JPH0339622A - Image sensor driving circuit - Google Patents

Abstract

PURPOSE:To reduce a switching noise to be superposed upon a reference voltage and variation by providing a subtracting circuit for a 1st buffer output and eliminating a switch which resets a 1st buffer output line to a reference voltage. CONSTITUTION:The output of an image sensor 10 which outputs a voltage corresponding to the quantity of incident light is received by a buffer 201 and a subtracting circuit 1 which compares its signal with the reference voltage C and outputs their difference. Then a shift register 6 is so driven that switches 401 turn on, bit by bit; and a detection signal is compared with the reference voltage and their difference is outputted. A sensor 10 which is not detected is charged to the reference voltage with a switch 301. Further, the output signal is set to GND with a switch 5 when an ENA signal is at low level. Thus, the subtracting circuit is provided to reduce the switching noise to be superposed on the reference voltage and the variation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a drive circuit for a contact type image sensor.

[Conventional technology]

FIG. 2 shows a circuit diagram of a conventional image sensor drive circuit. In the figure, (301) to (364) are switches for charging the image sensor, (401) to (464) are output switches of voltage followers (201) to (264), and (9) is line A ( 7〉 socket switch sw
p, (8) C0M terminal output control switch f SW
It is c. FIG. 3(a) is a waveform diagram of the function. When the ENA signal is high level, the output control switch S
Wc (8) is turned on and turned off when it is at low level.

Therefore, the C0M output is synchronized with the ENA signal, but
Output control switch SWc (8) and output switches (201) to (264) of voltage followers (201) to (264)
Line A (7> Glue set switch y-
swp (9) is provided, and the volume follower SWs
(201) ~'3iaa (264) (!: (D
By turning the delay switch on and off, even if the output control switch 5Wc (8) is delayed, an unstable output is prevented from being output to the C0M output.

[Problem to be solved by the invention]

Since the conventional image sensor drive circuit was configured as described above, there was a problem in that the switching noise of the reset switch fSWc affected the reference voltage VR, causing the reference voltage to fluctuate.

This invention was made to solve the above-mentioned problems, and its purpose is to reduce the noise on the reference voltage of the sensor.

[Means to solve the problem]

The image sensor drive circuit according to the present invention is provided with a subtraction circuit at the output of the first buffer, and the reference voltage is connected to the positive input of the subtraction circuit, and the output of the buffer is connected to the negative input of the subtraction circuit. .

[Function] The amount of delay caused by the subtraction circuit in this invention increases,
Since the delay amount of the output control switch SWc can be ignored, the reset switch SWp can be omitted.

〔Example〕

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

FIG. 1 is a circuit diagram of a contact type image sensor driving circuit according to an embodiment of the present invention. A buffer (201
), a subtraction circuit (1) that compares the signal with the reference voltage ■ and outputs the difference, a switch (301) that charges the image sensor α0 to the reference voltage, and a switch (301) that controls the output of the buffer. 401), a shift register (6) that controls these switches, and switches (5) and (8) that control the final output.

Next, the operation will be explained. FIG. 3(a) is a waveform diagram of the function. The shift register (6) is driven so that the switch:P (401) is turned on one bit at a time, the detection signal is compared with the reference voltage, and the difference is output. If the image sensor is not detected, switch (301)
) is charged to the reference voltage. The output signal is set to GND by switch y-(5) when the ENA signal is at low level.

〔Effect of the invention〕

As described above, according to the present invention, since the subtraction circuit is provided, the delay time of the linear circuit including the buffer increases, and this switch can be eliminated, thereby reducing switching noise and fluctuations on the reference voltage.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an image sensor drive circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional image sensor drive circuit, and FIGS. 3(a) and (b) are examples of function waveforms. FIG. In the figure, +1) is a subtraction circuit, (201) to (264
) is a buffer, (301) to (364) are image sensor charging switches P%, (401) to (464) are buffer output control switches, and (6) is a shift register. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] a first switch for charging an image sensor that outputs a voltage according to the amount of incident light to a reference voltage; a first buffer connected to the output of the image sensor; and controlling the output of the first buffer. a second switch to
In an image sensor driving circuit including a shift register for controlling the first and second switches and a switch for resetting the output line of the first buffer to the reference voltage, An image sensor driving circuit characterized in that a subtraction circuit is provided at the output, and a switch for resetting the output line of the first buffer to the reference voltage is omitted.