Patent Publication Number: US-9432046-B1

Title: Successive approximation analog-to-digital converter

Description:
This application claims the benefit of People&#39;s Republic of China Application Serial No. 201510306750.6, filed Jun. 4, 2015, the subject matter of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an analog-to-digital converter, and more particularly to a successive approximation analog-to-digital converter (SAR ADC). 
     BACKGROUND OF THE INVENTION 
     As is well known, an analog-to-digital converter (ADC) is used for converting the amplitude of an analog voltage (or current) into a digital value. Moreover, the analog-to-digital converter (ADC) has various circuitry configurations. According to the circuitry configurations, the analog-to-digital converters include a flash analog-to-digital converter (flash ADC), a pipeline analog-to-digital converter (pipeline ADC), a successive approximation analog-to-digital converter (SAR ADC), and so on. 
     Generally, the flash ADC is operated at the highest rate. However, the flash ADC has a complicated circuitry configuration and higher fabricating cost. The SAR ADC is operated at a slower rate. However, the SAR ADC has a simplified circuitry configuration and lower fabricating cost. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a successive approximation analog-to-digital converter. The successive approximation analog-to-digital converter includes a first capacitance bank, a second capacitance bank, a bridge capacitor, a switch set, a comparator and a successive approximation register logic circuit. The first capacitance bank includes j capacitors. First terminal of the j capacitors of the first capacitance bank are connected with a first node. The second capacitance bank includes (i+1) capacitors. First terminals of the (i+1) capacitors of the second capacitance bank are connected with a second node. The bridge capacitor is connected between the first node and the second node. The switch set includes j switch elements and (i+1) switch elements. First terminals of the j switch elements are respectively connected with second terminals of the j capacitors of the first capacitance bank. First terminals of the (i+1) switch elements are respectively connected with second terminals of the (i+1) capacitors of the second capacitance bank. The (i+j+1) switch elements are controlled according to a switching signal. Each of second terminals of the (i+j+1) switch elements of the switch set is selectively connected with one of a low reference level, a high reference level, an input level and an intermediate level. A first input terminal of the comparator is connected with the first node. A second input terminal of the comparator receives the intermediate level. An output terminal of the comparator generates a comparing signal. The successive approximation register logic circuit receives the comparing signal according to a clock signal, and generates the switching signal and a digital data signal. The switch set further includes a sampling switch element that is controlled according to the switching signal. A first terminal of the sampling switch element receives the intermediate level. A second terminal of the sampling switch element is connected with the first node. 
     Numerous objects, features and advantages of the present invention will be readily apparent upon a reading of the following detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings. However, the drawings employed herein are for the purpose of descriptions and should not be regarded as limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a schematic circuit diagram illustrating a successive approximation analog-to-digital converter according to an embodiment of the present invention; 
         FIG. 2A  is a schematic circuit diagram illustrating an example of the successive approximation analog-to-digital converter according to the embodiment of the present invention; and 
         FIG. 2B  is a schematic timing waveform diagram illustrating the signals associated with the SAR logic circuit of the successive approximation analog-to-digital converter as shown in  FIG. 2A . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic circuit diagram illustrating a successive approximation analog-to-digital converter according to an embodiment of the present invention. As shown in  FIG. 1 , the successive approximation analog-to-digital converter  100  comprises a first capacitance bank  110 , a second capacitance bank  120 , a comparator  130 , a successive approximation register logic circuit  140 , a switch set  150  and a bridge capacitor Cb. Hereinafter, the successive approximation register logic circuit  140  is also referred as a SAR logic circuit. 
     In this embodiment, the second capacitance bank  120  comprises (i+1) capacitors C 0 ˜Ci. The first terminals of the capacitors C 0 ˜Ci are connected with a second node b. The second terminals of the capacitors C 0 ˜Ci are respectively connected with the first terminals of the switch elements S 0 ˜Si of the switch set  150 . In the second capacitance bank  120 , the capacitor C 0  has a capacitance value c (i.e., a unit capacitance value), and the capacitance values of the other capacitors C 1 ˜Ci increase at a power of two. That is, Ck=c×2 (k-1) , wherein k is larger than or equal to 1, and k is smaller than or equal to i. In other words, the capacitance value of the capacitor C 1  is equal to c, the capacitance value of the capacitor C 2  is equal to 2c, . . . , and the capacitance value of the capacitor Ci is equal to c×2″). 
     The first capacitance bank  110  comprises j capacitors C 1 +1˜Ci+j. The first terminals of the capacitors Ci+1˜Ci+j are connected with a first node a. The second terminals of the capacitors Ci+1˜Ci+j are respectively connected with the first terminals of the switch elements Si+1˜Si+j of the switch set  150 . In the first capacitance bank  110 , the capacitance values of the capacitors Ci+1˜Ci+j increase at a power of two. That is, Ci+x=c×2 (x-1) , wherein x is larger than or equal to 1, and x is smaller than or equal to j. In other words, the capacitance value of the capacitor Ci+1 is equal to c, the capacitance value of the capacitor Ci+2 is equal to 2c, . . . , and the capacitance value of the capacitor Ci+j is equal to c×2 (j-1) . 
     The switch set  150  is connected with an input level Vin, a low reference level Vrb, a high reference level Vrt and an intermediate level Vcm. The difference between the high reference level Vrt and the low reference level Vrb is equal to a reference voltage Vref. The intermediate level Vcm is in the range between the low reference level Vrb and the high reference level Vrt, for example Vcm=(Vrb+Vrt)/2. 
     The switch set  150  is controlled according to a switching signal Sw. The second terminal of the switch element S 0  is selectively connected with one of the low reference level Vrb and the high reference level Vrt. The second terminal of the each of the switch elements S 1 ˜Si and Si+1˜Si+j is selectively connected with one of the input level Vin, the low reference level Vrb, the high reference level Vrt and the intermediate level Vcm. The switch set  150  further comprises a sampling switch element Ss. A first terminal of the sampling switch element Ss receives the intermediate level Vcm, and a second terminal of the sampling switch element Ss is connected with the first node a. 
     The bridge capacitor Cb is connected between the first node a and the second node b. A first input terminal (e.g., a positive input terminal) of the comparator  130  is connected with the first node a. A second input terminal (e.g., a negative input terminal) of the comparator  130  receives the intermediate level Vcm. An output terminal of the comparator  130  generates a comparing signal Out. 
     The SAR logic circuit  140  receives the comparing signal Out. According to the comparing signal Out, the switching signal Sw is successively changed by the SAR logic circuit  140 . Consequently, the switched positions of the switch elements S 0 ˜Si and Si+1˜Si+j of the switch set  150  are successively changed. After the switch elements S 0 ˜Si and Si+1˜Si+j of the switch set  150  are successively changed, the SAR logic circuit  140  generates a corresponding digital data signal Dout. 
     In this embodiment, the successive approximation analog-to-digital converter  100  further comprises a compensation capacitor Cc and a compensation switch element Sc. The first terminal of the compensation capacitor Cc is connected with the second node b. The second terminal of the compensation capacitor Cc is connected with the first terminal of the compensation switch element Sc. Moreover, the compensation switch element Sc is controlled according to the switching signal Sw. Consequently, the second terminal of the compensation switch element Sc is selectively connected with one of the low reference level Vrb and the high reference level Vrt. 
     Moreover, the bridge capacitor Cb and the compensation capacitor Cc are specially designed. Consequently, after the second capacitance bank  120  and the compensation capacitor Cc are connected with each other in parallel and serially connected with the bridge capacitor Cb, the equivalent capacitor Cth has the capacitance value c. 
     Hereinafter, the operations of the successive approximation analog-to-digital converter will be illustrated by referring to i=4 and j=5. 
       FIG. 2A  is a schematic circuit diagram illustrating an example of the successive approximation analog-to-digital converter according to the embodiment of the present invention. As shown in  FIG. 2A , the successive approximation analog-to-digital converter  200  comprises a first capacitance bank  210 , a second capacitance bank  220 , a comparator  230 , a successive approximation register logic circuit  240 , a switch set  250  and a bridge capacitor Cb. The successive approximation register logic circuit  240  is also referred as a SAR logic circuit. The successive approximation analog-to-digital converter  200  further comprises a compensation capacitor Cc and a compensation switch element Sc. 
     The second capacitance bank  220  comprises four capacitors C 0 ˜C 4 . The first terminals of the capacitors C 0 ˜C 4  are connected with a second node b. The second terminals of the capacitors C 0 ˜C 4  are respectively connected with the first terminals of the switch elements S 0 ˜S 4  of the switch set  250 . In the second capacitance bank  220 , the capacitor C 0  has the capacitance value c, the capacitor C 1  has the capacitance value c, and the capacitance values of the other capacitors C 2 ˜C 4  increase at a power of two. That is, the capacitance value of the capacitor C 1  is equal to c, the capacitance value of the capacitor C 2  is equal to 2c, the capacitance value of the capacitor C 3  is equal to 4c, and the capacitance value of the capacitor C 4  is equal to 8c. 
     The first capacitance bank  210  comprises five capacitors C 5 ˜C 9 . The first terminals of the capacitors C 5 ˜C 9  are connected with a first node a. The second terminals of the capacitors C 5 ˜C 9  are respectively connected with the first terminals of the switch elements S 5 ˜S 9  of the switch set  250 . In the first capacitance bank  210 , the capacitor C 5  has the capacitance value c, and the capacitance values of the other capacitors C 6 ˜C 9  increase at a power of two. That is, the capacitance value of the capacitor C 5  is equal to c, the capacitance value of the capacitor C 6  is equal to 2c, the capacitance value of the capacitor C 7  is equal to 4c, the capacitance value of the capacitor C 8  is equal to 8c, and the capacitance value of the capacitor C 9  is equal to 16c. 
     The switch set  150  is connected with an input level Vin, a low reference level Vrb, a high reference level Vrt and an intermediate level Vcm. The difference between the high reference level Vrt and the low reference level Vrb is equal to a reference voltage Vref. The intermediate level Vcm is in the range between the low reference level Vrb and the high reference level Vrt, for example Vcm=(Vrb+Vrt)/2. 
     The switch set  250  is controlled according to a switching signal Sw. The second terminal of the switch element S 0  is selectively connected with one of the low reference level Vrb and the high reference level Vrt. The second terminal of the each of the switch elements S 1 ˜S 9  is selectively connected with one of the input level Vin, the low reference level Vrb, the high reference level Vrt and the intermediate level Vcm. The switch set  250  further comprises a sampling switch element Ss. A first terminal of the sampling switch element Ss receives the intermediate level Vcm, and a second terminal of the sampling switch element Ss is connected with the first node a. Moreover, the compensation switch element Sc is also controlled according to the switching signal Sw. Consequently, the second terminal of the compensation switch element Sc is selectively connected with one of the low reference level Vrb and the high reference level Vrt. 
     The bridge capacitor Cb is connected between the first node a and the second node b. A first input terminal (e.g., a positive input terminal) of the comparator  230  is connected with the first node a. A second input terminal (e.g., a negative input terminal) of the comparator  230  receives the intermediate level Vcm. An output terminal of the comparator  230  generates a comparing signal Out. 
     The SAR logic circuit  240  receives the comparing signal Out. According to the comparing signal Out, the switching signal Sw is successively changed by the SAR logic circuit  240 . Consequently, the switched positions of the switch elements S 0 ˜S 9  of the switch set  250  are successively changed. After the switch elements S 0 ˜S 9  of the switch set  250  are successively changed, the SAR logic circuit  240  generates a corresponding digital data signal Dout. 
       FIG. 2B  is a schematic timing waveform diagram illustrating the signals associated with the SAR logic circuit of the successive approximation analog-to-digital converter as shown in  FIG. 2A . 
     In a sampling period between the time point t 0  and the time point t 11 , the sampling switch Ss is turned on to receive the intermediate level Vcm, the switch elements S 1 ˜S 9  are connected with the input level Vin, and the switch element S 0  and the compensation switch element Sc are connected with the low reference level Vrb. Consequently, at the time point t 1 , the voltage value of the input level Vin is sampled to the capacitors C 1 ˜C 9 . 
     After the sampling period is ended (i.e., at the time point t 1 ), the sampling switch Ss is tuned off and not connected with the intermediate level Vcm. Moreover, the switch elements S 1 ˜S 9  are connected with the intermediate level Vcm, and the switch element S 0  and the compensation switch element Sc are connected with the low reference level Vrb or the high reference level Vrt according to the practical requirements. 
     In a converting period between the time point t 1  and the time point t 3 , at least 10 (=i+j+1) clock cycles are used as the comparing cycles. In each comparing cycle, the comparator  230  compares the voltage of the first node a with the intermediate level Vcm and generates a comparing signal Out. According to the comparing signal Out, the switching signal Sw is successively changed by the SAR logic circuit  240  and then the comparing operation is done in the next comparing cycle. Moreover, according to the switching signal Sw, the switch elements S 1 ˜S 9  of the switch set  250  are successively changed from the highest-numbered switch element S 9  to the lowest-numbered switch element S 1 . That is, in each comparing cycle, a switched position of one switch element is changed according to the switching signal Sw, and the comparing signal Out from the comparator  230  is correspondingly changed. 
     That is, in a converting period between the time point t 1  and the time point t 3 , the SAR logic circuit  240  receives the comparing signal Out according to a clock signal CLK and successively change the switching signal Sw in order to control the switch elements S 1 ˜S 9 . In other words, by changing the logic levels of the bits D 1 ˜D 9 , the switched position of the switch elements S 9 ˜S 1  are controlled accordingly. 
     Firstly, in the first comparing cycle, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. Consequently, the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined. 
     For example, if the voltage of the first node a is lower than the intermediate level Vcm, the comparing signal Out generates a first logic level (e.g., logic level “1”), and the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined as “1”. Then, the switching signal Sw is changed, and the highest-numbered switch element S 9  is switched to the high reference level Vrt according to the switching signal Sw. Whereas, if the voltage of the first node a is higher than the intermediate level Vcm, the comparing signal Out generates a second logic level (e.g., logic level “0”), and the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined as “0”. Then, the switching signal Sw is changed, and the highest-numbered switch element S 9  is switched to the low reference level Vrb according to the switching signal Sw. 
     The operations of the successive approximation analog-to-digital converter in the subsequent comparing cycles are similar to those in the first comparing cycle. After the previous-numbered switch element Sx is switched to the determined level, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. Consequently, the logic level of the next bit Dx−1 of the digital data signal Dout is determined, and the next switch element Sx−1 is correspondingly controlled. In the embodiment of  FIG. 2A , x is successively decreased from 9 to 1. If the voltage of the first node a is lower than the intermediate level Vcm, the comparing signal Out generates the first logic level (e.g., logic level “1”), and the logic level of the next bit of the digital data signal Dout is determined as “1”. Then, the switching signal Sw is changed, and the next-numbered switch element is switched to the high reference level Vrt according to the switching signal Sw. Whereas, if the voltage of the first node a is higher than the intermediate level Vcm, the comparing signal Out generates a second logic level (e.g., logic level “0”), and the logic level of the next bit of the digital data signal Dout is determined as “0”. Then, the switching signal Sw is changed, and the next-numbered switch element is switched to the low reference level Vrb according to the switching signal Sw. 
     After the switch element S 1  is switched to the determined level, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. Consequently, the logic level of the least significant bit (LSB), i.e., D 0 , of the digital data signal Dout is determined. 
     After the switch elements S 9 ˜S 1  are successively switched according to the clock signal CLK, the logic levels of the bits D 9 ˜D 0  of the digital data signal Dout from the most significant bit (MSB) to the least significant bit (LSB) are successively acquired. Moreover, at the time point t 2 , the sampled digital data signal Dout is outputted. 
     From the above descriptions, the switch elements S 9 ˜S 1  of the successive approximation analog-to-digital converter  200  are successively switched from the intermediate level Vcm to the high reference level Vrt or switched from the intermediate level Vcm to the low reference level Vrb. Since the voltage swing amount is only one half of the amplitude of the reference voltage Vref, the power consumption is reduced. 
     Hereinafter, another operating method of the successive approximation analog-to-digital converter  200  of  FIG. 2A  will be illustrated as follows. 
     Firstly, in the first comparing cycle, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. Consequently, the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined. 
     For example, if the voltage of the first node a is lower than the intermediate level Vcm, the comparing signal Out generates a first logic level (e.g., logic level “1”), and the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined as “1”. Then, the switching signal Sw is changed, and the highest-numbered switch element S 9  is switched to the high reference level Vrt according to the switching signal Sw. Whereas, if the voltage of the first node a is higher than the intermediate level Vcm, the comparing signal Out generates a second logic level (e.g., logic level “0”), and the logic level of the most significant bit (MSB), i.e., D 9 , of the digital data signal Dout is determined as “0”. Then, the switching signal Sw is changed, and the highest-numbered switch element S 9  is switched to the low reference level Vrb according to the switching signal Sw. 
     In case that the D 9  of the digital data signal Dout is “1”, the other switch elements S 8 ˜S 1  are switched to the high reference level Vrt or the intermediate level Vcm according to the logic levels of the comparing signal Out in the subsequent comparing cycles. For example, after the previous-numbered switch element Sx is switched to the determined level, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. If the voltage of the first node a is lower than the intermediate level Vcm, the comparing signal Out generates the first logic level (e.g., logic level “1”), and the logic level of the next bit of the digital data signal Dout is determined as “1”. Then, the switching signal Sw is changed, and the next-numbered switch element is switched to the high reference level Vrt according to the switching signal Sw. Whereas, if the voltage of the first node a is higher than the intermediate level Vcm, the comparing signal Out generates a second logic level (e.g., logic level “0”), and the logic level of the next bit of the digital data signal Dout is determined as “0”. Then, the switching signal Sw is changed, and the next-numbered switch element is switched to the intermediate level Vcm according to the switching signal Sw. Similarly, x is successively decreased from 9 to 1. 
     In case that the D 9  of the digital data signal Dout is “0”, the other switch elements S 8 ˜S 1  are switched to the low reference level Vrb or the intermediate level Vcm according to the logic levels of the comparing signal Out in the subsequent comparing cycles. For example, after the previous-numbered switch element Sx is switched to the determined level, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. If the voltage of the first node a is lower than the intermediate level Vcm, the comparing signal Out generates the first logic level (e.g., logic level “1”), and the logic level of the next bit of the digital data signal Dout is determined as “1”. Then, the switching signal Sw is changed, and the next-numbered switch element is maintained at the intermediate level Vcm according to the switching signal Sw. Whereas, if the voltage of the first node a is higher than the intermediate level Vcm, the comparing signal Out generates a second logic level (e.g., logic level “0”), and the logic level of the next bit of the digital data signal Dout is determined as “0”. Then, the switching signal Sw is changed, and the next-numbered switch element is switched to the low reference level Vrb according to the switching signal Sw. Similarly, x is successively decreased from 9 to 1. 
     After the switch element S 1  is switched to the determined level, the comparator  230  generates the comparing signal Out according to the result of comparing the voltage of the first node a with the intermediate level Vcm. Consequently, the logic level of the least significant bit (LSB), i.e., D 0 , of the digital data signal Dout is determined. 
     In the above embodiment, the successive approximation analog-to-digital converter  200  comprises the first capacitance bank  210  and the second capacitance bank  220 , and the first capacitance bank  210  and the second capacitance bank  220  are connected with each other through the bridge capacitor Cb. Consequently, the capacitance values of the capacitances can be reduced. That is, the layout size of the capacitor is reduced. In the successive approximation analog-to-digital converter  200  of  FIG. 2A , i=4 and j=5. It is noted that the values of i and j are not restricted. As long as the value (j−i) is larger than or equal to 1, the converting performance is satisfied. 
     As mentioned above, the second terminal of each of the switch element S 0  and the compensation switch element Sc is selectively connected with one of the low reference level Vrb and the high reference level Vrt. It is noted that numerous modifications and alterations may be made while retaining the teachings of the invention. For example, the second terminal of each of the switch element S 0  and the compensation switch element Sc is selectively connected with one of the input level Vin, the low reference level Vrb, the high reference level Vrt and the intermediate level Vcm. However, each of the switch element S 0  and the compensation switch element Sc is selectively connected with one of the low reference level Vrb and the high reference level Vrt according to the switching signal Sw. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.