Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2002-08-07
2004-12-14
Jeanglaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S154000
Reexamination Certificate
active
06831580
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a digital-to-analog converter used in a complementary metal-oxide semiconductor (CMOS) image sensor circuit; and, more particularly, to a digital-to-analog converter that makes the amplification rate of a pixel signal change linearly according to digital control codes inputted thereto.
DESCRIPTION OF RELATED ART
Generally, an image sensor is a device that captures images by using a photo-reactive characteristic of a semiconductor. With different brightness and wavelength, every aspect of a subject existing in nature shows a different electrical value at each pixel of a sensing device. It is the image sensor that converts the electrical values into signals that can be processed.
Recently, image sensors have received additional attention as they are applied to a variety of security equipment, video conference cameras, digital still cameras, PC cameras, next-generation PDAs, with a function of transmitting image information and other data.
Image sensors are classified into two types: One type is a charge-coupled device (CCD) image sensor; and the other type is a complementary metal-oxide semiconductor (CMOS) image sensor. Compared to a CCD image sensor, a CMOS image sensor can be operated more easily and implement a variety of scanning methods. Also, because a CMOS image sensor is capable of integrating a single processing circuit in one chip, the CMOS image sensor may assist in miniaturizing products as well as reducing production costs with its CMOS technology. Additionally, with remarkably low power consumption when compared to the CCD, the CMOs image sensor's applicable fields continue to expand.
Conventionally, a CMOS image sensor comprises a comparator for comparing a ramp signal, as a reference signal, which decrease regularly, and an analog data signal from a photodiode; a counter to initiate counting when the ramp signal is outputted; and a latch for storing a counted value in a digital data value according to the value of the comparison result.
To perform the operation described above, the CMOS image sensor includes a digital control block for outputting various control signals. The digital control block sets up a range of storable digital data values, for example, from 0 to 255, according to the brightness of an image, by inputting a digital control code with a digital-to-analog converter. A digital control code is a control code a user inputs arbitrarily that controls the range of storable digital data values according to the brightness of an external light.
The CMOS image sensor equips the digital-to-analog converter at the digital control block so as to set up the range of digital data values (e.g. 0-255). According to the output voltage from the digital-to-analog converter, the unit voltage of a ramp signal according to the reference clock is determined.
For example, when an input digital control code is binary ‘00001’, the range of storable digital data values is binary ‘00000000-11111111’ (0-255). If the input digital control code is binary ‘00010’, the range of digital data values is binary ‘00000000-01111111’ (0-127). Similarly, if the input digital control code is binary ‘00100’, the range of digital data values becomes binary ‘00000000-00111111’ (0-63). When the input digital control code is binary ‘01000’, the range becomes binary ‘00000000-00011111’ (0-31).
The bigger the range of digital data values is, the brighter the image is stored. When a digital data value is 255, the screen becomes about twice as bright as when the digital data value is 127. Therefore, if a user wants to see a dark part brighter, the range of storable digital data values should be increased by making the value of a digital control code smaller.
FIG. 1
is a circuit diagram illustrating a digital-to-analog converter of a conventional CMOS image sensor, which is implemented in three bits.
Referring to
FIG. 1
, the digital-to-analog converter comprises a voltage divider
10
for equally distributing a reference voltage; a switching block
20
for switching the distributed resistances to an output block; and an output block
30
for outputting signals through the switching block
20
.
The voltage divider
10
is composed of eight resistors (R) serially arrayed between the reference voltage (Vref) and the ground voltage level. The switching block
20
is comprised of a plurality of N channel MOS transistors configured so that only one path should be selected by a decoded digital value. The output block
30
is comprised of an operation amplifier that feedbacks the output to a negative (−) input and thus serves as a buffer. When connected as shown in
FIG. 1
, an operation amplifier operates as a buffer with a gain of 1.
Referring to
FIG. 1
, the operation of a digital-to-analog converter will be described below.
When the reference voltage (Vref) from the circuit shown in
FIG. 1
is ‘1V’ and the digital control code ‘001’ is inputted into the switching block
20
, switches receiving the signals /b
1
, /b
2
, /b
3
inputted to the switching block
20
are turned on, and a voltage of ⅛V is output to the output block
30
. Also, when the digital control code is increased from ‘0001’ to ‘0100’, the output values of the digital-to-analog converter increase to ⅛V, {fraction (2/8)}V, ⅜V and {fraction (4/8)}V, respectively. That is, as the resistance values of the digital-to-analog converter is regular, the output voltage differences increase regularly as well according to input digital control codes.
A conventional digital-to-analog converter of a CMOS image sensor uses an array of 32 resistors and outputs 32 voltage values. The voltage values determine the unit voltage of a ramp signal input to the comparator of the CMOS image sensor. A ramp signal decreases as much as the unit voltage, e.g., {fraction (1/256)}, each clock pulse, so the size of the unit voltage is determined by the output value of the digital-to-analog converter before the output value is input to the comparator.
For instance, when the digital control code is binary ‘00001’, the output voltage of the digital-to-analog converter is {fraction (1/32)}V. Then, from the output value, the unit voltage of a ramp signal {fraction (1/256)}V is output. If the digital control code is binary ‘00010’, the voltage output from the digital-to-analog converter is {fraction (2/32)}V, and from this voltage, the unit voltage of a ramp signal is obtained to be {fraction (2/256)}V. After that, when the digital control code increases to ‘00011’ and ‘00100’, the unit voltages of the ramp signal increases to {fraction (3/256)}V and {fraction (4/256)}V, respectively, in the CMOS image sensor.
The unit voltage of a ramp signal is an amount of decreased voltage per clock cycle. As the unit voltage becomes smaller and smaller, the ramp signal decreases for many clock cycles, and as the unit voltage becomes bigger and bigger, the ramp signal decreases for fewer clock cycles. The number of clock cycles used while the ramp signals are supplied becomes the range of storable digital data values in the image sensor. That is, if the digital control code is 1000011, the ramp signal of the comparator decreases {fraction (1/256)}V per clock cycle, and thereafter, the ramp signal decreases for a total of 255 clock cycles. This means that the range of storable digital data values is from 0 to 255.
FIG. 2A
is a schematic diagram showing a ramp signal of a CMOS image sensor wherein the digital control code input to the digital-to-analog converter is binary ‘00001’.
Referring to
FIG. 2A
, it is shown that the ramp signal decreases as much as the unit voltage, i.e., {fraction (1/256)}V, per clock cycle when a digital control code is ‘00001’. Therefore, the range of digital data values that can be stored in the CMOS image sensor is ‘from 0 to 255’.
FIG. 2B
is a schematic diagram showing a ramp signal of a CMOS image sensor, wherein the digital control code inputted to the digital-to-analog converter is binary ‘00010’.
Referring to
FIG. 2B
, when the digital control code is ‘0001
Hynix / Semiconductor Inc.
Jeanglaude Jean Bruner
Marshall & Gerstein & Borun LLP
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