SAR ADC capable of reducing energy consumption

An SAR ADC capable of reducing energy consumption, including a voltage selecting circuit for configuring a capacitor circuit to form a first equivalent capacitor having a capacitance of (2m−1)C, a second equivalent capacitor having a capacitance of (2n−2m−1)C, a fourth equivalent capacitor having a capacitance of (2m−1)C, and a fifth equivalent capacitor having a capacitance of (2n−2m−1)C, wherein, the first equivalent capacitor has one terminal coupled to a reference voltage or a ground voltage, and the other terminal coupled to a positive input end of a comparator; the second equivalent capacitor is coupled between a common mode voltage and the positive input end; the fourth equivalent capacitor has one terminal coupled to the ground voltage or the reference voltage, and the other terminal coupled to a negative input end of the comparator; and the fifth equivalent capacitor is coupled between the common mode voltage and the negative input end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an SAR ADC (Successive Approximation Register Analog-to-Digital Converter), especially to an SAR ADC capable of minimizing energy consumption.

2. Description of the Related Art

As portable electronic products have evolved vigorously, low power consumption—capable of prolonging the battery life—has become a key issue in the design of electronic products; and among the varieties of analog-to-digital converters, the SAR ADC has been widely used in electronic products due to its power saving advantage—its analog-to-digital conversion is implemented by a simple architecture including only a comparator and few components.

Please refer toFIG. 1, which illustrates the block diagram of a prior art N-bit SAR ADC. As illustrated inFIG. 1, the SAR ADC includes a bit value determination unit100, and a successive approximation register140, wherein the bit value determination unit100includes a sample and hold circuit110, a digital-to-analog conversion circuit120, and a comparator130.

The sample and hold circuit110is used for performing a sample and hold operation on an analog input signal VAto generate a sampled signal VA1during a sampling stage.

The digital-to-analog conversion circuit120, biased between a reference voltage VREFand a ground voltage VGND, is used for generating a quantization voltage VA2according to switch control signals SWN. . . SW1.

The comparator130is used for comparing the sampled signal VA1with the quantization voltage VA2to generate a bit output B.

The successive approximation register140is used for successively changing the content of the switch control signals SWN. . . SW1during a voltage comparison stage to vary the quantization voltage VA2, and successively receiving the bit output B to generate a digital output code DOUT.

During the voltage comparison stage, first, the successive approximation register140will output a prediction code through the switch control signals SWN. . . SW1to cause a charging or discharging process happening in the digital-to-analog conversion circuit120, so as to generate a corresponding level for the quantization voltage VA2. Second, the comparator130will compare the sampled voltage VA1with the quantization voltage VA2, and thereby determine the content of the bit output B—“0” or “1”. Third, the successive approximation register140will store the bit output B in a register, and output a next prediction code according to the content of the bit output B to determine the next content of the bit output B. By repeating the three steps N times, an N bits content of the digital output code DOUTcorresponding to the sampled signal VA1will be generated.

In the process of generating the digital output code DOUT, the quantization voltage VA2generated by the digital-to-analog conversion circuit120will successively approach the sampled signal VA1in a binary-weighted manner; that is, an N bits SAR ADC will generate N values of the quantization voltage VA2—let them labeled as VA2(1), VA2(2), VA2(3), . . . , VA2(N)respectively, and the voltage difference between VA2(k)and VA2(k-1)will be half of the voltage difference between VA2(k-1)and VA2(k-2), wherein k=3 to N.

As the digital-to-analog conversion circuit120normally uses a plurality of switches to control the charging or discharging of a plurality of capacitors of different weightings, therefore, most energy will be dissipated in the charging or discharging of the capacitors.

For the architecture of general digital-to-analog conversion circuits, please refer toFIG. 2, which illustrates the block diagram of a prior art bit value determination unit including a digital-to-analog conversion circuit. As illustrated inFIG. 2, the prior art bit value determination unit includes a switch unit210, a digital-to-analog conversion circuit220, and a comparator230, wherein the digital-to-analog conversion circuit220has a first capacitor array221, a second capacitor array222, a voltage selecting circuit223, and a voltage selecting circuit224.

The switch unit210has a pair of sampling switches, of which one side has a first contact and a second contact, and both the first contact and the second contact are coupled to a reference voltage VREF; and the other side has a third contact and a fourth contact coupled to the first capacitor array221and the second capacitor array222respectively.

The first capacitor array221and the second capacitor array222both have N+1 capacitors, with capacitances of C, C, 2C, 4C, 8C, . . . , 2N-1C respectively. The N+1 capacitors of the first capacitor array221has a common terminal coupled to the third contact of the switch unit210, and N+1 bias contacts coupled to the voltage selecting circuit223. The N+1 capacitors of the second capacitor array222has a common terminal coupled to the fourth contact of the switch unit210, and N+1 bias contacts coupled to the voltage selecting circuit224.

The voltage selecting circuit223is used for outputting N+1 bias voltages for the N+1 bias contacts of the first capacitor array221according to switch control signals SWN. . . SW1, wherein, each of the bias voltages outputted by the voltage selecting circuit223is selected from a group consisting of a negative analog input voltage VAN, the reference voltage VREF, and a ground voltage VGND. The voltage selecting circuit224is used for outputting N+1 bias voltages for the N+1 bias contacts of the second capacitor array222according to the switch control signals SWN. . . SW1, wherein, each of the bias voltages outputted by the voltage selecting circuit224is selected from a group consisting of a positive analog input voltage VAP, the reference voltage VREF, and the ground voltage VGND.

The comparator230has a positive input end, a negative input end, and an output end, wherein the positive input end is coupled to the third contact of the switch unit210, and the negative input end is coupled to the fourth contact of the switch unit210. The comparator230is used for generating a bit output B according to the voltage difference between the positive input end and the negative input end—the voltage difference can be expressed as VAP−VAN−γVREF, wherein 0≦γ<1. When VAP−VAN−γVREF>0, B=1; and when VAP−VAN−γVREF<0, B=0.

Please refer toFIG. 3, which illustrates a circuit of the prior art bit value determination unit ofFIG. 2formed in a sampling stage. As illustrated inFIG. 3, when in the sampling stage, both the common contact of the first capacitor array221and the common contact of the second capacitor array222are coupled to VREF, the N+1 bias contacts of the first capacitor array221are coupled to the negative analog input voltage VAN, and the N+1 bias contacts of the second capacitor array222are coupled to the positive analog input voltage VAP. By the end of the sampling stage, the first capacitor array221will store a voltage of (VREF−VAN), and the second capacitor array222will store a voltage of (VREF−VAP).

Please refer toFIG. 4, which illustrates another circuit of the prior art bit value determination unit ofFIG. 2formed in a voltage comparison stage. As illustrated inFIG. 4, when in the voltage comparison stage, the switch unit210is switched off; the first capacitor array221has a first equivalent capacitor with a capacitance of KC and a second equivalent capacitor with a capacitance of (2N−K)C; the second capacitor array222has a third equivalent capacitor with a capacitance of KC and a fourth equivalent capacitor with a capacitance of (2N−K)C, wherein, both K and N are positive integers, K=1 to 2N−1; the bias contact of the first equivalent capacitor is coupled to VREF; the bias contact of the second equivalent capacitor is coupled to VGND; the bias contact of the third equivalent capacitor is coupled to VGND; the bias contact of the fourth equivalent capacitor is coupled to VREF.

During the voltage comparison stage, the comparator230will see a voltage of VREF−VAN+(K/2N)VREFat the positive input end, and a voltage of VREF−VAP+(1−K/2N)VREFat the negative input end, i.e., a voltage difference of VAP−VAN−(1−K/2N)VREFbetween the positive input end and the negative input end. Taking N=4 as an example, wherein:

when K=1, the voltage difference will be VAP−VAN−(⅞)VREF;

when K=2, the voltage difference will be VAP−VAN−(¾)VREF;

when K=3, the voltage difference will be VAP−VAN−(⅝)VREF;

when K=4, the voltage difference will be VAP−VAN−(½)VREF;

when K=5, the voltage difference will be VAP−VAN−(⅜)VREF;

when K=6, the voltage difference will be VAP−VAN−(¼)VREF;

when K=7, the voltage difference will be VAP−VAN−(⅛)VREF;

when K=8, the voltage difference will be VAP−VAN−0;

when K=9, the voltage difference will be VAP−VAN−(−⅛)VREF;

when K=10, the voltage difference will be VAP−VAN−(−¼)VREF;

when K=11, the voltage difference will be VAP−VAN−(−⅜)VREF;

when K=12, the voltage difference will be VAP−VAN−(−½)VREF;

when K=13, the voltage difference will be VAP−VAN−(−⅝)VREF;

when K=14, the voltage difference will be VAP−VAN−(−¾)VREF;

when K=15, the voltage difference will be VAP−VAN−(−⅞)VREF.

During the voltage comparison stage, K is initially set at 8 to compare VAP−VANwith 0, and if VAP−VANis higher than 0, K will be changed to 4 to compare VAP−VANwith (½)VREF, and if VAP−VANis lower than (½)VREF, K will be changed to 6 to compare VAP−VANwith (¼)VREF, and so forth. When K=8, (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (8C, 8C); when K=4, (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (4C, 12C); and when K=6, (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (6C, 10C).

Therefore, assume VAP−VAN=( 9/32)VREF, then the comparator230will output “1” when K is initially set at 8—because ( 9/32)VREF−0 is higher than 0, and K will then be changed to 4; when K is at 4, the comparator230will output “0”—because ( 9/32)VREF−(½)VREFis lower than 0, and K will then be changed to 6; when K is at 6, the comparator230will output “1”—because ( 9/32)VREF−(¼)VREFis higher than 0, and K will then be changed to 5; when K is at 5, the comparator230will output “0”—because ( 9/32)VREF−(⅜)VREFis lower than 0. In this manner, a digital code (1010) corresponding to ( 9/32)VREFwill be generated.

Besides, as there are N occurrences of dynamic energy consumption in the N bits SAR ADC during the generation of each N bits digital code, wherein the energy consumed in each occurrence of the dynamic energy consumption is equal to the product of the reference voltage VREFand the charge flowing out of the reference voltage VREF, therefore, given a specified value of the reference voltage VREF, the charge flowing out of the reference voltage VREFhas to be reduced to minimize the dynamic energy consumption in the SAR ADC. According to the architecture of the prior art SAR ADC, the only way to reducing the dynamic energy consumption therein seems to be reducing the basic capacitance C. However, reducing the basic capacitance C can deteriorate SNR (Signal to Noise Ratio), and thereby impact the resolution of the analog to digital conversion of the SAR ADC.

In view of the foregoing problem, a novel SAR analog to digital conversion architecture is needed to reduce the dynamic energy consumption without sacrificing SNR.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide an SAR ADC capable of minimizing energy consumption by a novel capacitor array architecture, of which the largest capacitance is 2N-2times of the smallest capacitance, wherein N represents the number of bits of the digital output of the SAR ADC.

Another objective of the present invention is to provide an SAR ADC capable of minimizing energy consumption by a novel input voltage sampling mode and a novel voltage comparison mode, wherein the novel voltage comparison mode enables most of the capacitors in two capacitor arrays to have a floating option.

Still another objective of the present invention is to provide an SAR ADC capable of minimizing energy consumption by a novel input voltage sampling mode and a novel voltage comparison mode, wherein both the novel input voltage sampling mode and the novel voltage comparison mode utilize a common mode voltage, which is lower than a reference voltage and higher than a ground voltage.

To attain the foregoing objectives, the present invention proposes an SAR ADC capable of reducing energy consumption, the SAR ADC including:

a comparator, having a positive input end, a negative input end, and a comparison output end;

a capacitor circuit, having a first capacitor array and a second capacitor array, and both the first capacitor array and the second capacitor array including N groups of capacitors, in which a first group has one capacitor with a capacitance of C, a second group has one capacitor with a capacitance of C, a third group has one capacitor with a capacitance of 2C, and a K-th group has K−2 capacitors with capacitances of 2C, 21C, 22C, . . . 2K-3C, K=4 to N, and each of the capacitors of the first capacitor array has an electrode coupled to the positive input end of the comparator, and each of the capacitors of the second capacitor array has an electrode coupled to the negative input end of the comparator;

a sampling switch, having one side coupled to a positive input voltage and a negative input voltage, and the other side coupled to the positive input end and the negative input end of the comparator;

a logic circuit, having a bit input terminal, N bit output terminals, and a plurality of switch control output terminals, wherein the bit input terminal is coupled to the comparison output end of the comparator, and the switch control output terminals are used for outputting a plurality of switch control signals; and

a voltage selecting circuit, used for making the other electrodes of the capacitors of the capacitor circuit floating or coupled to a reference voltage, a common mode voltage, or a ground voltage.

To make it easier for our examiner to understand the objective of the invention, its structure, innovative features, and performance, we use preferred embodiments together with the accompanying drawings for the detailed description of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more detail hereinafter with reference to the accompanying drawings that show the preferred embodiment of the invention.

Please refer toFIG. 5, which illustrates the block diagram of an SAR ADC capable of reducing energy consumption according to a preferred embodiment of the present invention. As illustrated inFIG. 5, the SAR ADC includes a switch unit510, a first capacitor array521, a second capacitor array522, a voltage selecting circuit523, a comparator530, and a logic circuit540, wherein the switch unit510, the first capacitor array521, the second capacitor array522, the voltage selecting circuit523, and the comparator530are used to form a bit value determination unit.

The switch unit510has a pair of sampling switches, of which one side has a first contact and a second contact coupled respectively to a positive analog input voltage VAPand a negative analog input voltage VAN, and the other side has a third contact and a fourth contact coupled respectively to the first capacitor array521and the second capacitor array522.

Both the first capacitor array521and the second capacitor array522have N groups of capacitors, in which a first group has a capacitor with a capacitance of C, a second group has a capacitor with a capacitance of C, a third group has a capacitor with a capacitance of 2C, and a K-th group has K−2 capacitors with capacitances of 2C, 21C, 22C, . . . 2K-3C, K=4 to N, and each of the capacitors of the first capacitor array521has an electrode coupled to the third contact of the switch unit510, and the other electrodes of the capacitors of the first capacitor array521are bias contacts coupled respectively to bias signals VU(1), VU(2), VU(3), VU(4,1), VU(4,2), VU(5,1), VU(5,2), VU(5,3) . . . , VU(N,1), VU(N,2), . . . VU(N,N−3), VU(N,N−2); and each of the capacitors of the second capacitor array522has an electrode coupled to the fourth contact of the switch unit510, and the other electrodes of the capacitors of the second capacitor array522are bias contacts coupled respectively to bias signals VD(1), VD(2), VD(3), VD(4,1), VD(4,2), VD(5,1), VD(5,2), VD(5,3) . . . , VD(N,1), VD(N,2), . . . VD(N,N−3), VD(N,N−2).

The voltage selecting circuit523is used for outputting the bias signals VU(1), VU(2), VU(3), VU(4,1), VU(4,2), VU(5,1), VU(5,2), VU(5,3) . . . , VU(N,1), VU(N,2), . . . VU(N,N−3), VU(N,N−2) to the bias contacts of the first capacitor array521, and the bias signals VD(1), VD(2), VD(3), VD(4,1), VD(4,2), VD(5,1), VD(5,2), VD(5,3) . . . , VD(N,1), VD(N,2), . . . VD(N,N−3), VD(N,N−2) to the bias contacts of the second capacitor array522according to switch control signals SWN. . . SW1, so as to make the bias contacts of the first capacitor array521and the second capacitor array522floating or coupled to a reference voltage VREF, a common mode voltage VCM, or a ground voltage VGND. The common mode voltage VCMis lower than the reference voltage VREFand higher than the ground voltage VGND, and is preferably equal to VREF/2.

The comparator530has a positive input end, a negative input end, and an output end, wherein the positive input end is coupled to the third contact of the switch unit510, the negative input end is coupled to the fourth contact of the switch unit510, and the output end is for generating a bit output B according to the voltage difference between the positive input end and the negative input end.

The logic circuit540has a bit input terminal, N bit output terminals, and a plurality of switch control output terminals, wherein the bit input terminal is coupled to the output end of the comparator530, the N bit output terminals are used for outputting a digital code DOUT, the switch control output terminals are used for outputting the switch control signals SWN. . . SW1, and the logic circuit540has a register for registering the digital code DOUT.

Please refer toFIG. 6, which illustrates the bit value determination unit ofFIG. 5operating in a sampling stage. As illustrated inFIG. 6, when in the sampling stage, the first capacitor array521has a common contact coupled to VAP, the second capacitor array522has a common contact coupled to VAN, and all the bias contacts of the first capacitor array521and the second capacitor array522are coupled to the common mode voltage VCM. By the end of the sampling stage, the first capacitor array521will store a voltage of (VAP−VCM) the second capacitor array522will store a voltage of (VAN−VCM), and the comparator will see a voltage difference of (VAP−VAN−0) between the positive input end and the negative input end of the comparator530.

When the bit value determination unit ofFIG. 5is during a voltage comparison stage, a first circuit will be formed to compare VAP−VANwith ((2m−1)/2n)VREFif VAP−VAN>0, and a second circuit will be formed to compare VAP−VANwith ((1-2m)/2n)VREFif VAP−VAN<0, wherein m, n are positive integers, m≦2n-1, and n≦N−1.

Please refer toFIG. 7(a), which illustrates the first circuit. As illustrated inFIG. 7(a), when the bit value determination unit forms the first circuit, the switch unit510is switched off; the first capacitor array521has a first equivalent capacitor with a capacitance of (2m−1)C, a second equivalent capacitor with a capacitance of (2n−2m+1)C, and a third equivalent capacitor with a capacitance of (2N-1−2n)C; and the second capacitor array522has a fourth equivalent capacitor with a capacitance of (2m−1)C, a fifth equivalent capacitor with a capacitance of (2n−2m+1)C, and a sixth equivalent capacitor with a capacitance of (2N-1−2n)C. The bias contact of the first equivalent capacitor is coupled to VGND, the bias contact of the second equivalent capacitor is coupled to VCM, the bias contact of the third equivalent capacitor is kept floating, the bias contact of the fourth equivalent capacitor is coupled to VREF, the bias contact of the fifth equivalent capacitor is coupled to VCM, and the bias contact of the sixth equivalent capacitor is kept floating.

In the first circuit, the comparator530will see a voltage of VAP−((2m−1)/2n)VCMat the positive input end, a voltage of VAN+((2m−1)/2n)(VREF−VCM) at the negative input end, that is, a voltage difference of VAP−VAN−((2m−1)/2n)VREFbetween the positive input end and the negative input end. Taking N=4 as an example, wherein:

when (n, m)=(1,1), the voltage difference will be VAP−VAN−(½)VREF;

when (n, m)=(2,1), the voltage difference will be VAP−VAN−(¼)VREF;

when (n, m)=(2,2), the voltage difference will be VAP−VAN−(¾)VREF;

when (n, m)=(3,1), the voltage difference will be VAP−VAN−(⅛)VREF;

when (n, m)=(3,2), the voltage difference will be VAP−VAN−(⅜)VREF;

when (n, m)=(3,3), the voltage difference will be VAP−VAN−(⅝)VREF;

when (n, m)=(3,4), the voltage difference will be VAP−VAN−(⅞)VREF.

Please refer toFIG. 7(b), which illustrates the second circuit. As illustrated inFIG. 7(b), when the bit value determination unit forms the second circuit, the switch unit510is switched off; the first capacitor array521has a first equivalent capacitor with a capacitance of (2m−1)C, a second equivalent capacitor with a capacitance of (2n−2m+1)C, and a third equivalent capacitor with a capacitance of (2N-1−2n)C; and the second capacitor array522has a fourth equivalent capacitor with a capacitance of (2m−1)C, a fifth equivalent capacitor with a capacitance of (2n−2m+1)C, and a sixth equivalent capacitor with a capacitance of (2N-1−2n)C. The bias contact of the first equivalent capacitor is coupled to VREF, the bias contact of the second equivalent capacitor is coupled to VCM, the bias contact of the third equivalent capacitor is kept floating, the bias contact of the fourth equivalent capacitor is coupled to VGND, the bias contact of the fifth equivalent capacitor is coupled to VCM, and the bias contact of the sixth equivalent capacitor is kept floating.

In the second circuit, the comparator530will see a voltage of VAP+((2m−1)/2n)(VREF−VCM) at the positive input end, a voltage of VAN−((2m−1)/2n)VCMat the negative input end, that is, a voltage difference of VAP−VAN−((1−2m)/2n)VREFbetween the positive input end and the negative input end. Taking N=4 as an example, wherein:

when (n, m)=(1,1), the voltage difference will be VAP−VAN−(−½)VREF;

when (n, m)=(2,1), the voltage difference will be VAP−VAN−(−¼)VREF;

when (n, m)=(2,2), the voltage difference will be VAP−VAN−(−¾)VREF;

when (n, m)=(3,1), the voltage difference will be VAP−VAN−(−⅛)VREF;

when (n, m)=(3,2), the voltage difference will be VAP−VAN−(−⅜)VREF;

when (n, m)=(3,3), the voltage difference will be VAP−VAN−(−⅝)VREF;

when (n, m)=(3,4), the voltage difference will be VAP−VAN−(−⅞)VREF.

During the voltage comparison stage, if VAP−VAN>0, then (n, m) will be set in an order to first compare VAP−VANwith (½)VREF, and then compare VAP−VANwith (¼)VREFor (¾)VREF, and finally compare VAP−VANwith (⅛)VREFor (⅜)VREFor (⅝)VREFor (⅞)VREF. If VAP−VAN<0, then (n, m) will be set in an order to first compare VAP−VANwith (−½)VREF, and then compare VAP−VANwith (−¼)VREFor (−¾)VREF, and finally compare VAP−VANwith (−⅛)VREFor (−⅜)VREFor (−⅝)VREFor (−⅞)VREF.

Therefore, assume VAP−VAN=( 9/32)VREF, then the comparator530will output “1” because ( 9/32)VREFis higher than 0, and (n, m) will be set as (1,1); when (n, m)=(1,1), the comparator530will output “0” because ( 9/32)VREF−(½)VREFis lower than 0, and (n, m) will be set as (2,1); when (n, m)=(2,1), the comparator530will output “1” because ( 9/32)VREF−(¼)VREFis higher than 0, and (n, m) will be set as (3,2); when (n, m)=(3,2), the comparator530will output “0” because ( 9/32)VREF−(⅜)VREFis lower than 0, wherein, when (n, m)=(1,1), (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (C, C); when (n, m)=(2,1), (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (C, 3C); when (n, m)=(3,2), (the first equivalent capacitor, the second equivalent capacitor) will form a combination of (3C, 8C). In this manner, a digital output code (1010) corresponding to ( 9/32)VREFwill be generated.

As can be seen in the above specification, the present invention has disclosed an SAR ADC capable of reducing energy consumption, which is different from the prior art as follows:

1. The capacitance of the largest capacitor of the present invention is (2N-3C), which is only one fourth of the corresponding capacitance (2N-1C) of the prior art SAR ADC; and the total capacitance of the present invention is (2×2N-1C), which is only one half of that of the prior art, so the present invention is capable of greatly reducing the chip area.

2. The prior art SAR ADC uses the larger capacitors to perform voltage comparisons during the earlier period of a voltage comparison stage, while the present invention uses the smaller ones. As the voltage comparisons during the earlier period of a voltage comparison stage of an SAR analog-to-digital conversion (including comparing an analog input voltage with ±(½)VREF, and ±(¼)VREF) involve larger voltage variations, therefore, by using smaller capacitors for voltage comparisons during the earlier period, the present invention can greatly reduce the charge flowing out of the reference voltage VREF, and thereby greatly reduce the energy consumption.

3. The prior art SAR ADC has to use all the capacitors when performing the voltage comparisons, while the present invention is capable of letting some capacitors floating to further reduce energy consumption.

4. The prior art SAR ADC uses a reference voltage VREFand a ground voltage to define a comparison range from −VREFto VREF, while the present invention inserts a common mode voltage between the reference voltage VREFand the ground voltage in defining the comparison range, so as to reduce the voltage across a capacitor circuit, and therefore further reduce the energy consumption.

The derived average energy consumption of the prior art N bits SAR ADC is as follows:

Taking N=10 as an example, the average energy consumption of the prior art is equal to 1363.33 CVREF2; while the average energy consumption of the present invention is as follows:

With N=10, the average energy consumption of the present invention is equal to 31.88 CVREF2. Comparing to the prior art, the present invention saves 97.66% of the energy consumption.

In summation of the above description, the present invention herein enhances the performance than the conventional structure and further complies with the patent application requirements and is submitted to the Patent and Trademark Office for review and granting of the commensurate patent rights.