JPH04256278A - Linear image sensor - Google Patents

Abstract

PURPOSE:To obtain an output with a high S/N without a fixed pattern noise by using a specific transmission pulse for a chip. CONSTITUTION:The period of pulses sent between chips being component of a long image sensor consists at least of an output period of an output at a bright state and an output at a dark state for at least one or over of consecutive picture elements and the period has the output periods of the bright state and the dark state of one and same picture element in pairs. Thus, the effect as noise of a transmission pulse (b) is given onto the output at a bright state and the output at a dark state of the same picture element equally. In this case, the effect of the transmission pulse (b) is given on both the output at a bright state and the output at a dark state or no effect is given onto the both. Thus, even when the effect of the transmission pulse (b) is given onto the both, the noise in the transmission pulse (b) is resultingly eliminated in the output (c) after the subtraction.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear image sensor comprising a plurality of IC chips capable of reading document information at high speed and high resolution.

0002

2. Description of the Related Art Conventionally, there are CCD type and MOS type linear image sensors capable of high-speed reading, and the CCD type has been considered to be more advantageous in terms of S/N. On the other hand, the MOS type had the advantage of being easier to manufacture, but in recent years, the addition of an internal amplification function has caused issues with S/N.
is also being greatly improved. FIG. 3 shows the components of one pixel of this amplified MOS image sensor. In this figure, 1 is a photodiode, and 2 is an amplification MO that receives the potential change of photodiode 1 as a gate potential and amplifies the optical signal.
S transistor; 3 is a readout switch that sequentially reads out the amplified optical information to the output line for each pixel; 4 is a readout switch that sets the gate potential of the amplification MOS transistor 2 to a predetermined reference value after reading out the optical information for each pixel; This is a reset switch that performs a reset. The optical information read out to the output line is read out as a drain current or as a source follower output voltage of the amplification MOS transistor 2. In FIG. 3, the anode of the photodiode is connected to the gate terminal of the amplification MOS transistor, but the cathode of the photodiode may be connected to the gate. This amplified MOS image sensor causes large variations in output, especially in dark times, due to manufacturing variations that always exist. Therefore, after reading out the bright output of each pixel, read out the output value immediately after resetting the gate terminal potential of the amplification MOS transistor, that is, the value corresponding to the dark output, and combine the dark output value with the bright output value. The difference must be taken to obtain the true video output. The situation at this time is shown in a timing chart as shown in FIG. In the output before subtraction in this figure, the bright output and the dark equivalent output from the first pixel to the fourth pixel are shown in order. Between the bright output readout timing and the dark equivalent output readout timing of each pixel, the reset switch 4 in FIG. 3 is turned on to reset the gate potential of the amplification MOS transistor to a predetermined reference value. ing. The output after subtraction is the output obtained by subtracting the output equivalent to the dark time immediately after reset from the bright output. In the output after subtraction, the dark output is corrected to zero as shown in the third pixel.

0003

However, when a long image sensor is constructed by arranging a plurality of chips in series, it is necessary to transfer transmission pulses from chip to chip. This means that a lead line to the outside of the chip is essential in order to transmit the transmission pulse from chip to chip, and this external lead line for the transmission pulse is capacitively coupled with the external lead line for the video output from each chip. It becomes easier. Various pulses exist within the chip and affect the output signal line as a noise source. However, what ultimately has the greatest influence on the output signal line drawn out to the outside of the chip is the transmission pulse between chips drawn out to the outside. Therefore, the transmitted pulses give capacitive coupling noise to the video output signal at the readout timing of pixels near the connections of each chip, and this also has an adverse effect on the output after subtraction, which is the difference between the bright output and the dark equivalent output. This reduces the applied S/N.

0004

[Means for solving the problem] The period of a pulse transmitted between chips constituting an elongated image sensor is from the output period of a bright output and a dark equivalent output of at least one or more consecutive pixels. The means for solving the above problem is to ensure that this period always includes a pair of output periods of bright output and dark output of the same pixel.

[0005]

[Operation] According to the above means, the influence of the transmitted pulse as noise equally affects both the bright output and the dark output of the same pixel. At this time, the bright-time output and the dark-time equivalent output of each pixel are either influenced by the transmitted pulse or not. Therefore, since the bright output noise and the dark equivalent output noise of the same pixel are equal, even if there is an influence of the transmitted pulse, the transmitted pulse noise is eventually removed in the output after subtraction.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a timing diagram of both inter-chip transmission pulses and output signals according to an embodiment of the present invention. a, b in this figure
, c indicate the output before subtraction, the inter-chip transmission pulse, and the output after subtraction, respectively. In this figure, the transmission pulse is formed so as to exactly include the bright output and the dark output of the Nth pixel. Here, it is not essential which pixel corresponds to the joint between chips. A feature of the present invention is that while the state of the transmitted pulse is changing, a pair of reading periods of a bright output and a dark output are always provided for any particular pixel output. What is drawn as a solid line in the output period of the Nth pixel in Figure 1a is the output that has caused a capacitive coupling output fluctuation due to the disturbance caused by the transmitted pulse in Figure 1b, and is drawn with a dashed-dotted line. This is the output when there is no disturbance. However, if the output after subtraction is obtained by sampling both the bright output and the output corresponding to dark regarding each pixel output, the influence of the transmitted pulse will not have any adverse effect on the output after subtraction, as shown in FIG. 1c. In other words, according to the present invention, the fixed pattern noise caused at the chip connection by the inter-chip transmission pulses extracted outside the chip in order to continuously scan from chip to chip is subtracted between the bright output and the dark equivalent output. This can be canceled out in the subsequent output.

For comparison, FIG. 4 shows an example in which the transmitted pulse includes only the dark output period of the Nth pixel and the bright output period of the N+1th pixel. a, b in Fig. 4,
c indicates the output before subtraction, the inter-chip transmission pulse, and the output after subtraction, respectively. What is drawn with a solid line in the dark output period of the N-th pixel and the bright output period of the N+1-th pixel in FIGS. 4a and 4c is the capacitive coupling caused by the disturbance caused by the transmitted pulse in FIG. 4b. This is the output that caused the output fluctuation, and the one drawn with a dashed-dotted line is the output when there is no disturbance. In this case, even if both the bright output and the dark equivalent output are sampled for each pixel output and the subtracted output is obtained, the effect of capacitive coupling noise of the inter-chip transmission pulse is It remains in the output after subtraction. No. N
For the th pixel, the capacitive coupling noise of the inter-chip transmission pulse is not superimposed on the bright output, but it is superimposed on the dark equivalent output, so only the dark equivalent output is increased by the noise. . Also, for the N+1th pixel, the capacitive coupling noise of the transmitted pulse is superimposed on the bright output, but it is not superimposed on the dark equivalent output, so only the bright output is increased by the amount of noise. . That is, in the output after subtraction, the output of the Nth pixel is smaller than the original value, and the output of the N+1th pixel is larger than the original value. As a result, for example, as can be seen from the dot-dashed line of the output before subtraction, the N+1-th pixel in FIG. As the result remains, the output after subtraction is not zero. As described above, in this comparative example, the capacitively coupled noise component superimposed by the chip-to-chip transmission pulse causes fixed pattern noise in the chip connection output.

Therefore, according to the present invention described above, even in a multi-chip elongated image sensor configured to serially scan a plurality of sensor chips, a high S/N output without fixed pattern noise can be achieved at the chip connection portion. This allows signal reading. Note that in the above embodiment, a transmission pulse that includes only a bright output readout period and a dark equivalent output readout period of one pixel is used, but a transmission pulse that includes only a bright output readout period and a dark equivalent output readout period of one pixel is used. There is no problem in including an output period of two or more pixels as long as it includes both an output reading period, and this does not depart from the spirit of the present invention. Further, the present invention can be applied to a case where the amplification MOS transistor is a junction FET, a Schottky gate FET, or a bipolar transistor.

[0009]

As described above, the present invention provides the output of one or more pixels, in which the period during which the state changes as a transmission pulse between chips includes both the bright output readout period and the dark output readout period. By using a period consisting of a period of
This makes it possible to obtain N outputs.

[Brief explanation of the drawing]

FIG. 1 is a timing diagram showing an embodiment of the present invention.

FIG. 2 is a timing diagram showing how the dark-time equivalent output is subtracted from the bright-time output.

FIG. 3 is a configuration diagram of one pixel of an amplified MOS image sensor.

FIG. 4 is a timing diagram showing an example of the present invention and a comparative example.

[Explanation of symbols]

1 Photodiode 2 MOS transistor for amplification 3 Output readout switch 4 Reset switch

Claims (1)

[Claims] 1. A photodiode, an amplification MOS transistor whose gate receives one terminal of the photodiode, a switch connected in series with the amplification MOS transistor, and a gate potential of the amplification MOS transistor set to a predetermined reference value. This is an amplified image sensor in which a switch for resetting is set as a pixel unit, and in a linear image sensor that is made up of multiple IC chips that perform serial scanning, it is a long linear image sensor that continuously scans between chips. The period during which the chip-to-chip transmission pulse changes its state is always the output period of one or more consecutive pixels that includes both the bright output period and the dark output period of the same pixel. A linear image sensor characterized by comprising: