JPH03148959A - Scanning circuit for photo diode array - Google Patents

Abstract

PURPOSE:To reduce the influence of a voltage drop owing to blocking diodes and to cancel the saturation of an output signal at low illuminance by constituting the circuit in such a way that the cathode of a second diode which is connected in serial connects to a common line and a scanning voltage is outputted to respective photo diodes from the connection point of the second diode and a second resistance. CONSTITUTION:When a sawtooth voltage Vdr comes to a two-fold threshold 2VF, a first diode Dw2 is turned on and a connection point tp12 starts rising. At that time, the voltage Vtp11 of a connection point tp11 is not saturated yet and it continues rising. When the sawtooth voltage Vdr comes to a three-fold threshold 3VF, the voltage Vtp11 of the connection point tp11 comes to the two- fold threshold 2VF and it is saturated. Then, first diodes Dw4-Dw10 are sequentially switched and scanning voltages supplied to the photo diode array 2 are respectively saturated by the two-fold threshold 2VF. At that time, inverse biases (=Vtp1i-Vtp2i) added to photo diodes PD1-PD10 come to the same voltages as the threshold VF by the voltage drop (=VF) of the blocking diodes BD1-BD10.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Industrial Application Field The present invention is directed to a photodiode array constituting a contact image sensor used in an optical image reading device such as a facsimile machine or an image scanner. The present invention relates to a scanning circuit for a photodiode array for scanning with one saw*i pressure.

(B) Prior Art Conventionally, as shown in FIG. 3, in this type of photodiode array scanning circuit, diodes D1 to DIG having the same forward characteristics are connected in series, and each diode D,
~ D Between the cathode of 10 and the ground GND, resistor R,
~Rue, resistor R11--R2G and diode DI
A scanning voltage is output from the anode of each diode D11-010 to each photodiode D□-D31) of the photodiode array. D31
~D4. is a probing diode for crossstroke prevention.

In such a circuit configuration, a sawtooth voltage Vdr is applied to the anode of the diode D1. Assuming that the threshold voltage of each diode D = "-D t o is VF, when the voltage of the drive power supply that outputs the saw tII711 pressure Vdr exceeds the threshold voltage vF, the diode D sh (o N j,, A voltage appears on the cathode side of diode D.Next, when the voltage of drive power supply Vdr reaches twice the threshold voltage 2Vt, diode D2 becomes O
Do N. From then on, diode D3 is applied sequentially up to diode D1°.
~DIo is ON l, go. In addition, the diode D11
--Dt. If the threshold voltage of is also VF, then the voltage vP appearing on the cathode side of diode D.

gradually increases at the anode side connection point P of the diode D and saturates at the threshold voltage VF of the diode D I+. At the anode side connection point P of the next stage diode I)tt, the voltage V□ is saturated at the threshold voltage Vt, and furthermore, the anode side connection point P, ~P1. The voltage waveform appearing on the voltage waveform turns ON sequentially, and the voltage V□ becomes saturated at the threshold voltage vF. This diode D, ~D,. Anode side connection points P1 to P,. The voltage waveform of is shown in FIG. Assuming that the ramp rate of the sawtooth voltage Vdr at this time is constant, the time when the voltage vPI at the connection point P is saturated and the time at which the voltage vPI at the connection point P,
The time when the voltage vPt appears at is the same, and each connection point Pt
~pro switching has good isolation characteristics because the voltage at the connection point in the next stage rises after the voltage at the connection point in the previous stage reaches the threshold voltage VF.

By using this threshold voltage V as a reverse bias voltage to the photodiode, each photodiode D t
= ”= D s. is sequentially driven to read the original.

(c) Problems to be solved by the invention However, in the above conventional example, the photodiode D□
A voltage V, □ (however, i −
l, 2...--10) are diodes DIN~D,
. The threshold voltage is V.

If the scanning circuit section and the sensor part using series diodes are formed simultaneously on the same substrate, each diode D, ~D
z6. Dll~D2°, photodiode Dt=
” D s. and blocking diodes D31 to D
The threshold voltage vF at 4° is the same. At this time, the voltage VPL-V□ at each connection point P, ~PIG. Due to the voltage drop VF of the blocking diodes D31 to D4°, the voltage at the connection point P l l - P f (l becomes a very small voltage VPL-IO. As a result, the voltage is accumulated in the capacitance of the photodiodes D□ to D. The amount of charge generated is smaller than the amount of charge discharged by the photocurrent, the signal output is saturated at low illuminance, and a sufficient contrast ratio of the signal cannot be obtained.

The present invention has been made in consideration of the above circumstances, and it is an object of the present invention to provide a scanning circuit for a photodiode array that can increase the scanning voltage supplied to the photodiodes.

(D) Means for Solving the Problems The present invention provides a scanning circuit for a photodiode array that sequentially scans a photodiode array composed of a plurality of photodiodes, the cathode being common to at least the same number of first diodes as the number of photodiodes. 1st between road and road
a plurality of second diodes connected in series are connected to the cathode of the first diode through the second resistor, and the cathodes of the second diodes connected in series are connected to a common path. A scanning circuit for a photodiode array is configured to output a scanning voltage to each photodiode from a connection point between a second diode and a second resistor.

(E) Effect Since a plurality of second diodes are connected in series, the scanning voltage to the photodiodes increases in proportion to the number of second diodes connected in series, so that the saturation of the photodiode output signal at low illuminance is eliminated. act.

(F) EXAMPLES Below, examples of the invention will be described in detail with reference to the drawings, but the invention is not limited to the following examples.

In Fig. 1, photodiodes PD, -PD, and blocking diode B are connected to the anodes of each photodiode.
D,~BD,. the anode is connected. Probking diode BD, -BD,.

The cathode of is connected to the output common path 1.

A photodiode array 2 is constituted by photodiodes PD, -PD, and blocking diodes BD, -BD, 6.

3 is a scanning circuit for a photodiode array, which includes a first diode DV+O, a first resistor Rv1 to Rvto, and a second resistor RrI=R connected in series.
t t o and the second diode Dt t t + D
t□・-・・-DTIO,、D tI、D? □・・・・・・
-・DTI. It consists of 2. A first resistor Rv□ is connected between the cathode of the first diode D6 and the common path 4. The second diode D T l + and the second diode D7□ are connected in series with their respective cathodes and anodes in common, and the anode of the second diode D1□ is connected to the first diode D7□ through a resistor RT t. Diode Dv
connected to the cathode of t, and also a second diode D-r
The cathode of tt is connected to the common path 4. Further, a connection point T between the second resistor R0 and the anode of the second diode DT□.

The cathode of the photodiode PD is connected to the photodiode PD.

In the above, i is an integer from 1 to 10.

In the above configuration, first, the saw*i pressure Vdr is applied to the input terminal 5. When the sawtooth voltage Vdr exceeds the threshold value VF of the first diode DwH, the voltage V, 1 at the connection point 11+□ rises at the first diode Dv+ ONL. Sawtooth voltage Vdr is twice the threshold value 2V
When F, the first diode I)vt becomes ONt, .
The connection point , begins to rise, but at this time, the voltage V tp 11 at the connection point □ is not saturated and continues to rise. When the sawtooth t11 voltage Vdr reaches three times the threshold value 3VF, the first diode Dwffih (ON) is connected to the connection point, and the voltage of 3 begins to rise.At this time, the connection point is The voltage V tpH becomes twice the threshold value 2VF and becomes saturated.Hereafter, the diode D
As w4 to Dw16 shift, the scanning voltages supplied to the photodiode array 2 are each saturated at twice the threshold value 2VF. At this time, the photodiode PD,
~PD,. Reverse bias applied to (=V, □□−V ,,
, , ) becomes the same voltage as the threshold value VF due to the voltage drop (=VF) of the blocking diodes BD, -BD, and 0. Also, the separation of the voltages V, , , at the connection point □ is not sufficient compared to the case where there is only one second diode, but at the connection point L ttl, the blocking diode B
D.

The point in time when the voltage Vtttl is saturated due to the voltage drop is immediately before the rise of the next stage voltage VtpHel.
The peak value of the output signal of the photodiode PD to be detected does not overlap with that of an adjacent pixel, and separation becomes good.

(G) Effects of the Invention According to this invention, by connecting a plurality of second diodes in series, the reverse bias of the photodiode is increased to reduce the effect of voltage drop caused by the blocking diode, and the output at low illuminance is improved. Signal saturation can be eliminated.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing the configuration of an embodiment of the present invention;
Fig. 2 is a waveform diagram showing voltage waveforms at each connection point in the embodiment, Fig. 3 is an electric circuit diagram showing the configuration of a conventional column, and Fig. 4 is a diagram corresponding to Fig. 2 of the conventional example. ...Photodiode array, 3...Photodiode scanning circuit,
4-... Common path, 0 w I-D w r o... First diode, DTII~DrIo+* Dr□~D r +. .・・・・-Second diode, R%#1-RMjO"・-"First resistor, Rr --R
□. ...Second resistance. + Power JE VTP1izVTP2i-ReG9- Output h Vp; , Vpi++. Gi

Claims (1)

[Claims] 1. A scanning circuit for a photodiode array that sequentially scans a photodiode array composed of a plurality of photodiodes, the scanning circuit having at least the same number of first photodiodes as the number of photodiodes.
A first resistor is connected between the cathodes of the diodes and the common path, and a plurality of second diodes connected in series are connected to the cathodes of the first diodes through the second resistors, and further connected in series. A scanning circuit for a photodiode array configured to connect the cathodes of the connected second diodes to a common path and output a scanning voltage to each photodiode from a connection point between the respective second diode and a second resistor. .