Abstract:
Disclosed is a signal processing circuit which outputs a digital word corresponding to a current source controlled by a physical response. The signal processing circuit includes an analog integrated circuit for generating an analog signal in response to a time varying reference signal and a signal corresponding to the current source controlled by the physical response, a reference signal generator for generating a reference signal, a comparator for comparing the analog signal with the reference signal, an output circuit for generating the digital word indicating a time interval defined by a start signal and an end signal indicating a transition of an output of the comparator, and a controller inactivating the comparator in response to the end signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims priority to Korean Patent Application No. 2002-44984, filed on Jul. 30, 2002, which is herein incorporated by reference in its entirety.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to electronic components for image sensing, capturing, and signal processing and, in particular, to an active image device which can be fabricated using standard CMOS (Complementary Metal Oxide Semiconductor) processes.  
         BACKGROUND  
         [0003]    Charge Coupled Device (CCD) imaging arrays have made possible high quality imagers now used in consumer camcorder equipment, scanners for FAX machines, and video cameras for a wide range of applications including video-conferencing, and portable equipment for professional TV broadcasting.  
           [0004]    With the advent of multimedia communications, there arises a need for low cost solid state image sensors to complement computers and communication devices and thus realize practical video telephones and the like. An image input device is central to any teleconferencing and multimedia application. Recently, CMOS image sensors have been recognized as a viable candidate for the image input device. CMOS image sensors also have utility in other fields such as robotics, machine vision, security surveillance, automotive applications and personal ID systems through fingerprint/retina scan. A distinct advantage of CMOS image sensors (or imagers) is that signal processing circuits can be readily integrated on the same chip as the image, thus enabling design of smart, single-chip image acquisition systems. CMOS imagers can be manufactured at lower cost than that of conventional charge coupled devices (CCDs) using conventional, preinstalled CMOS fabrication lines without any process modification.  
           [0005]    Since portable electronic equipment operates using batteries, it is preferable to design such equipment to provide low-power consumption. The use of a low-power image device enables portable electronic equipment to consume less power.  
           [0006]    As is well known, light is analog data that varies continuously. For discrete signal processing, analog data is converted into digital data. CMOS image devices (or imagers) incorporate a device that detects the light as an analog signal and converts a detected analog signal into digital data. For this, CMOS image devices typically incorporate an analog-to-digital converter. In this respect, one approach to realize a low-power image device is to reduce power consumption of analog-to-digital converters incorporated in the image device. Accordingly, there is a need for an analog-to-digital converter capable of reducing power consumption, which can be used for a CMOS image device.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention is directed to an analog-to-digital (AD) converter that is capable of reducing power consumption, and in particular, to a low-power CMOS image device that comprises an AD converter that provides reduced power consumption.  
           [0008]    According to one aspect of the present invention, an analog-to-digital converter circuit comprises a comparator for comparing an analog input signal with a reference signal; an output circuit for generating a digital word indicating a time interval defined by a start signal and an end signal, wherein the end signal indicates a transition of an output of the comparator; and a controller for inactivating the comparator in response to the end signal. For example, the controller inactivates the comparator when the output of the comparator transitions from an active state to an inactive state.  
           [0009]    In another aspect of the present invention, a signal processing circuit outputs a digital word corresponding to a current source controlled by a physical response. The signal processing circuit comprises: an analog integrated circuit for generating an analog signal in response to a time varying reference signal and a signal corresponding to the current source controlled by the physical response; a reference signal generator for generating a reference signal; a comparator for comparing the analog signal with the reference signal; an output circuit for generating the digital word indicating a time interval defined by a start signal and an end signal, wherein the end signal indicates a transition of an output of the comparator; and a controller for inactivating the comparator in response to the end signal. For example, the controller inactivates the comparator when the output of the comparator transitions from an active state to an inactive state. The controller includes an S-R latch that generates a first enable signal in response to an output of the comparator and a second enable signal, the comparator being inactivated or activated by the first enable signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as preferred embodiments become better understood by reference to the following detailed description when considered in conjuction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0011]    [0011]FIG. 1 is a schematic diagram of a CMOS image device according to an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a schematic diagram of a correlated double sampling (CDS) circuit and an output circuit according to embodiments of the invention;  
         [0013]    [0013]FIG. 3 is a circuit diagram of a comparator according to an embodiment of the invention, which is preferably used in the circuit of FIG. 2;  
         [0014]    [0014]FIG. 4 is a circuit diagram of an enable controller according to an embodiment of the invention, which is preferably used in the circuit of FIG. 2; and  
         [0015]    [0015]FIG. 5 is a timing diagram for describing an operation of a CMOS image device according to an embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    Preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. FIG. 1 shows a CMOS image device according to a preferred embodiment of the present invention. A CMOS image device includes a sensor array  10 , a timing and control logic  20 , a ramp voltage generator circuit  30 , a counter circuit  40 , a plurality of correlated double sampling (CDS) circuits  60   1 - 60   N , and a plurality of output circuits  70   1 - 70   N .  
         [0017]    The sensor array  10  incorporates a plurality of active cells (or pixels)  12  that are arranged in rows R 1 -R M  and columns C 1 -C N . Active cells in a row are simultaneously activated to read out an image from a row of active cells. The timing and control logic  20  provides row select signals onto corresponding row select lines RSL 1 -RSL M  to select and activate any row. The logic  20  also provides reset signals onto corresponding reset lines RST 1 -RST m . Charges induced from respective active cells  12  by light are transferred onto corresponding column data lines  14   1 - 14   N  that are connected with the active cells  12  in respective columns C 1 -C N . At any time, voltage on each column is determined by image charges from one active cell in a corresponding column and a selected row. Signal lines  16   1 - 16   N  are connected with active cells  12  in corresponding rows R 1 -R M , and transfer control signals VTG1-VTG M  for driving corresponding active cells  12 .  
         [0018]    The CDS circuits  60   1 - 60   N  are connected with ends of column data lines  14   1 - 14   N , respectively. Each of the CDS circuit  60   1 - 60   N  receives voltage on a corresponding column data line and a ramp voltage VRAMP from the ramp voltage generator circuit  30 , and generates an analog signal in response to received voltages. For example, the CDS circuit  60   1  of the first column receives voltage VPXL 1  on a column data line  14   1  and the ramp voltage VRAMP, and generates an analog signal VA 1  in response to received voltages VPXL 1  and VRAMP. The CDS circuit  60   N  of the last column receives voltage VPXL N  on a column data line  14   N  and the ramp voltage VRAMP, and generates an analog signal VA N  in response to received voltages VPXL N  and VRAMP. The ramp voltage generator circuit  30  generates the ramp voltage VRAMP in response to a ramp enable signal RAMP_EN from the timing and control logic  20 . The ramp voltage VRAMP is a time varying reference voltage that varies with a predetermined slope.  
         [0019]    Each of the output circuits  70   1 - 70   N  receives an analog signal from a corresponding CDS circuit, a reference voltage VREF from the timing and control logic  20 , an output CNT of the counter  40 , and an enable signal C_ENb from the timing and control logic  20 , and generates a digital word corresponding to a received analog signal. For example, the output circuit  70   1  in the first column receives an analog signal VA 1  from a CDS circuit  60   1 , the reference voltage VREF, the output CNT of the counter  40 , and the enable signal C_ENb, and generates a digital word D 1  corresponding to the received analog signal VA 1 . The output circuit  70   N  in the last column receives an analog signal VA N  from a CDS circuit  60   N , the reference voltage VREF, the output CNT of the counter  40 , and the enable signal C_ENb, and generates a digital word D N  corresponding to the received analog signal VA N .  
         [0020]    [0020]FIG. 2 is a circuit diagram of a CDS circuit and an output circuit according to embodiments of the invention, which correspond to one column of a sensor array in FIG. 1. A CDS circuit  60   1  and an output circuit  70   1  corresponding to the first column  14   1  are illustrated in FIG. 2, but it is well understood to one of ordinary skill in the art that circuits corresponding to remaining columns are constructed in the same way as in FIG. 2.  
         [0021]    Referring to FIG. 2, an active cell  12  includes four NMOS transistors ( 101 ,  102 ,  103  and  104 ) and a photodiode PD 1 . The NMOS transistor  101  whose gate is connected with a reset line RST 1  has its current path formed between a power supply voltage VDD and an internal node  110 . A reset signal RESET is transferred via the reset line RST 1 . The NMOS transistor  102  has its gate connected to a signal line  16   1  and its current path formed between the internal node  110  and a cathode of the photodiode PD 1 . An anode of the photodiode PD 1  is grounded, and a control signal VTG 1  is transferred via the signal line  16   1 . The NMOS transistors  103  and  104  are connected between the power supply voltage VDD and the column data line  14   1 . A gate of the NMOS transistor  103  is connected with the internal node  110 , and a gate of the NMOS transistor  104  is connected to receive a row select signal ROWSEL on a row select line RSL 1 .  
         [0022]    In the aforementioned active cell structure, when the photodiode PD 1  is exposed to light, voltage VPXL 1  of the column data line  14   1  will be determined according to the intensity of the light. For example, when the light is intense the voltage VPXL 1  becomes lower in level than that when the light is weak.  
         [0023]    The CDS circuit  60   1  incorporates two switches ( 120 ,  122 ) and two capacitors ( 121 ,  123 ). The switch  120  is operatively connected to the column data line  14   1  and the capacitor  121 . The switch  122  is operatively connected to a ramp voltage VRAMP input and the capacitor  123 . The capacitor  121  is operatively connected to the capacitor  123  and the output circuit  70   1 . The switches ( 120 ,  122 ) are controlled by corresponding control signals (S 1 , S 2 ) that are provided from the timing and control logic  20  in FIG. 1.  
         [0024]    The output circuit  70   1  includes a comparator  71 , a switch  72 , an enable signal generator  73 , and a latch  74 . The comparator  71  has its non-inverting input terminal connected to a reference voltage VREF, which is received from the timing and control logic  20  and its inverting input terminal connected to an analog signal VA 1 , which is received from the CDS circuit  60   1 . The reference voltage VREF, for example, is half a power supply voltage VDD/2. The comparator  71  compares a voltage of the analog signal VA 1  with the reference voltage VREF to output a signal VOUT based on a comparison result. The switch  72  is connected between inverting input and output terminals of the comparator  71 , and is switched on or off by a control signal S 3  that is provided from the timing and control logic  20 . The enable signal generator  73  generates an enable signal CMP_EN in response to an output VOUT of the comparator  71  and a control signal C_ENb from the timing and control logic  20 . The enable signal generator  73  functions as a controller for activating or inactivating the comparator  71 . The latch  74  latches an output value CNT of the counter  40  (FIG. 1) when the output VOUT transitions from an active state to an inactive state.  
         [0025]    [0025]FIG. 3 is a preferred embodiment of the comparator  71  illustrated in FIG. 2. The comparator  71  is preferably a differential amplifier that includes two PMOS transistors ( 201 ,  202 ) and four NMOS transistors ( 203 ,  204 ,  205 , and  206 ). The PMOS transistor  201  has its source connected with a power supply voltage VDD. The PMOS transistor  202  has its source connected with the power supply voltage VDD, its gate connected to a gate of the PMOS transistor  201 , and its drain connected to an output terminal VOUT. A drain of the NMOS transistor  203  is connected in common with the drain and gate of the transistor  201 , and a gate thereof is connected to receive a reference voltage VREF. The NMOS transistor  204  whose gate is connected with an analog signal VA 1  has its drain connected to the output terminal VOUT. The NMOS transistors ( 205 ,  206 ) are connected between a common-source node of the transistors ( 203 ,  204 ) and a ground voltage. A gate of the transistor  205  is connected to a bias voltage BIAS, and a gate of the transistor  206  is connected to receive an enable signal CMP_EN from the enable signal generator  73  in FIG. 2.  
         [0026]    When the enable signal CMP_EN is at a high level, the comparator  71  compares the reference voltage VREF with the analog voltage VA 1  to output a signal VOUT as a comparison result. On the other hand, when the enable signal CMP_EN is at a low level, the comparator  71  does not operate.  
         [0027]    [0027]FIG. 4 is a preferred embodiment of the enable signal generator  73  illustrated in FIG. 2. The enable signal generator  73  preferably includes an S-R latch that receives an output VOUT of a comparator  71  in FIG. 2 and a control signal C_ENb to generate an enable signal CMP_EN. The S-R latch includes two NAND gates ( 301 ,  302 ) which are connected as illustrated in FIG. 4. In accordance with this structure, the enable signal CMP_EN is inactivated low when the output VOUT transitions from a high level to a low level after a low-to-high transition of the control signal C_ENb.  
         [0028]    [0028]FIG. 5 is a timing diagram for describing an operation of a CMOS image device according to an embodiment of the present invention. An operation of the present CMOS image device will be more fully described with reference to FIGS.  2  to  5 . It is assumed that a row select signal ROWSEL connected to an active cell  12  in the first row R 1  and column C 1  is activated.  
         [0029]    In a reset sampling period, when a reset signal RESET on a signal line RST 1  is at a high level, the node  110  is charged to a voltage of (VDD-Vth) via NMOS transistor  101  (wherein Vth is a threshold voltage of the NMOS transistor  101 ). At this time, voltage VPXL 1  on column data line  14   1  increases in proportion to voltage of the internal node  110 . For instance, since the amount of current flowing through NMOS transistor  103  as a source follower is determined by voltage of the internal node  110 , the voltage VPXL 1  on column data line  14   1  increases in proportion to the voltage of the internal node  110 . On the other hand, voltage variation of the internal node  110  is reflected on the column data line  14   1  through the NMOS transistors ( 103 ,  104 ). The voltage VPXL 1  on the column data line  14   1  will be detected by CDS circuit  60   1 .  
         [0030]    As illustrated in FIG. 5, control signals (S 1 , S 2 , and S 3 ) have a “high” logic level during a reset sampling period, so that switches ( 120 ,  122 , and  72 ) are activated, respectively. As the inverting input and output terminals of comparator  71  are interconnected via the switch  72 , the inverting input terminal of the comparator  71  has a reference voltage VREF(=VDD/2). For example, as an input signal of the inverting input terminal of the comparator  71 , an analog signal VA 1  is equal to the reference voltage VREF. When the control signals (S 1 , S 2 , and S 3 ) transition to a low level, the analog signal VA 1  continues to be equal to the reference voltage VREF due to charges in capacitor  121 .  
         [0031]    In a signal sampling period, as signal line VTG 1  of a selected row is pulsed high, charges on the internal node  110  are transferred to photodiode PD 1 . The voltage across the photodiode PD 1  corresponds to the intensity of light, and voltage of the internal node  110  becomes a gate voltage of source follower transistor  103 . Therefore, voltage VPXL 1  of column data line  14   1  becomes the voltage corresponding to the voltage of the internal node  110 . In the signal sampling period, the switches ( 120 ,  122 ) are turned on in response to high-level signals (S 1 , S 2 ), respectively.  
         [0032]    At this time, voltage of analog signal VA 1  is lowered to the same as varied amplitude of the voltage VPXL 1 . Enable signal generator  73  activates enable signal CMP_EN having a high logic level in response to a control signal C_ENb of a low logic level, which activates the comparator  71 .  
         [0033]    And then, the control signal S 1  transitions from a high logic level to a low logic level and the control signal S 2  is maintained high. After the control signal S 1  transitions from a high level to a low level, control signals RAMP_EN and CNT_EN all are activated high, as illustrated in FIG. 5. At this time, the C_ENb signal is inactivated high. A ramp voltage generator  30  generates a ramp voltage VRAMP in response to activation of the signal RAMP_EN. As illustrated in FIG. 5, the ramp voltage VRAMP increases with a constant slope. Since the control signal S 2  is at a high level, the voltage of the analog signal VA 1  also increases in proportion to increase the ramp voltage VRAMP. Meanwhile, the counter  40  (FIG. 1) is activated by activation of the signal CNT_EN and counts cycles of a clock signal CLK from a timing and control logic  20 .  
         [0034]    The comparator  71  compares the voltage of the analog signal VA 1  with the reference voltage VREF. If the voltage of the analog signal VA 1  is higher than the reference voltage VREF, latch  74  receives and latches an output value CNT from the counter  40  when an output signal VOUT transitions from a high level to a low level. Data in the latch  74  will be provided to an image input device (or an image data processing device) as a digital word D 1  corresponding to the analog signal VA 1 .  
         [0035]    Meanwhile, the enable signal generator  73  inactivates the enable signal CMP_EN low in response to a high-to-low transition of the signal VOUT. This inactivation of the enable signal CMP_EN causes the comparator  71  to be inactivated. At this time, data in the latch  74  continues to be maintained without modification.  
         [0036]    An operating time interval of the comparator  71  is measured from an activation point of the enable signal CMP_EN to an inactivation point thereof, for example, until voltage of the analog signal VA 1  becomes higher than the reference voltage, as illustrated in FIG. 5. An inactivated state of the comparator  71  is maintained until the enable signal CMP_EN is activated again. By so doing, power consumption is reduced as compared with the case that the comparator  71  is always activated while a CMOS image device operates. The intensity of light received by the photodiode PD 1  corresponds to a time until the voltage of the analog signal VA 1  becomes higher than the reference voltage VREF after starting to increase with a constant slope. For example, an inactive period of the comparator  71  is in inverse proportion to the intensity of the light received to the photodiode PD 1 . Also, although input signals VA 1  and VREF to the comparator  71  are changed owing to unwanted noise, the digital word D 1  in the latch  74  is not modified. Accordingly, there is reduced the affect on the digital word due to noise caused after a latch operation is completed.  
         [0037]    The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the preferred embodiments disclosed through the specification. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation to encompass all such modifications and similar arrangements.