Patent Publication Number: US-10763875-B2

Title: Switched capacitor circuit and analog-to-digital converter device

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 62/791,128, filed Jan. 11, 2019, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a switched capacitor circuit. More particularly, the present disclosure relates to a switched capacitor circuit that provides a noise shaping function and an analog-to-digital converter using the same. 
     Description of Related Art 
     An analog-to-digital converter (ADC) has been widely applied to various electronic devices, in order to covert an analog signal to a digital signal for subsequent signal processing. As the need of processing data with high resolution (for example, video data) rises, the ADC is often the key component in the system. However, in practical applications, performance of the ADC is affected by several non-ideal factors, such as process variations, quantization noise, thermal noise, and so on. 
     SUMMARY 
     Some aspects of the present disclosure are to provide a switched capacitor circuit that includes a first capacitor, a second capacitor, and a switching circuit. The first capacitor is configured to receive a first signal. The second capacitor is configured to receive a second signal. The switching circuit is configured to selectively couple the first capacitor and the second capacitor to an input terminal of a quantizer according to at least one clock signal. In a first configuration of the switching circuit, the first capacitor is configured to store the first signal, and the second capacitor is configured to store the second signal. In a second configuration of the switching circuit, the first capacitor and the second capacitor are stacked in series, in order to transmit a combination of the first signal and the second signal to the input terminal of the quantizer. 
     Some aspects of the present disclosure are to provide an analog-to-digital converter (ADC) device that includes a switched capacitor circuit and a successive approximation register (SAR) circuitry. The switched capacitor circuit is configured to sample an input signal according to a plurality of clock signals. The SAR circuitry is configured to perform an analog-to-digital conversion on a sampled input signal according to a conversion clock signal, in order to generate a digital output. The switched capacitor circuit includes a first capacitor and a second capacitor. The first capacitor is configured to store a first residue signal associated with the sample input signal. The second capacitor configured to store a second residue signal that is generated based on the first residue signal in a previous conversion phase, in which the first capacitor and the second capacitor are stacked in series, in order to provide a combination of the first residue signal and the second residue signal to the SAR circuitry in the analog-to-digital conversion. 
     As described above, the switched capacitor circuit and the ADC device of embodiments of the present disclosure are able to provide a circuit architecture that has a noise-shaping function. As a result, the overall performance of the ADC device can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a switched capacitor circuit according to some embodiments of the present disclosure. 
         FIG. 2A  is a schematic diagram of an analog-to-digital converter (ADC) device according to some embodiments of the present disclosure. 
         FIG. 2B  is a schematic diagram illustrating waveforms of signals in  FIG. 2A  or  FIG. 3  according to some embodiments of the present disclosure. 
         FIG. 3  is a schematic diagram of an ADC device according to some embodiments of the present disclosure. 
         FIG. 4  is a schematic diagram of the comparator circuit in  FIG. 2A  or  FIG. 3  according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following embodiments are disclosed with accompanying diagrams for detailed description. For illustration clarity, many details of practice are explained in the following descriptions. However, it should be understood that these details of practice do not intend to limit the present disclosure. That is, these details of practice are not necessary in parts of embodiments of the present embodiments. Furthermore, for simplifying the drawings, some of the conventional structures and elements are shown with schematic illustrations. 
     In this document, the term “coupled” may also be termed as “electrically coupled,” and the term “connected” may be termed as “electrically connected.” “Coupled” and “connected” may mean “directly coupled” and “directly connected” respectively, or “indirectly coupled” and “indirectly connected” respectively. “Coupled” and “connected” may also be used to indicate that two or more elements cooperate or interact with each other. 
     In this document, the term “circuitry” may indicate a system formed with one or more circuits. The term “circuit” may indicate an object, which is formed with one or more transistors and/or one or more active/passive elements based on a specific arrangement, for processing signals. 
     For ease of understanding, like elements in each figure are designated with the same reference number. 
       FIG. 1  is a schematic diagram of a switched capacitor circuit  100  according to some embodiments of the present disclosure. In some embodiments, the switched capacitor circuit  100  may be applied to mixed signal circuit. 
     For example, the switched capacitor circuit  100  may be applied to an analog-to-digital converter (ADC), in order to provide a noise shaping function. In some embodiments, the noise shaping function is a feedback of the quantization error to an input of the quantizer (e.g., quantizer  100 A in  FIG. 1  or comparator circuit  220  in  FIG. 2A ). With the noise shaping function, the spectral characteristics of the quantization noise can be changed (e.g., shaped), and the power of the quantization noise is at a lower level in lower frequency band and is at a higher level in higher frequency band. Thus, a desired signal in the lower frequency band may present higher signal-to-noise ratio. 
     As shown in  FIG. 1 , the switched capacitor circuit  100  is coupled to a quantizer  100 A (e.g., a comparator in an ADC (not shown)). The switched capacitor circuit  100  includes capacitors C 1 -C 2  and a switching circuit  120 . In some embodiments, each of the capacitor C 1  and the capacitor C 2  may be formed with a single capacitive element or an array of capacitive elements. The capacitors C 1 -C 2  are coupled to the switching circuit  120 . The switching circuit  120  may include switches (e.g., switches in  FIG. 2A ), in order to selectively couple the capacitor C 1  and/or the capacitor C 2  to an input terminal of the quantizer  100 A according to at least one clock signal (e.g., clock signals in  FIG. 2B ). 
     For example, in response to the at least one clock signal, the switching circuit  120  may operate in a first configuration CF 1  or a second configuration CF 2 . In the first configuration CF 1 , the capacitors C 1  and C 2  may be disconnected from the input terminal of the quantizer  100 A. Under this condition, the capacitor C 1  is configured to store a signal V in1 , and the capacitor C 2  is configured to store a signal V in2 . 
     In some embodiments, the signal V in1  may be a signal from an input signal Vin sampled in k-th conversion phase (hereinafter refer to as “signal Vin(k)”). In some embodiments, the signal V in1  may be a signal processed based on the signal Vin(k). For example, the signal V in1  may be a residue signal that is generated by an ADC based on the signal Vin(k). 
     In some embodiments, the signal V in2  may be a signal from a signal Vin sampled in a conversion phase prior to the k-th conversion phase. For example, the signal V in2  may be a signal Vin(k−1) (e.g., signal sampled from the input signal Vin in a (k−1)-th conversion phase). In some embodiments, the signal V in2  may be a signal processed based on the signal Vin(k−1). For example, the signal V in2  may be a residue signal that is generated by an ADC based on the signal Vin(k−1). In some embodiments, the signal V in1  may be a signal processed based on the signal Vin(k−1), Vin(k−2), . . . , and Vin(k−n), where k&gt;n&gt;0. 
     In the second configuration CF 2 , the capacitors C 1  and C 2  may be coupled in series and coupled to the input terminal of the quantizer  100 A. Under this condition, a combination of the signals V in1  and V in2  are transmitted to the quantizer  100 A for subsequent processing (e.g., analog-to-digital (A/D) conversion). Equivalently, a model of noise shaping may be introduced to the quantizer  100 A. As a result, a signal-to-noise ratio of the output of the quantizer  100 A can be increased. 
     In some embodiments, the combination of the signal V in1  and V in2  may be an integration of the signals V in1  and V in2 . In some embodiments, the combination of the signals V in1  and V in2  may be a summation of the signals V in1  and V in2 . In some embodiments, the combination of the signals V in1  and V in2  may be a difference between the signals V in1  and V in2 . The above configurations of the signals V in1  and V in2  are given for illustrative purposes only, and the present disclose is not limited thereto. 
     Reference is made to  FIG. 2A .  FIG. 2A  is a schematic diagram of an ADC device  200  according to some embodiments of the present disclosure. In some embodiments, the switched capacitor circuit  100  in  FIG. 1  may be applied to the ADC device  200 . 
     In this example, the ADC device  200  operates as a successive approximation register (SAR) ADC. The ADC device  200  includes the switched capacitor circuit  100  and a SAR circuitry  201  that includes a comparator circuit  220 , and a control logic circuit  240 . 
     In this example, the switched capacitor circuit  100  includes capacitors C 1 -C 3  and the switching circuit  120 . The capacitor C 1  is formed with a binary capacitor array, and the binary capacitor array includes capacitors and switches controlled by the control logic circuit  240 . A first terminal of the capacitor C 1  is configured to receive the input signal Vin and is coupled to a node N 1  that is between the first terminal of the capacitor C 1  and a first terminal of the capacitor C 2 . A second terminal of the capacitor C 1  is configured to selectively receive common mode voltages Vrefn or Vrefp under control of the control logic circuit  240 . The second terminal of the capacitor C 2  is coupled a first input terminal (e.g., a positive input terminal) of the comparator circuit  220 . A second input terminal (e.g., a negative input terminal) of the comparator circuit  220  is coupled to ground. In some embodiments, the ground may be AC ground. 
     The capacitor C 1  is configured to sample the input signal Vin and to generate reference voltages to the first input terminal of the comparator circuit  220  based on a binary search algorithm and the common voltages Vrefn and Vrefp. In some embodiments, the binary search algorithm is performed under control of the control logic circuit  240 . The comparator circuit  220  and the control logic circuit  240  are enabled by a clock signal ϕ C  (e.g., a conversion clock signal) to perform operations of the binary search algorithm, in order to perform an analog-to-digital (A/D) conversion on the sampled signal Vin to decide a digital output Dout. 
     In some embodiments, the control logic circuit  240  may be implemented with a digital processing circuit and/or a digital logic circuit that performs the binary search algorithm, but the present disclosure is not limited thereto. 
     The switching circuit  120  includes switches S 1 -S 5 . A first terminal of the switch S 1  is configured to receive the input signal Vin, a second terminal of the switch S 1  is coupled to the first terminal of the capacitor C 1 , and a control terminal (not shown) of the switch S 1  is configured to receive a clock signal ϕ S . A first terminal of the switch S 2  is coupled to ground, a second terminal of the switch S 2  is coupled to the node N 1  via the switch S 3 , and a control terminal (not shown) of the switch S 2  is configured to receive the clock signal ϕ S . A first terminal of the capacitor C 3  is coupled to the second terminal of the switch S 2 , and a second terminal of the capacitor C 3  is coupled to ground. 
     With this configuration, the switches S 1 -S 2  are closed (e.g., conducted) in response to an enabling level of the clock signal ϕ S . Under this condition, the input signal Vin is sampled on the capacitor C 1 , and the capacitor C 3  is reset to a ground level. 
     In response to an enabling level of the clock signal ϕ c , the comparator circuit  220  and the control logic circuit  240  perform the A/D conversion. In some embodiments, when the clock signal ϕ S  has the enabling level, the clock signal ϕ c  has a disabling level. Under this condition, the comparator circuit  220  is disabled, and thus provides high impedance at the first input terminal of the comparator circuit  220 . Accordingly, when the input signal Vin is sampled to the capacitor C 1  in response to the enabling level of the clock signal ϕ S , a signal path from the capacitor C 2  to the comparator circuit  220  may be considered as an open circuit, and thus the sampling of the signal Vin is not affected by the capacitor C 2 . 
     In some alternative embodiments, an additional switch (not shown) may be employed to provide the above high impedance. For example, the additional switch is coupled between the node N 1  and the first terminal of the capacitor C 2  (or between the node N 1  and the first terminal of the switch S 5 ), and is open (e.g., not conducted) in response to the enabling level of the clock signal ϕ S  to provide the above high impedance. The additional switch is closed during the A/D conversion. 
     In some embodiments, the clock signal ϕ c  may be a group of synchronous clock signals. In some embodiments, the clock signal ϕ c  may be a group of asynchronous clock signals. Various settings of the clock signal ϕ c  are within the contemplated scope of the present disclosure. 
     A first terminal of the switch S 3  is coupled to the node N 1 , a second terminal of the switch S 3  is coupled to the first terminal of the capacitor C 3 , and a control terminal (not shown) of the switch S 3  is configured to receive a clock signal ϕ cs0 . With this configuration, the switch S 3  is closed in response to an enabling level of the clock signal ϕ cs0 . Under this condition, the capacitor C 3  is coupled to the capacitor C 1  via the conducted switch S 3 , in order to store a residue signal Vres 1 . In some embodiments, the residue signal Vres 1  is generated in the A/D conversion or after the A/D conversion is completed. 
     A first terminal of the switch S 4  is coupled to the second terminal of the capacitor C 2 , a second terminal of the switch S 4  is coupled to the first terminal of the capacitor C 3 , and a control terminal (not shown) of the switch S 4  is configured to receive a clock signal Φ cs1 . A first terminal of the switch S 5  is coupled to the node N 1 , a second terminal of the switch S 5  is coupled to ground, and a control terminal (not shown) of the switch S 5  is configured to receive a clock signal Φ cs1 . With this configuration, the switches S 4  and S 5  are closed in response to an enabling level of the clock signal Φ cs1 . Under this condition, the capacitor C 2  is coupled to the capacitor C 3  via the conducted switch S 4 , and the capacitor C 3  carrying the residue signal Vres 1  and the capacitor C 2  are configured to share charges. After the charge sharing of the capacitors C 2  and C 3  is settled, each of the capacitors C 2  and C 3  stores a residue signal Vres 2  (e.g., residue signal Vres 2 ( k −1) shown in  FIG. 2A ). 
     Reference is made to both of  FIGS. 2A and 2B .  FIG. 2B  is a schematic diagram illustrating waveforms of signals in  FIG. 2A  or  FIG. 3  according to some embodiments of the present disclosure. 
     As shown in  FIG. 2B , in some embodiments, a time interval of the clock signal ϕ c  having the enabling level (e.g., high level) is configured to follow a time interval of the clock signal ϕ s  having the enabling level (e.g., high level). In other words, the time interval of the SAR circuitry  201  performing the A/D conversion follows the time interval of the switches S 1  and S 2  being conducted (e.g., the time interval of the input signal Vin being sampled). 
     In some embodiments, a time interval of the clock signal ϕ cs0  having the enabling level (e.g., high level) is configured to follow the time interval of the clock signal ϕ c  having the enabling level. In other words, a time interval of the switch S 3  being conducted follows the time interval of the SAR circuitry  201  performing the A/D conversion. 
     In some embodiments, a time interval of the clock signal ϕ cs1  having the enabling level (e.g., high level) is configured to follow the time interval of the clock signal ϕ cs0  having the enabling level. In other words, a time interval of the switches S 4 -S 5  being conducted follows the time interval of the switch S 3  being conducted. 
     In phase k−1, when the clock signal ϕ cs1  has the enabling level, the switches S 4  and S 5  are closed. Under this condition, a residue signal Vres 2 ( k− 1) is applied to the capacitors C 2  and C 3 . In some embodiments, the residue signal Vres 2 ( k− 1) is generated from the result of charge sharing of the residue signal Vres 1 ( k− 1) on the capacitor C 3  and the residue signal Vres 2 ( k− 2) on the capacitor C 2 . The residue signal Vres 1 ( k− 1) indicates the aforementioned residue signal Vres 1  generated in phase k−1. By this analogy, the residue signal Vres 2 ( k− 1) indicates the aforementioned residue signal Vres 2  generated in phase k−1, and the residue signal Vres 2 ( k− 2) indicates the aforementioned residue signal Vres 2  generated in phase k−2 (e.g., a phase prior to the phase k−1). 
     In phase k, when the clock signal ϕ s  has the enabling level (e.g., high level), the switches S 1  and S 2  are closed. Under this condition, the signal Vin(k) is sampled by the capacitor C 1 . Then, when the clock signal ϕ c  has the enabling level, the comparator circuit  220  and the control logic circuit  240  are enabled to perform the A/D conversion on the sampled signal Vin(k). In the A/D conversion, the sampled signal Vin(k) is then processed to be the residue signal Vres 1 ( k ). Under this situation, as shown in  FIG. 2A , the capacitors C 1  and C 2  are stacked to provide a summation of the residue signals Vres 1 ( k ) and Vres 2 ( k− 1) to the first input terminal of the comparator circuit  220 . Equivalently, the comparator circuit  220  quantizes the summation of the residue signals Vres 1 ( k ) and Vres 2 ( k− 1) to generate the corresponding digital output Dout(k). As a result, a noise transfer function having the characteristic of noise shaping of the ADC device  200  can be obtained. 
     In some embodiments, the residue signal Vres 1 ( k ) is varied in the A/D conversion. In some embodiments, in phase k, the residue signal Vres 1 ( k ) is varied when the clock signal ϕ c  has the enabling level. 
     Reference is made to  FIG. 3 .  FIG. 3  is a schematic diagram of an ADC device  300  according to some embodiments of the present disclosure. 
     Compared to  FIG. 2A , the switching circuit  120  in  FIG. 3  further includes a switch S 6 , and the connections of the switches S 4 -S 5  are changed. A first terminal of the switch S 6  is coupled to the node N 1 , a second terminal of the switch S 6  is coupled to the first terminal of the capacitor C 2 , and a control terminal (not shown) of the switch S 6  is configured to receive a clock signal ϕ p . 
     In some embodiments, the clock signal ϕ p  may be a result of logic AND operation of the inverse of the signal ϕ s  and the inverse of the clock signal ϕ cs1 . For example, as shown in  FIG. 2B , when both of the clock signal ϕ s  and the clock signal ϕ cs1  have disabling levels (e.g., low level), the clock signal ϕ p  has an enabling level (e.g., high level). As shown in  FIG. 2B , a time interval of the switch S 6  being conducted is configured to follow a time interval of the switches S 1 -S 2  being conducted. 
     In this example, the second terminal of the switch S 4  is coupled to the first terminal of the capacitor C 2 . The first terminal of the switch S 5  is coupled to the second terminal of the capacitor C 2 . With this configuration, when the clock signal ϕ cs1  has to the enabling level, the switch S 6  is open, and the switches S 4  and S 5  are closed. 
     The operations of the ADC device  300  are similar with the operations of the ADC device  200 . For example, in phase k−1, after the charge sharing of the capacitors C 2  and C 3  is settled, each of the capacitors C 2  and C 3  stores the residue signal Vres 2 ( k− 1), in which the polarity of the residue signal Vres 2  in  FIG. 3  is different from the polarity of the residue signal Vres 2  in  FIG. 2A . 
     In phase k, when the clock signal ϕ s  has the enabling level, the switches S 1 -S 2  are closed. Under this condition, the signal Vin(k) is sampled by the capacitor C 1 . Then, when the clock signal ϕ c  has the enabling level, the comparator circuit  220  and the control logic circuit  240  are enabled to perform the A/D conversion on the sampled signal Vin(k) to generate the residue signal Vres 1 ( k ). Under this situation, as shown in  FIG. 3 , the capacitors C 1  and C 2  are stacked to provide a difference between the residue signals Vres 1 ( k ) and Vres 2 ( k− 1) to the input terminal of the comparator circuit  220 . Equivalently, the comparator circuit  220  quantizes the difference between the residue signals Vres 1 ( k ) and Vres 2 ( k− 1), in order to generate the corresponding digital output Dout(k). Similar to  FIG. 2A , a noise transfer function having the characteristic of noise shaping of the ADC device  300  can be obtained. 
     The level configurations of each clock signal in  FIG. 2B  are given for illustrative purposes only, and the present disclosure is not limited thereto. 
     Reference is made to  FIG. 4 .  FIG. 4  is a schematic diagram of the comparator circuit  220  in  FIG. 2A  or  FIG. 3  according to some embodiments of the present disclosure. 
     In some embodiments, the comparator circuit  220  may operate as the quantizer  100 A. In  FIG. 4 , the comparator circuit  220  includes transistors M 1 -M 11 . The transistors M 1 -M 2  operate as an input pair, in which a gate terminal of the transistor M 1  receives a signal V 1 , and a gate terminal of the transistor M 2  receives a signal V 2 . In some embodiments, the signal V 1  may be a signal transmitted from the switched capacitor circuit  100 , and the signal V 2  may be a ground voltage. 
     The transistors M 3 -M 6  operate as a latch circuit and an output stage circuit, in order to generate output signals VO 1  and VO 2  based on the operations of the transistors M 1 -M 2 . In some embodiments, one of the output signals VO 1  and VO 2  may be the digital output Dout in  FIGS. 1, 2A, and 3 . 
     The transistors M 7 -M 10  operate as a reset circuit. For example, the transistors M 7 -M 8  are configured to reset voltage levels of output terminals of the comparator circuit  220  in response to the disabling level of the clock signal ϕ C . The transistors M 9 -M 10  are configured to reset the voltage levels of drain nodes of the input pair in response to the disabling level of the clock signal ϕ C . 
     The transistor M 11  operates as a tail current source circuit, in order to bias the transistors M 1 -M 10 . 
     The above configuration of the comparator circuit  220  is given for illustrative purposes, and the present disclosure is not limited thereto. Various types of the comparator circuit  220  are within the contemplated scope of the present disclosure. 
     As described above, the switched capacitor circuit and ADC device of embodiments of the present disclosure are able to provide a circuit architecture that has a noise-shaping function. As a result, the overall performance of the ADC device can be improved. 
     Various functional components or blocks have been described herein. As will be appreciated by persons skilled in the art, in some embodiments, the functional blocks will preferably be implemented through circuits (either dedicated circuits, or general purpose circuits, which operate under the control of one or more processors and coded instructions), which will typically comprise transistors or other circuit elements that are configured in such a way as to control the operation of the circuitry in accordance with the functions and operations described herein. As will be further appreciated, the specific structure or interconnections of the circuit elements will typically be determined by a compiler, such as a register transfer language (RTL) compiler. RTL compilers operate upon scripts that closely resemble assembly language code, to compile the script into a form that is used for the layout or fabrication of the ultimate circuitry. Indeed, RTL is well known for its role and use in the facilitation of the design process of electronic and digital systems. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.