JPH03131170A - A/D converter for image sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an A/D converter for an image sensor that converts an analog image signal obtained by an image sensor into a digital image signal.

[Conventional technology]

In recent years, contact type image sensors that have been widely used as image reading devices such as small facsimiles and barcode leagues use A/D converters that convert analog image signals into digital image signals. A contact image sensor usually has a large number of photodiodes and blocking diodes connected in a matrix.
Signals are sequentially extracted from the plurality of output circuits. Therefore, it is necessary to provide a large number of A/D converters corresponding to the number of output circuits.

The successive approximation type A/D converter, which is the most common type of A/D converter, has the advantage of being low cost, but has the disadvantage that its circuit configuration inevitably results in slow operation speed.
It is not suitable for use in image sensors that require high-speed operation, and for this reason, flash type A/D converters (parallel comparison type A/D converters) are often used in conventional image sensors.

The circuit configuration when a flash type A/D converter is used as an image sensor is shown in FIG. Photodiode (
A signal supplied from an amplifier 1 (not shown) is amplified by an amplifier 1, and the peak voltage value of its output is held by a sample hold circuit 2 and supplied to a flash type A/D converter 7. The flash type A/D converter has a number of comparators corresponding to the quantization level, and compares the supplied voltage value with these quantization levels in parallel to determine which level of the comparator is operating. The digital output is obtained by Therefore, it is possible to operate much faster than a successive approximation type A/D converter.

[Problem to be solved by the invention]

However, since the flash type A/D converter has a large number of circuit elements, it is expensive even if a commercially available LSI is used. In particular, when a large number of A/D converters are used, such as in an image sensor arranged in a matrix, there is a problem in that the overall cost increases.

The present invention has been made based on the above circumstances, and is an A/C for an image sensor that is low cost and capable of high speed operation.
The purpose of this invention is to provide a D converter.

[Means to solve the problem]

In order to achieve the above object, the present invention provides an A/D converter for an image sensor that converts an image signal obtained as an analog signal into a digital signal, comprising: a voltage holding circuit that holds the voltage value of the analog signal; , a discharge circuit that changes the voltage value of the analog signal held in the voltage holding circuit by discharge at a predetermined rate; a voltage comparison circuit that compares the voltage value changed by discharge with a reference voltage; and the voltage holding circuit. and a counting circuit that counts pulse signals supplied until the voltage held at the reference voltage becomes equal to the reference voltage, and a digital image signal is extracted from the output of the counting circuit. .

[Effect]

In the present invention, with the above configuration, the peak voltage of the image signal input as a pulsed analog voltage signal is held by the voltage holding circuit. This voltage changes at a predetermined rate as the discharge circuit discharges at a substantially constant current value. Therefore, the time it takes for this voltage to change and become equal to the predetermined reference voltage is proportional to the peak voltage input as an analog voltage signal.

The voltage comparison circuit compares the voltage that changes due to discharge with a reference voltage, and outputs a predetermined signal when this voltage becomes equal to the reference voltage. The count circuit receives this signal and
The pulses (superimpositions) supplied from when the analog voltage signal is input until it becomes equal to the reference voltage are counted. The count value output from this counting circuit is taken out as an image signal converted to a digital signal.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an A for an image sensor which is an embodiment of the present invention.
FIG. 2 is a block diagram of the /D converter and a timing chart showing signal waveforms at each point in FIG. 1.

In FIG. 1, the amplifier 1 includes, for example, a photodiode (
(not shown), the image signal of the original that has been photoelectrically converted is input and amplified. The signal output from the amplifier 1 is a pulsed signal as shown in FIG. 2(a), and the magnitude of this pulse changes depending on the brightness and darkness of the original.

The sample and hold circuit 2 holds the peak voltage of the pulsed signal output from the amplifier 1 and makes it a constant voltage, as shown in FIG. 2(b). The discharge circuit 3 discharges the voltage held in the sample hold circuit 2 by flowing a substantially constant current.

As a result, the output voltage of the discharge circuit 3 gradually decreases at a predetermined rate, that is, at a constant angle, from the voltage held in the sample and hold circuit 2, as shown in FIG. 2(C). Therefore, the time for the output voltage of the discharge circuit 3 to decrease from the peak voltage to the reference voltage is longer if the peak voltage held in the sample-and-hold circuit 2 is higher, and shorter if the peak voltage is lower.

The output voltage of the discharge circuit B3 is compared with the reference voltage 8 in the comparator 4, and as shown in FIG. 2(d), when this voltage is higher than the reference voltage, it becomes "high", When it is also low, it is "low". Therefore, the pulse width of the output signal of the comparator 4 is proportional to the peak value of the analog signal input to the amplifier 1.

The output signal of the comparator 4 is input to an AND circuit (logical product) 5, and the other terminal of the AND circuit 5 is as shown in FIG. 2(a).
A clock pulse signal with a constant frequency shown in is input.

Therefore, the output signal of the AND circuit 5, as shown in FIG. ”.

The output of the AND circuit 5 is input to an 8-bit counter 6, where the clock pulses shown in FIG. 2(f) are counted. Therefore, the output of the counter 6 becomes an 8-bit digital image signal corresponding to the brightness and darkness of the original. Figure 2 (f
”), 8 pulses, 3 pulses, 5 pulses in left order.
As the pulses are counted, the outputs of the counter 6 are respectively rooooloooJ and rooooooo in binary.
llJ, roooooololJ.

Now, if we consider the case where the A/D converter shown in Fig. 1 is applied to an image sensor with 40 blocks x 16 outputs (each output of 16 sets has a set of 40 elements of photodiodes and blocking diodes), the total number of blocks is 16. A/D converters are required. Here, if the pulse width of the drive pulse that operates the - pair of photodiodes and bronking diodes is 16 μsec, and the clock frequency is 20 MHz (one period is 0.05 μ5 ec), the number of pulses is the maximum value of 8 bits. up to 2 corresponding to
0. even during 5G pulse. 05/1sec X256=
12° is 8 μsec, which is well within 16 μsec. At this time, the time required to read the l line is 16 μs.
It is extremely short, ec x40=0.64m5ec, and can support high-speed reading.

By the way, the brightness of the background of the document read by the image sensor differs depending on the type of document, so if you read a newspaper with the white level set to a document with a bright background, such as photographic paper, the background of the newspaper will be It is read as gray, and there is a wasted part in the dynamic range. For this reason, the image sensor needs to be provided with a shading correction function that changes the white level depending on the original to be read.

The circuit shown in FIG. 1 can easily perform this shading correction. First, when the value of the current discharged by the discharge circuit 3 is changed, the slope of the voltage shown in FIG. 2(c) changes, and the pulse width of the pulse shown in FIG. 2(d) can be changed.

Furthermore, by changing the reference voltage input to the comparator 4, the pulse width can be changed in the same way. Therefore, when the original is bright, the image shown in Fig. 2(d)
Shading correction can be performed by changing the current value of the discharge circuit 3 or the reference voltage v11 of the comparator 4 so as to narrow the pulse width of the original, and widen the pulse width when the original is dark. Furthermore, AND circuit 5
Since the output level of the counter 6 can also be changed by changing the frequency of the clock pulse supplied to the counter 6, shading correction becomes possible.

All of the elements constituting the circuit shown in FIG. 1 are of the polar type, and the number of elements is small. Therefore, it can be constructed at about half the cost of a flash type A/D converter used in conventional devices. In addition, since each of the circuit elements shown in Figure 1 has been established as a standard cell, a custom LSI consisting of the circuit shown in Figure 1 can be designed,
It is also easy to manufacture.

〔Effect of the invention〕

As explained above, according to the present invention, the successive approximation type A/D
Since it operates faster than a transducer, the image sensor itself can be made faster. Further, since the number of elements used is small and the configuration can be made using circuit elements that are independent of each other, costs can be significantly reduced, especially when a large number of A/D conversion circuits are provided. Furthermore, by having multiple adjustable points, it is possible to perform shading correction that flexibly corresponds to the type of document.
/D converter can be provided.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG. 2 is a block diagram of an embodiment of the present invention.
A timing chart showing signal waveforms at each point in the circuit shown in the figure, and FIG. 3 is a block diagram of a conventional device. 1...Amplifier, 2...Sample hold circuit, 3...Discharge circuit, 4...Comparator, 5...A
ND circuit, 6...Counter, 7...Flash type A
/D converter.

Claims (1)

[Claims] An A/D converter for an image sensor that converts an image signal obtained as an analog signal into a digital signal, comprising: a voltage holding circuit that holds the voltage value of the analog signal; and a voltage holding circuit that holds the voltage by discharging. a discharge circuit that changes the voltage value of an analog signal held in the circuit at a predetermined rate; a voltage comparison circuit that compares the voltage value that changes due to discharge with a reference voltage; An A/D converter for an image sensor, comprising a counting circuit that counts pulse signals supplied until the pulse signals become equal to the reference voltage, and extracting a digital image signal from the output of the counting circuit. .