal TL 3  to the positive side and connecting the fourth input terminal TL 4  to the negative side. 
     The first comparison device  24  includes N-type MOS transistors Q 41 , Q 42 . The first differential input signal Vin_p is input to a gate of the transistor Q 41 . The gate of the transistor Q 41  is the positive side. The first differential input signal Vin_n is input to a gate of the transistor Q 42 . The gate of the transistor Q 42  is the negative side. 
     An N-type MOS transistor Q 43  is connected between respective sources of the transistors Q 41 , Q 42  and a ground node. A clock signal Clk is input to a gate of the transistor Q 43 . The transistors Q 41 , Q 42  perform a comparison operation of the first differential input signal pair Vin_p, Vin_n when the clock signal Clk is at a high level, and stop the comparison operation when the clock signal Clk is at a low level. A drain of the transistor Q 41  is connected to the first output node n 1 , and a drain of the transistor Q 42  is connected to the second output node n 2 . 
     Furthermore, a P-type MOS transistor Q 68  is connected between the sources of the transistors Q 41 , Q 42  and a power supply voltage node (second reference voltage node). An enable signal En is input to a gate of the transistor Q 68 . The transistor Q 68  performs the same operation as the transistor Q 4  in  FIG.  1   . 
     The second comparison device  25  includes N-type MOS transistors Q 44 , Q 45 . The first differential input signal Vns_p is input to a gate of the transistor Q 44 . The gate of the transistor Q 44  is the positive side. The second differential input signal Vns_n is input to a gate of the transistor Q 45 . The gate of the transistor Q 45  is the negative side. 
     An N-type MOS transistor Q 46  is connected between sources of the transistors Q 44 , Q 45  and a ground node. A clock signal Clk is input to a gate of the transistor Q 46 . The transistors Q 41 , Q 42  perform a comparison operation of the first differential input signal pair Vin_p, Vin_n when the clock signal Clk is at a high level, and stop the comparison operation when the clock signal Clk is at a low level. Respective drains of the transistors Q 44 , Q 45  are connected to the first output node n 1  and the second output node n 2 . 
     Furthermore, a P-type MOS transistor Q 69  is connected between the sources of the transistors Q 44 , Q 45  and a power supply voltage node (second reference voltage node). An enable signal En is input to a gate of the transistor Q 69 . The transistor Q 69  performs the same operation as the transistor Q 4  in  FIG.  1   . 
     A pull-up circuit (first voltage setting circuit)  26  is connected to the first output node n 1  and the second output node n 2 . The pull-up circuit  26  pulls up the first output node n 1  and the second output node n 2  to a high level when the clock signal Clk is at a low level, that is, during a period in which the comparison circuit  22  does not perform a comparison operation. The pull-up circuit  26  includes a P-type MOS transistor Q 47  connected to the first output node n 1 , and a P-type MOS transistor Q 48  connected to the second output node n 2 . A clock signal Clk is input to gates of the transistors Q 47 , Q 48 . 
     A latch circuit  30  includes P-type MOS transistors Q 57  to Q 60 , and N-type MOS transistors Q 61  to Q 66 . The first output node n 1  is connected to each gate of the transistors Q 57 , Q 61 , and Q 62 . The second output node n 2  is connected to each gate of the transistors Q 58 , Q 64 , and Q 65 . Each gate of the transistors Q 60  and Q 66  and each drain of the transistors Q 59 , Q 63  are connected to an output terminal TL 5  that outputs a differential output voltage Vout_p of the comparator  1   e . Each gate of the transistors Q 59  and Q 63  and each drain of the transistors Q 65 , Q 66  are connected to an output terminal TL 6  that outputs a differential output voltage Vout_n of the comparator  1   e.    
     In the comparator  1   e  in  FIG.  11   , similar to the comparators  1  to  1   d  according to the first to fourth embodiments described above, the tail node in each comparison device is pulled up during a period in which the comparator  1   e  does not perform a comparison operation. Accordingly, the voltage dependence of the input parasitic capacitances Cin_p and Cin_n of the comparator  1   e  can be sufficiently reduced, so that deterioration of various characteristics such as distortion of the ADC  4   b  can be prevented. 
     Note that the present technology can have the following configurations. 
     (1) A comparator including: a first transistor and a second transistor that include two sources connected to each other, two gates to which a differential input signal pair are input, and two drains that output a differential output signal pair corresponding to a difference signal of the differential input signal pair; 
     a third transistor that is connected between both the sources of the first transistor and the second transistor and a first reference voltage node, the third transistor being switched on or off in accordance with logic of a first signal; and 
     a fourth transistor that is connected between both the sources of the first transistor and the second transistor and a second reference voltage node, the fourth transistor being switched on or off in accordance with logic of a second signal having logic different from the logic of the first signal. 
     (2) The comparator described in (1), in which the fourth transistor is turned on in a period in which the first transistor and the second transistor do not perform a comparison operation of the difference signal of the differential input signal pair. 
     (3) The comparator described in (1) or (2), in which the third transistor is intermittently turned on during a period in which the fourth transistor is off. 
     (4) The comparator described in any one of (1) to (3), further including a latch circuit that holds the differential output signal pair. 
     (5) The comparator described in (4), further including a waveform shaping circuit that performs waveform shaping of the differential output signal pair output from both the drains of the first transistor and the second transistor, 
     in which a signal after waveform shaping performed by the waveform shaping circuit is input to the latch circuit. 
     (6) The comparator described in (5), in which the waveform shaping circuit includes two inverters that invert logic of the differential output signal pair. 
     (7) The comparator described in any one of (4) to (6)), further including a fifth transistor that switches whether or not to perform a holding operation by the latch circuit in accordance with the logic of the first signal, 
     in which the fifth transistor is intermittently turned on during a period in which the fourth transistor is off. 
     (8) The comparator described in any one of (1) to (7), in which the fourth transistor has a conductivity type different from a conductivity type of the third transistor. 
     (9) The comparator described in any one of (1) to (8), in which the first transistor, the second transistor, and the third transistor include N-type MOS transistors, and 
     the fourth transistor includes a P-type MOS transistor. 
     (10) The comparator described in any one of (1) to (8), in which the first transistor, the second transistor, and the third transistor include P-type MOS transistors, and 
     the fourth transistor includes an N-type MOS transistor. 
     (11) The comparator described in any one of (1) to (10), further including: 
     a first input terminal and a second input terminal to which a first differential input signal pair are input; 
     a third input terminal and a fourth input terminal to which a second differential input signal pair are input; and 
     a comparison circuit that outputs a signal corresponding to a difference signal of the first differential input signal pair input to the first input terminal and the second input terminal and a difference signal of the second differential input signal pair input to the third input terminal and the fourth input terminal, 
     in which the comparison circuit includes: 
     a first comparison device that includes sixth to ninth transistors having a same circuit configuration as a circuit configuration of the first to fourth transistors; and 
     a second comparison device that includes tenth to thirteenth transistors having the same circuit configuration as the circuit configuration of the first to fourth transistors. 
     (12) An analog to digital converter including: a first sampling switch that switches whether or not to sample one signal of a differential input signal pair; 
     a first digital to analog converter that converts the one signal sampled into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to an unconverted bit; 
     a second sampling switch that switches whether or not to sample another signal of the differential input signal pair; 
     a second digital to analog converter that converts the another signal sampled into a digital signal including a plurality of bits bit by bit in order, and outputs a signal having a voltage level corresponding to an unconverted bit; 
     a comparator that outputs a signal corresponding to a difference signal of a first differential input signal pair in which an output signal of the first digital to analog converter and an output signal of the second digital to analog converter form a pair; and 
     a control circuit that controls the first digital to analog converter and the second digital to analog converter on the basis of an output signal of the comparator, 
     in which the comparator includes: 
     a first transistor and a second transistor that include two sources connected to each other, two gates to which the first differential input signal pair are input, and two drains that output a differential output signal pair corresponding to the difference signal of the first differential input signal pair; 
     a third transistor that is connected between both the sources of the first transistor and the second transistor and a first reference voltage node, the third transistor being switched on or off in accordance with logic of a first signal; and 
     a fourth transistor that is connected between both the sources of the first transistor and the second transistor and a second reference voltage node, the fourth transistor being switched on or off in accordance with logic of a second signal having logic different from the logic of the first signal. 
     (13) The analog to digital converter described in (12), further including a filter circuit that samples and outputs the output signal of the first digital to analog converter and the output signal of the second digital to analog converter, 
     in which the comparator outputs a signal corresponding to the difference signal of the first differential input signal pair in which the output signal of the first digital to analog converter and the output signal of the second digital to analog converter form a pair and a difference signal of a second differential input signal pair output from the filter circuit, 
     the comparator includes: 
     a first input terminal and a second input terminal to which the first differential input signal pair are input; 
     a third input terminal and a fourth input terminal to which the second differential input signal pair are input; and 
     a comparison circuit that outputs a signal corresponding to the difference signal of the first differential input signal pair input to the first input terminal and the second input terminal and the difference signal of the second differential input signal pair input to the third input terminal and the fourth input terminal, and 
     the comparison circuit includes the first to fourth transistors. 
     Aspects of the present disclosure are not limited to individual embodiments described above, but include various modifications that can be conceived by those skilled in the art, and advantageous effects of the present disclosure are not limited to the contents described above. That is, various additions, modifications, and partial deletions can be made without departing from the conceptual idea and spirit of the present disclosure derived from the contents defined in claims and equivalents thereof. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1   a ,  1   b ,  1   c ,  1   d ,  1   e  Comparator 
           2  Pull-up circuit 
           3  Latch circuit 
           4 ,  4   a ,  4   b  ADC 
           5  First sampling switch 
           6  Second sampling switch 
           7  First DAC 
           8  Second DAC 
           9   a ,  9   b  Inverter 
           10  Control circuit 
           11  AND gate 
           12  NOR gate 
           13  AND gate 
           14  Inverter 
           20  Capacitive DAC 
           21  Filter circuit 
           22  Comparison circuit 
           30  Latch circuit