US20110221618A1

US20110221618A1 - Continuous-time delta-sigma adc with compact structure

Info

Publication number: US20110221618A1
Application number: US12/723,680
Authority: US
Inventors: Jen-Che Tsai
Original assignee: MediaTek Inc
Current assignee: MediaTek Inc
Priority date: 2010-03-14
Filing date: 2010-03-14
Publication date: 2011-09-15
Anticipated expiration: 2030-03-14
Also published as: US20110285562A1; TW201136188A; US8018365B1; US8629793B2; CN102270989A; CN102270989B

Abstract

A continuous-time delta-sigma Analog to Digital Converter (ADC) with a compact structure comprises a loop filter, a summing circuit, a quantizer, and a current Digital to Analog Converter (DAC). The loop filter is utilized for receiving and noise-shaping an analog input signal, and accordingly outputting a positive and a negative loop voltages. The summing circuit comprises a positive and a negative summing resistors. The summing resistors are utilized for transforming a positive and negative feedback currents to be a positive and a negative feedback voltages, and summing the loop voltages and the feedback voltages so as to generate a positive and a negative summing voltages, respectively. The quantizer is utilized for outputting a digital output signal according to a difference between the positive and the negative summing voltages. The current DAC is utilized for generating the positive and the negative feedback currents according to the digital output signal.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a continuous-time delta-sigma Analog to Digital Converter (ADC), and more particularly, to a continuous-time delta-sigma ADC with a compact structure.
2. Description of the Prior Art
The delta-sigma ADC, also referred as the Δ/Σ ADC, has a major advantage of shaping the quantization noise spectrum for efficiently removing the noise from the output. More specifically, the delta-sigma ADC can move the noise from low frequencies into high frequencies so that the noise of the output can be filtered out by a low-pass filter. Since the continuous-time delta-sigma ADC is able to operate at a higher sampling frequency than the discrete-time delta-sigma ADC, the continuous-time delta-sigma ADC is more applicable in wireless communication receivers.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional continuous-time delta-sigma ADC 100. The continuous-time delta-sigma ADC 100 comprises a loop filter 110, a summing circuit 120, a quantizer 130, and a current Digital to Analog Converter (DAC) 140. The loop filter 110 noise-shapes an analog input signal S_AlN, and then accordingly outputs a positive loop voltage V_L+ and a negative loop voltage V_L−. The summing circuit 120 comprises a positive input resistor R_I+, a negative input resistor R_I+, a positive feedback resistor R_F+, a negative feedback resistor R_F−, and a fully differential amplifier 121. The resistors R_I+, R_I−, R_F+, and R_F− all have the same resistance R. The summing circuit 120 generates a positive summing voltage V_S+ and a negative summing voltage V_S− according to a positive feedback current I_FB+, a negative feedback current I_FB−, and the loop voltages V_L+ and V_L−. The quantizer 130 outputs a digital output signal S_DOUTaccording the difference between the summing voltages V_S+ and V_S−. The current DAC 140 drains/sources the positive feedback current I_FB+ from/to the summing circuit 120, and sources/drains the negative feedback current I_FB− to/from the summing circuit 120 according to the value of the digital output signal S_DOUT. For example, when the value of the digital output signal S_DOUTis larger than zero, the current DAC 140 sources the positive feedback current I_FB+ to the summing circuit 120 and drains the negative feedback current I_FB− from the summing circuit 120; when the value of the digital output signal S_DOUTis smaller than zero, the current DAC converter 140 drains the positive feedback current I_FB+ from the summing circuit 120 and sources the negative feedback current I_FB− to the summing circuit 120. The magnitudes of the feedback currents I_FB+ and I_FB− are both (N_DOUT×I_DAC), wherein I_DACis the magnitude of the Least Significant Bit (LSB) current of the current DAC 140, and N_DOUTis a value represented by the digital output signal S_DOUT. For example, when the value N_DOUTrepresented by the digital output signal S_DOUTis (+3), the current DAC 140 drains the positive feedback current I_FB+ from the summing circuit 120 and sources the negative feedback current I_FB− to the summing circuit 120 with the magnitude 3×I_DAC; when the value N_DOUTrepresented by the digital output signal S_DOUTis (−2), the current DAC 140 sources the positive feedback current I_FB+ to the summing circuit 120 and drains the negative feedback current I_FB− from the summing circuit 120 with the magnitude 2×I_DAC. The difference between the summing voltages V_S+ and V_S−can be represented by the following formula:
(V _S+ −V _S−)=(V _L+ −V _L−)−2×N _DOUT ×I _DAC ×R (1).
Since the structure of the conventional continuous-time delta-sigma ADC is complicated, the layout area is wasted and the loop delay is increased, causing a higher cost and worse system stability.

SUMMARY OF THE INVENTION

The present invention provides a continuous-time delta-sigma Analog to Digital Converter (ADC) with a compact structure. The continuous-time delta-sigma ADC comprises a loop filter, a summing circuit, a quantizer, and a current Digital to Analog Converter (DAC). The loop filter is utilized for receiving and noise-shaping an analog input signal, and accordingly outputting a positive loop voltage and a negative loop voltage. The summing circuit comprises a positive summing resistor, and a negative summing resistor. The positive summing resistor is utilized for transforming a positive feedback current to be a positive feedback voltage, and summing the positive loop voltage and the positive feedback voltage so as to generate a positive summing voltage. The negative summing resistor is utilized for transforming a negative feedback current to be a negative feedback voltage, and summing the negative loop voltage and the negative feedback voltage so as to generate a negative summing voltage. The quantizer is utilized for outputting a digital output signal according to a difference between the positive summing voltage and the negative summing voltage. The current DAC is utilized for generating the positive and the negative feedback currents according to the digital output signal.
The present invention further provides a continuous-time delta-sigma ADC with a compact structure. The continuous-time delta-sigma ADC comprises a loop filter, a quantizer, and a current DAC. The loop filter comprises a delta-sigma modulator, and an output stage. The delta-sigma modulator is utilized for receiving and noise-shaping an analog input signal and accordingly generating a positive and a negative analog signals. The output stage is utilized for outputting a positive and a negative loop voltages. The output stage comprises a positive output transistor, and a negative output transistor. The positive output transistor comprises a control end, a first end, and a second end. The control end of the positive output transistor is utilized for receiving the positive analog signal. The first end of the positive output transistor is utilized for receiving a first reference current. The positive output transistor outputs the positive loop voltage at the first end of the positive output transistor according to the positive analog signal and the first reference current. The negative output transistor comprises a control end, a first end, and a second end. The control end of the negative output transistor is utilized for receiving the negative analog signal. The first end of negative output transistor is utilized for receiving the first reference current. The negative output transistor outputs the negative loop voltage at the first end of the negative output transistor according to the negative analog signal and the first reference current. The quantizer is utilized for outputting a digital output signal according to the difference between the positive loop voltage and the negative loop voltage. The quantizer comprises a first reference current module, a positive voltage divider, a negative voltage divider, and a comparing module. The first reference current module is utilized for providing the first reference current. The positive voltage divider is coupled between the first end of the positive output transistor and the first reference current module. The positive voltage divider is utilized for receiving the first reference current. The positive voltage divider comprises (N₁−1) positive voltage dividing resistors for generating N₁positive comparing voltages. An A^thpositive comparing voltage can be represented by a following formula:
V _CA+ =V _C1+ +I _REF×(A−1)×R _Q;
the negative voltage divider is coupled between the first end of the negative output transistor and the first reference current module. The negative voltage divider is utilized for receiving the first reference current. The negative voltage divider comprises (N₁−1) negative voltage dividing resistors for generating N₁negative comparing voltages according to the first reference current. A B^thnegative comparing voltage can be represented by a following formula:
V _CB− =V _C1− +I _REF×(B−1)×R _Q;
wherein N₁, A, and B represents positive integers, 1≦A≦N₁, 1≦B≦N₁, V_CA+ represents the A^thpositive comparing voltage, V_CB−represents the B^thnegative comparing voltage, V_C1+ represents a first positive comparing voltage, V_C1−represents a first negative comparing voltage, I_REFrepresents magnitude of the first reference current, and R_Qrepresents resistance of one of the (N₁−1) positive or the (N₁−1) negative voltage dividing resistors. Resistance of the (N₁−1) positive voltage dividing resistors and the (N₁−1) negative voltage dividing resistors are same. The comparing module comprises (N₁−1) comparators for generating (N₁−1) compared result signals. An M^thcomparator of the (N₁−1) comparators compares an M^thpositive comparing voltage of the (N₁−1) positive comparing voltages and an (N₁−M)^thnegative comparing voltage of the (N₁−1) negative comparing voltages for generating an M^thcompared result signal and 1≦M≦(N₁−1). The digital output signal is obtained by combining the (N₁−1) compared result signals. The current DAC is coupled to the positive voltage divider, the negative voltage divider, and the first reference current module. The current DAC is utilized for generating the positive and the negative feedback currents according to the digital output signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a conventional continuous-time delta-sigma ADC.

FIG. 2 is a diagram illustrating a continuous-time delta-sigma ADC having a compact structure according to a first embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating the quantizer of the present invention.

FIG. 4 is a diagram illustrating a continuous-time delta-sigma ADC according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating the loop filter of the continuous-time delta-sigma ADC.

FIG. 6 is a diagram illustrating a continuous-time delta-sigma ADC according to a third embodiment of the present invention.

FIG. 7 is a diagram illustrating a current DAC according to a first embodiment of the current DAC.

FIG. 8 is a diagram illustrating a current DAC according to a second embodiment of the current DAC.

DETAILED DESCRIPTION

Please refer to FIG. 2. FIG. 2 is a diagram illustrating a continuous-time delta-sigma ADC 200 having a compact structure according to a first embodiment of the present invention. The continuous-time delta-sigma ADC 200 comprises a loop filter 210, a summing circuit 220, a quantizer 230, and a current DAC 240. The structures and the functions of the loop filter 210, the quantizer 230, and the current DAC 240 are similar to those of the loop filter 110, the quantizer 130, and the current DAC 140 and will not be repeated again for brevity.
The summing circuit 220 comprises a positive summing resistor R_S+ and a negative summing resistor R_S−. The summing resistors R_S+ and R_S− have the same resistance R. The summing resistors R_S+ and R_S− transform the feedback currents I_FB+ and I_FB− to the feedback voltages V_FB+ and V_FB−, respectively, and sum the loop voltages V_L+ and V_L− to the feedback voltages V_FB+ and V_FB−, respectively, so as to generate the summing voltages V_S+ and V_S−. The details are explained as follows.
The positive feedback voltage V_FB+, which is the voltage drop cross the positive summing resistor R_S+, is generated by the positive feedback current I_FB+ passing through the positive summing resistor R_S+. The direction of the positive feedback current I_FB+ is determined by the value of the digital output signal S_DOUT. For example, when the value N_DOUTrepresented by the digital output signal S_DOUTis larger than zero, the current DAC 240 sources the positive feedback current I_FB+ to the summing circuit 220, which means the positive feedback current I_FB+ passes from the nodes P₂to P₁; when the value N_DOUTrepresented by the digital output signal S_DOUTis smaller than zero, the current DAC 240 drains the positive feedback current I_FB+ to the summing circuit 220, which means the positive feedback current I_FB+ passes from the nodes P₁to P₂. The magnitude of the positive feedback current I_FB+ is (N_DOUT×I_DAC), wherein I_DACis the magnitude of the LSB current of the current DAC 240. In this way, the voltage level of the positive feedback voltage V_FB+ is (−N_DOUT×I_DAC×R_S+). The positive summing voltage V_S+ is obtained by summing the positive feedback voltage V_FB+ and the positive loop voltage V_L+.
The negative feedback voltage V_FB−, which is the voltage drop cross the negative summing resistor R_S−, is generated by the negative feedback current I_FB− passing through the negative summing resistor R_S−. The direction of the negative feedback current I_FB− is determined by the value of the digital output signal S_DOUT. For example, when the value N_DOUTis larger than zero, the current DAC 240 drains the negative feedback current I_FB− to the summing circuit 220, which means the negative feedback current I_FB+ passes from the nodes N₁to N₂; when the value N_DOUTis smaller than zero, the current DAC 240 sources the negative feedback current I_FB− to the summing circuit 220, which means the negative feedback current I_FB− passes from the nodes N₂to N₁. The magnitude of the negative feedback current I_FB− is (N_DOUT×I_DAC), wherein I_DACis the magnitude of the LSB current of the current DAC 240. In this way, the voltage level of the negative feedback voltage V_FB−is (N_DOUT×I_DAC×R_S−). The negative summing voltage V_S− is obtained by summing the negative feedback voltage V_FB− and the negative loop voltage V_L−.
As a result, the difference between the summing voltages V_S+ and V_S− can be represented by the following formula according to the above-mentioned description:
$\begin{matrix} \begin{matrix} (V_{s +} - V_{s -}) = (V_{L +} + V_{FB +}) - (V_{L -} + V_{FB +}) \\ = (V_{L +} - I_{FB +} \times R_{S +}) - (V_{L -} + I_{FB -} \times R_{S -}) \\ = (V_{L +} - I_{FB +} \times R) - (V_{L -} + I_{FB -} \times R) \\ = (V_{L +} - N_{DOUT} \times I_{DAC} \times R) - \\ = (V_{L -} + N_{DOUT} \times I_{DAC} \times R) \\ = (V_{L +} - V_{L -}) - 2 \times S_{DOUT} \times I_{DAC} \times R; \end{matrix} & (2) \end{matrix}$
Comparing formulas (1) with (2), the difference between the summing voltages V_S+ and V_S− remains the same value. Therefore, the continuous-time delta-sigma ADC 200 can operate with a compact summing circuit 220, saving the consumption of layout area.
Please refer to FIG. 3. FIG. 3 is a schematic diagram illustrating the quantizer 230 of the present invention. The quantizer 230 comprises a positive input transistor Q_I+, a negative input transistor Q_I−, a positive voltage divider 231, a negative voltage divider 232, a reference current module 233, and a comparing module 234. The reference current module 233 comprises two current sources 2331 and 2332. The current sources 2331 and 2332 respectively provide a reference current I_REFto the voltage dividers 231 and 232. The magnitude of the reference current is constant. The voltage dividers 231 and 232 respectively comprise a plurality of positive voltage dividing resistors R_QX1˜R_QX(N-1)and a plurality of negative voltage dividing resistors R_QY1˜R_QY(N-1), wherein the voltage dividing resistors R_QX1˜R_QX(N-1)and R_QY1˜R_QY(N-1)all have the same resistance R_Q. The resistance R_Qrelates to the resistance R, and the relation between the resistance R and the resistance R_Qcan be represented by the following formula:
R=(N−1)×R _Q (3);
the voltage dividing resistors R_QX1˜R_QX(N-1)and R_QY1˜R_QY(N-1)are utilized for generating the positive comparing voltages V_X1˜V_XNon the nodes X₁˜X_Nand the negative comparing voltages V_Y1˜V_YNon the nodes Y₁˜Y_N, respectively. The comparing voltages V_X1˜V_XNand V_Y1˜V_YNcan be represented by the following formulas:
V _XA =V _X1 +I _REF×(A−1)×R _Q (4);
V _YB =V _Y1 +I _REF×(B−1)×R _Q (5);
wherein 1≦A≦N, 1≦B≦N, A, B, and N represent positive integers, V_XAand V_YBrepresent the A^thpositive comparing voltage (the voltage on the node X_Aof the positive voltage divider 231) and the B^thnegative comparing voltage (the voltage on the node Y_Bof the voltage divider 232) respectively. The first ends of the input transistors Q_I+ and Q_I− are coupled to the voltage divider 231 and 232, respectively; the control ends of the input transistors Q_I+ and Q_I− are coupled to the positive summing resistor R_S+ (the node P₂) and the negative summing resistor R_S− (the node N₂) of the summing circuit 220. The input transistors Q_I+ and Q_I− output the 1^stpositive comparing voltage V_X1and the 1^stnegative comparing voltage V_Y1by the first ends of the input transistors Q_I+ and Q_I− to the voltage dividers 231 and 232 as the bias voltages of the voltage dividers 231 and 232, respectively. The voltage drop (the positive transforming voltage) V_T+ between the first end and the control end of the positive input transistor Q_I+ and the voltage drop (the negative transforming voltage) V_T− between the first end and the control end of the negative input transistor Q_I− are fixed because the magnitudes of the reference currents passing through the transistors Q_I+ and Q_I− are the same. Since the voltage level of the 1^stpositive comparing voltage V_X1equals (V_S++V_T+) and the voltage level of the 1^stnegative comparing voltage V_Y1equals (V_S−+V_T−), the transforming voltages V_T+ and V_T− are designed to be equal so that (V_X1−V_Y1) equals to (V_S+−V_S−).
The comparing module 234 comprises a plurality of comparators CMP₁˜CMP_(N-1). The comparators CMP₁˜CMP_(N-1)compare the comparing voltages V_X1˜V_XNand V_Y1˜V_YNfor generating the compared result signals S_B1˜S_B(N-1), respectively. For example, when the positive comparing voltage V_X1is higher than the negative comparing voltage V_Y(N-1), the comparator CMP₁outputs the compared result signal S_B1with logic 1; when the positive comparing voltage V_X1is lower than the negative comparing voltage V_Y(N-1), the comparator CMP₁outputs the compared result signal S_B1with logic 0. When the positive comparing voltage V_X2is higher than the negative comparing voltage V_Y(N-2), the comparator CMP₂outputs the compared result signal S_B2with logic 1; when the positive comparing voltage V_X2is lower than the negative comparing voltage V_Y(N-2), the comparator CMP₂outputs the compared result signal S_B2with logic 0. The rest comparators CMP₃˜CMP_(N-1)operate in the similar way. By combining the compared result signals S_B1˜S_B(N-1), the digital output signal S_DOUTis obtained. For example, assuming S_DOUTis a 3-bit signal and the value N_DOUTrepresented by the digital output signal S_DOUTby means of 1's complement method is between −3 and 3. The compared result signal S_B1represents the Most Significant Bit (MSB), and the compared result signal S_B(N-1)represents the LSB. In this way, the digital output signal S_DOUTcan be represented by [S_B1,S_B2,S_B3]. When the digital output signal S_DOUTis [1,0,0], the value N_DOUTis −3. When the digital output signal S_DOUTis [0,1,1], the value N_DOUTis 3 and so on. Therefore, an A^thcompared result signal SBA is determined by the difference between the A^thpositive comparing voltage V_XAand the (N−A)^thnegative comparing voltage V_Y(N-A)and the difference between the comparing voltages V_XAand V_Y(N-A)can be represented by the following formula according to the formulas (3), (4), and (5):
$\begin{matrix} \begin{matrix} V_{XA} - V_{Y (N - A)} = [V_{X 1} + I_{REF} \times (A - 1) \times R_{Q}] - \\ [V_{Y 1} + I_{REF} \times (N - A - 1) \times R_{Q}] \\ = (V_{X 1} - V_{Y 1}) - I_{REF} \times (N - 2 \times A) \times R_{Q} \\ = (V_{L +} - V_{L -}) - 2 \times N_{DOUT} \times I_{DAC} \times R - \\ I_{REF} \times (N - 2 - A) \times R_{Q} \\ = (V_{L +} - V_{L -}) - 2 \times N_{DOUT} \times I_{DAC} \times (N - 1) \times \\ R_{Q} - I_{REF} \times (N - 2 - A) \times R_{Q}; \end{matrix} & (6) \end{matrix}$
wherein 1≦A≦(N−1), A represents a positive integer. Since the digital output signal S_DOUTis obtained by the compared result signals S_B1˜S_B(N-1)and the compared result signals S_B1˜S_B(N-1)can be determined by the formula (6), the digital output signal S_DOUTcan be obtained by the formula (6).
Please refer to FIG. 4. FIG. 4 is a diagram illustrating a continuous-time delta-sigma ADC 400 according to a second embodiment of the present invention. The continuous-time delta-sigma ADC 400 comprises a loop filter 210, a current DAC 240, and a quantizer 430. The functions of loop filter 210 and the current DAC 240 are described as above-mentioned and will not be repeated again for brevity. It is noticeable that the quantizer 430 is integrated with summing function so that the summing circuit is no longer required. Compared to the quantizer 230, the quantizer 430 does not comprise the input transistors Q_I+ and Q_I−. In other words, in the continuous-time delta-sigma ADC 200, the output ends O₁and O₂of the loop filter 210 are coupled to the control ends of the input transistors Q_I+ and Q_I−, respectively. However, in the continuous-time delta-sigma ADC 400, the output ends O₁and O₂of the loop filter 210 are, instead, coupled to the voltage dividers 231 and 232, respectively. Hence, in the continuous-time delta-sigma ADC 400, the comparing voltages V_X1and V_Y1equal to the loop voltages V_L+ and V_L−, respectively. In addition, in the continuous-time delta-sigma ADC 200, the output ends O₁and O₂of the current DAC 240 are coupled to the control ends of the input transistors Q_I+ and Q_I−, respectively. However, in the continuous-time delta-sigma ADC 400, the output end O₁of the current DAC 240 are coupled to the current source 2331 and the node X_Nof the voltage divider 231, and the output end O₂of the current DAC 240 are coupled to the current source 2332 and the node Y_Nof the voltage divider 232. In this way, the difference between the A^thpositive comparing voltage V_XAand the (N−A)^thnegative comparing voltage V_Y(N-A)can be represented by the following formula:
$\begin{matrix} \begin{matrix} V_{XA} - V_{Y (N - A)} = [\begin{matrix} V_{X 1} - N_{DOUT} \times I_{DAC} \times (N - 1) \times R_{Q} + \\ I_{REF} \times (A - 1) \times R_{Q} \end{matrix}] - \\ [\begin{matrix} V_{Y 1} + N_{DOUT} \times I_{DAC} \times (N - 1) \times R_{Q} + \\ I_{REF} \times (N - A - 1) \times R_{Q} \end{matrix}] \\ = (V_{L +} - V_{L -}) - 2 \times N_{DOUT} \times I_{DAC} \times (N - 1) \times \\ R_{Q} - I_{REF} \times (N - 2 \times A) \times R_{Q}; \end{matrix} & (7) \end{matrix}$
by comparing formulas (6) and (7), it can be derived that the compared result signal S_BAdetermined by the difference between the comparing voltage V_XAand the comparing voltage V_Y(N-A)remains the same value. That is, the continuous-time delta-sigma ADC 400 is equivalent to the continuous-time delta-sigma ADC 200. In addition, since no summing circuit exists between the quantizer 430 and the loop filter 210, the delay from the output of the loop filter 210 to the quantizer 430 is reduced so that the system stability of the continuous-time delta-sigma ADC 400 is improved.
Please refer to FIG. 5. FIG. 5 is a diagram illustrating the loop filter 210 of the continuous-time delta-sigma ADC 400. The loop filter 210 comprises an output stage 211 and a delta-sigma modulator 212. The delta-sigma modulator 212 receives and noise-shapes the analog input signal SAN, and accordingly generates a positive analog signal S_A+ and a negative analog signal S_A−. The output stage 211 comprises a reference current module 2111, a positive output transistor Q_O+, and a negative output transistor Q_O−. The reference current module 2111 comprises two current sources 21111 and 21112. The current sources 21111 and 21112 provide enough currents to output transistors Q_O+ and Q_O−, so that the output transistors Q_O+ and Q_O− can operate in the saturation region and can be utilized as source followers. The control end of the positive output transistor Q_O+ receives the positive analog signal S_A+; the first end (source) of the positive output transistor Q_O+ outputs the positive loop voltage V_L+ according to the positive analog signal S_A+. The control end of the negative output transistor Q_O− receives the negative analog signal S_A−; the first end (source) of the negative output transistor Q_O− outputs the negative loop voltage V_L− according to the negative analog signal S_A−. The node O₁between the current source 21111 and the positive output transistor Q_O+, and the node O₂between the current source 21112 and the negative output transistor Q_O− are the output ends of the loop filter 210 for outputting the loop voltages V_L+ and V_L−.
Please refer to FIG. 6. FIG. 6 is a diagram illustrating a continuous-time delta-sigma ADC 600 according to a third embodiment of the present invention. The structure and operational principles of the continuous-time delta-sigma ADC 600 are similar to those of the continuous-time delta-sigma ADC 400. The difference is that, in the continuous-time delta-sigma ADC 600, the output stage 611 of the loop filter 610 does not comprise the reference current module 2111. The first ends of the output transistors Q_O+ and Q_O− are coupled to the current sources 2331 and 2332 through the voltage dividers 231 (the node X₁) and 232 (the node Y₁), respectively. Since the currents provided by the current sources 2331 and 2332 are enough for the output transistors Q_O+ and Q_O− operating in the saturation region, the current sources 21111 and 21112 from FIG. 5 are not required in the output stage 611 of the loop filter 610. The output transistors Q_O+ and Q_O− can operate as source followers for outputting the loop voltages V_L+ and V_L− so that the continuous-time delta-sigma ADC 600 is equivalent to the continuous-time delta-sigma ADC 400 and the current consumption is reduced because the current sources 21111 and 21112 are saved.
Please refer to FIG. 7. FIG. 7 is a diagram illustrating a current DAC 340 according to a first embodiment of the current DAC 240. The current DAC 340 comprises a decoder 341 and a cell circuit 342. The decoder 341 receives the digital output signal S_DOUT(the compared result signals S_B1˜S_B(N-1)) from the input ends I₁˜I_(N-1)of the decoder 341. The decoder 341 outputs the switch control signals S_C1˜S_CMaccording to the digital output signal S_DOUT. The cell circuit 342 generates the feedback current I_FB+ and I_FB− according to the switch control signals S_C1˜S_CM. The cell circuit 342 comprises a souring current module 3421, a draining current module 3422, and a plurality of cells C₁˜C_M. The sourcing current module 3421 comprises a plurality of sourcing current source I_DACP1˜I_DACPM. The draining current module 3422 comprises a plurality of draining current source I_DACN1˜I_DACNM. The sourcing current sources I_DACP1˜I_DACPMand the draining current source I_DACN1˜I_DACNMprovide currents with the same magnitude I_DAC. Each cell of the plurality of the cells C₁˜C_Mcomprises four switches. For example, the cell C₁comprises four switches SW_1PA, SW_1PB, SW_1NA, and SW_1NB. The control ends C of the switches SW_1PA, SW_1PB, SW_1NA, and SW_1NBare coupled to the output end Z₁of the decoder for receiving the switch control signal S_C1; the first ends 1 of the switches SW_1PA, SW_1PB, SW_1NA, and SW_1NBare coupled to the second ends 2 of the switches SW_1PA, SW_1PB, SW_1NA, and SW_1NBaccording to the switch control signal S_C1. More particularly, when the switch control signal S_C1indicates “draining”, the switches SW_1NAand SW_1PBcouple their first ends 1 to their second ends 2, but the switches SW_1NBand SW_1PAdo not couple their first ends 1 to their second ends 2. In this way, the draining current source I_DACN1drains a current with a magnitude I_DACthrough the cell C₁from the output end O₁of the current DAC 340, and the sourcing current source I_DACP1sources a current with a magnitude I_DACthrough the cell C₁to the output end O₂of the current DAC 340. When the switch control signal S_C1indicates “sourcing”, the switches SW_1NBand SW_1PAcouple their first ends 1 to their second ends 2, but the switches SW_1NAand SW_1PBdo not couple their first ends 1 to their second ends 2. In this way, the draining current source I_DACN1drains a current with a magnitude I_DACthrough the cell C₁from the output end O₂of the current DAC 340, and the sourcing current source I_DACP1sources a current with a magnitude I_DACthrough the cell C₁to the output end O₁of the current DAC 340. The structures and operation principles of the rest cells C₂˜C_Mare similar to the cell C₁and are not repeated again for brevity. By means of the decoder 341 transforming the digital output signal S_DOUTto be the switch control signals S_C1˜S_CM, and the current modules 3421 and 3422 draining/sourcing currents through the cells C₁˜C_Maccording to the switch control signals S_C1˜S_CM, the current DAC 340 generates the feedback currents I_FB+ and I_FB− according to the digital output signal S_DOUT.
Please refer to FIG. 8. FIG. 8 is a diagram illustrating a current DAC 440 according to a second embodiment of the current DAC 240. The current DAC 440 comprises a decoder 441 and a cell circuit 442. The structure and the operational principle of the decoder 441 are similar to the decoder 341 and are not repeated again for brevity. Comparing the cell circuits 442 with 342, each cell C₁˜C_Mof the cell circuit 442 comprises only two switches, and the cell circuit 442 comprises only a sourcing current module 4421. For example, the cell C₁comprises two switches SW_1PAand SW_1PB. The sourcing current source I_DACT1provides a cell current I_DACT1. It is noticeable that the magnitudes of the cell currents I_DACT1˜I_DACTMall equal to I_DAC2, and the magnitude I_DAC2is different from the above-mentioned magnitude I_DAC1. In this way, the magnitude of the currents provided by the current sources 2331 and 2332 of the quantizers 230 and 430 are adjusted according to the magnitude I_DAC2so as to keep the digital output signal S_DOUTcorrectly outputted. The switches SW_1PA, SW_1PBcouple their first ends 1 to their second ends 2 according to the switch control signal S_C1. For example, when the switch control signal S_C1indicates “draining”, the first end 1 of the switch SW_1PBis coupled to the second end 2 of the switch SW_1PB, but the first end 1 of the switch SW_1PAis not coupled to the second end 2 of the switch SW_1PA. In this way, the sourcing current source I_DACT1sources a current with a magnitude I_DAC2through the cell C₁from the output end O₂of the current DAC 440. When the switch control signal S_C1indicates “sourcing”, the first end 1 of the switch SW_1PAis coupled to the second end 2 of the switch SW_1PA, but the first end 1 of the switch SW_1PBis not coupled to the second end 2 of the switch SW_1PB. In this way, the sourcing current source I_DACT1sources a current with a magnitude I_DAC2through the cell C₁to the output end O₁of the current DAC 440. The structures and operation principles of the rest cells C₂˜C_Mare similar to the cell C₁and are not repeated again for brevity. By means of the decoder 441 transforming the digital output signal S_DOUTto be the switch control signals S_C1˜S_CM, and the cells C₁˜C_Mdraining/sourcing currents according to the switch control signals S_C1˜S_CM, the current DAC 440 generates the feedback currents I_FB+ and I_FB− according to the digital output signal S_DOUT.
In conclusion, the present invention provides a continuous-time delta-sigma ADC with a compact summing circuit. In addition, the present invention further integrates the summing function into the quantizer, reduces the current sources of the output stage of the loop filter, and reduces the current sources and switches in the current DAC. In this way, the structure of the continuous-time delta-sigma ADC becomes compact, saving the layout area and the current consumption, and improving the system stability.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

Claims

1-8. (canceled)

9. A continuous-time delta-sigma Analog to Digital Converter (ADC) with a compact structure, comprising:

a loop filter, comprising:

a delta-sigma modulator, for receiving and noise-shaping an analog input signal and accordingly generating a first and a second analog signals; and

an output stage, for outputting a first and a second loop voltages, comprising:

a first output transistor, comprising:

a control end, for receiving the first analog signal;

a first end, for receiving a first reference current; and

a second end;

wherein the positive output transistor outputs the first loop voltage at the first end of the first output transistor according to the first analog signal and the first reference current; and

a second output transistor, comprising:

a control end, for receiving the second analog signal;

a first end, for receiving the first reference current; and

a second end;

wherein the second output transistor outputs the second loop voltage at the first end of the second output transistor according to the second analog signal and the first reference current;

a quantizer, for outputting a digital output signal according to the difference between the first loop voltage and the second loop voltage, the quantizer comprising:

a first reference current module, for providing the first reference current;

a first voltage divider, coupled between the first end of the first output transistor and the first reference current module, for receiving the first reference current, the first voltage divider comprising (N₁−1) positive voltage dividing resistors for generating N₁positive comparing voltages according to the first reference current, and an A^thpositive comparing voltage can be represented by a following formula:

V _CA+ =V _C1+ +I _REF×(A−1)×R _Q;

a second voltage divider, coupled between the first end of the second output transistor and the first reference current module, for receiving the first reference current, the second voltage divider comprising (N₁−1) second voltage dividing resistors for generating N₁second comparing voltages according to the first reference current, a B^thsecond comparing voltage can be represented by a following formula:

V _CB− =V _C1− +I _REF×(B−1)×R _Q;

wherein N₁, A, and B represents positive integers, 1≦A≦N₁, 1≦B≦N₁, V_CA+ represents the A^thfirst comparing voltage, V_CB− represents the B^thsecond comparing voltage, V_C1+ represents a first comparing voltage, V_C1− represents a first second comparing voltage, I_REFrepresents magnitude of the first reference current, and R_Qrepresents resistance of one of the (N₁−1) first or the (N₁−1) second voltage dividing resistors;

wherein resistance of the (N₁−1) first voltage dividing resistors and the (N₁−1) second voltage dividing resistors are same; and

a comparing module, comprising (N₁−1) comparators for generating (N₁−1) compared result signals;

wherein an M^thcomparator of the (N₁−1) comparators compares an M^thfirst comparing voltage of the (N₁−1) first comparing voltages and an (N₁−M)^thsecond comparing voltage of the (N₁−1) second comparing voltages for generating an M^thcompared result signal and 1≦M≦(N₁−1);

wherein the digital output signal is obtained by combining the (N₁−1) compared result signals; and

a current DAC, coupled to the first voltage divider, the second voltage divider, and the first reference current module, for generating the first and the second feedback currents according to the digital output signal.

10. The continuous-time delta-sigma ADC of claim 9, wherein the current DAC comprises:

a decoder, for outputting N₂switch control signals according to the digital output signal, N₂representing a positive integer; and

a cell circuit, for generating the first and the second feedback currents according to the N₂switch control signals.

11. The continuous-time delta-sigma ADC of claim 9, wherein the loop filter further comprises:

a second reference current module, for providing a second reference current to the first output transistor and the second output transistor.

12. A continuous-time delta-sigma Analog to Digital Converter (ADC) with a compact structure, comprising:

a loop filter, for receiving and noise-shaping an analog input signal, and accordingly outputting a first loop voltage and a second loop voltage;

a summing circuit, comprising:

a first summing resistor, for transforming a first feedback current to be a first feedback voltage, and summing the first loop voltage and the first feedback voltage so as to generate a first summing voltage, wherein the first summing voltage is equal to a sum of the first loop voltage and the first feedback voltage; and

a quantizer, for outputting a digital output signal according to a difference between the first summing voltage and the second loop voltage; and

a current Digital to Analog Converter (DAC), for generating the first feedback current according to the digital output signal.

13. The continuous-time delta-sigma ADC of claim 12, wherein the current DAC generates a second feedback current according to the digital output signal;

wherein the summing circuit further comprises:

a second summing resistor for transforming the second feedback current to be a second feedback voltage, and summing the second loop voltage and the second feedback voltage so as to generate a second summing voltage;

wherein the quantizer outputs the digital output signal according to a difference between the first summing voltage and the second summing voltage.

14. The continuous-time delta-sigma ADC of claim 13, wherein the first summing resistor and the second summing resistor have same resistance.

15. The continuous-time delta-sigma ADC of claim 14, wherein the difference between the first summing voltage and the second summing voltage can be represented by a following formula:

(V _S+ −V _S−)=(V _L+ −I _FB+ ×R)−(V _L− +I _FB− ×R);

wherein V_S+ represents the first summing voltage, V_S− represents the second summing voltage, V_L+ represents the first loop voltage, V_L− represents the second loop voltage, I_FB+ represents magnitude of the first feedback current, I_FB− represents magnitude of the second feedback current, and R represents resistance of one of the first or the second summing resistors.

16. The continuous-time delta-sigma ADC of claim 15, wherein magnitude of the first or the second feedback current can be represented by a following formula:

I _FB =N _DOUT ×I _DAC;

wherein I_FBrepresents the magnitudes the first or the second feedback current, N_DOUTrepresents a value represented by the digital output signal, I_DACrepresent magnitude of a Least Significant Bit (LSB) current of the current DAC.

17. The continuous-time delta-sigma ADC of claim 16, wherein when the value represented by the digital output signal is larger than a predetermined value, the current DAC sources the first and the second feedback currents to the summing circuit; when the value represented by the digital output signal is smaller than the predetermined value, the current DAC drains the first and the second feedback currents to the summing circuit.

18. The continuous-time delta-sigma ADC of claim 13, wherein the first feedback voltage is generated by the first feedback current passing through the first summing resistor; the second feedback voltage is generated by the second feedback current passing through the second summing resistor.

19. The continuous-time delta-sigma ADC of claim 13, wherein the current DAC comprises:

a decoder, for outputting N₂switch control signals according to the digital output signal; and

a cell circuit, for generating the first and the second feedback currents according to the N₂switch control signals;

wherein N₂represents a positive integer.

20. The continuous-time delta-sigma ADC of claim 19, wherein the cell circuit comprises:

a sourcing current module, for sourcing a predetermined current;

N₂cells, wherein a K^thcell comprises:

a first switch, comprising:

a first end, coupled to the sourcing current module;

a second end, coupled to the first summing resistor and the quantizer; and

a control end, coupled to the decoder for receiving a K^thswitch signal of the N₂switch control signals;

wherein when the K^thswitch control signal indicates sourcing, the first switch couples the first end of the first switch to the second end of the first switch;

wherein when the K^thswitch control signal indicates draining, the first switch does not couple the first end of the first switch to the second end of the first switch; and

a second switch, comprising:

a first end, coupled to the sourcing current module;

a second end, coupled to the negative summing resistor and the quantizer; and

wherein when the K^thswitch control signal indicates draining, the second switch couples the first end of the second switch to the second end of the second switch;

wherein when the K^thswitch control signal indicates sourcing, the second switch does not couple the first end of the second switch to the second end of the second switch;

wherein K represents a positive integer, and 1≦K≦N₂.

21. A summing circuit of saving layout area of a continuous-time delta-sigma Analog to Digital Converter (ADC), the continuous-time delta-sigma ADC having a loop filter, a quantizer, and a current Digital to Analog Converter (DAC), the loop filter receiving and noise-shaping an analog input signal, and accordingly outputting a first loop voltage and a second loop voltage, the quantizer outputting a digital output signal according to a difference between a first summing voltage and the second loop voltage, the current DAC generating a first feedback current according to the digital output signal, the summing circuit comprising:

a first summing resistor, for transforming the first feedback current to be a first feedback voltage, and summing the first loop voltage and the first feedback voltage so as to generate the first summing voltage;

wherein the first summing voltage is equal to a sum of the first loop voltage and the first feedback voltage.

22. The summing circuit of claim 21, wherein the current DAC generates a second feedback current according to the digital output signal;

wherein the summing circuit further comprises:

23. The summing circuit of claim 22, wherein the first summing resistor and the second summing resistor have same resistance.

24. The summing circuit of claim 23, wherein the difference between the first summing voltage and the second summing voltage can be represented by a following formula:

(V _C+ −V _C−)=(V _L+ −I _FB+ ×R)−(V _L− +I _FB− ×R);

25. The summing circuit of claim 24, wherein magnitude of the first or the second feedback current can be represented by a following formula:

I _FB =N _DOUT ×I _DAC;

26. The summing circuit of claim 25, wherein when the value represented by the digital output signal is larger than a predetermined value, the current DAC sources the first and the second feedback currents to the summing circuit; when the value represented by the digital output signal is smaller than the predetermined value, the current DAC drains the first and the second feedback currents to the summing circuit.

27. The summing circuit of claim 22, wherein the first feedback voltage is generated by the first feedback current passing through the first summing resistor; the second feedback voltage is generated by the second feedback current passing through the second summing resistor.

28. The summing circuit of claim 22, wherein the current DAC comprises:

wherein N₂represents a positive integer.

29. The continuous-time delta-sigma ADC of claim 28, wherein the cell circuit comprises:

a sourcing current module, for sourcing a predetermined current;

N₂cells, wherein a K^thcell comprises:

a first switch, comprising:

a first end, coupled to the sourcing current module;

a second end, coupled to the first summing resistor and the quantizer; and

a second switch, comprising:

a first end, coupled to the sourcing current module;

a second end, coupled to the negative summing resistor and the quantizer; and

wherein K represents a positive integer, and 1≦K≦N₂.