JPH0779385A - Charge detection method for ccd image pickup element - Google Patents

Abstract

PURPOSE:To obtain the best S/N in response to an incident luminous quantity without giving a hindrance of a charge voltage conversion function by using selectively a horizontal transfer register depending on a received luminous quantity so as to switch the charge detection circuit. CONSTITUTION:A separate charge detection circuit is connected to each of horizontal transfer registers 3, 4 respectively. A capacitor 6, a reset switch 11 and an output extract amplifier 8 as components of the charge detection circuit are connected to the horizontal transfer circuit 3. Similarly a capacitor 7, a reset switch 12 and an output extract amplifier 9 as components of the charge detection circuit are connected to the horizontal transfer circuit 4. The capacitors 6, 7 store the signal charge sent through the horizontal transfer registers 3, 4 respectively and converts the signal into a voltage. The capacitance of the capacitors 3, 4 differs from each other. Thus, an output voltage is obtained via the charge detection circuit whose charge voltage conversion efficiency is different by selecting either of the two horizontal transfer registers 3, 4 to provide an output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image pickup device, and more particularly to a CCD (Charge Coupled Device).
e) relates to the charge detection method.

[0002]

2. Description of the Related Art A CCD image pickup device includes a large number of light-receiving pixel portions arranged in a vertical and horizontal matrix for receiving light and performing photoelectric conversion, and a plurality of columns for vertically transferring the charges photoelectrically converted in each pixel portion. Of the vertical transfer registers arranged in parallel with each other, a horizontal transfer register provided at an end of the vertical transfer registers of the plurality of columns, and a charge detection circuit connected to the horizontal transfer registers. The charge detection circuit is composed of a capacitor for accumulating charges corresponding to the amount of received light transferred through the horizontal transfer register and converting it into a voltage, a reset switch, and an amplifier for extracting a voltage output from the capacitor and outputting it to the outside. It is composed of

The light incident on each pixel portion forming the light receiving portion is converted into electric charges according to the amount of light received at each pixel portion, and transferred to the detection circuit via the vertical transfer register and the horizontal transfer register. In the detection circuit, the transferred charge is stored in the capacitor, and it is taken out through the amplifier.
The output of the image sensor is obtained as a voltage according to the amount of received light. In this case, the charge amount is Q, the capacitor capacitance is C, and the output voltage is V
Then, V = Q / C, and the value of the output voltage varies depending on the capacitance C of the capacitor used even if the amount of charge is the same. On the contrary, even if the charges are different, the same output voltage can be obtained by changing the capacitance of the capacitor.

On the other hand, as the noise of the CCD image pickup device, there are shot noise which depends on the input signal amount (light amount) and reset amplifier noise which is unrelated to the input signal amount. A graph of such a noise occurrence state is shown in FIG. a is a signal output according to the amount of light, b is shot noise, and c is reset amplifier noise. As you can see from the figure, C
The noise of the CD image pickup device is dominated by shot noise when the amount of incident light is large, and reset amplifier noise is dominated when the amount of incident light is small. Shot noise is proportional to the square root of the signal amount. Therefore, when the signal charge amount increases, the noise amount itself increases, but the ratio to the signal charge amount at that time decreases and the S / N ratio improves. Therefore, when the amount of incident light is large, the S / N ratio can be improved by increasing the capacitance of the capacitor and increasing the amount of charge.

On the other hand, in a place where the amount of incident light is small, almost constant reset amplifier noise is dominant regardless of the signal output. Therefore, by increasing the output voltage, the ratio of noise to the output is reduced and S / The N ratio can be improved.

[0006]

However, in the conventional CCD image pickup device, since only one charge detection circuit connected to the horizontal transfer register is provided, the capacitance of the capacitor is fixed and therefore the output charge amount. It was not possible to change the charge-voltage conversion efficiency so as to obtain the different optimum S / N ratio.

In this case, continuously changing the capacitor capacity of the charge-voltage converter by applying a voltage from the outside generally has a very small capacitor capacity. Therefore, the capacity of the variable mechanism is smaller than that of the capacitor capacity. If it becomes too large, the influence on the conversion unit becomes large, and the charge-voltage conversion function of the capacitor is hindered.

Further, if the conversion circuits having a plurality of different capacities are connected to one horizontal transfer register and the conversion circuits are switched and used, the capacity of the switching mechanism is the same as in the case described above. The capacitance of the capacitor becomes too large and the influence on the conversion portion becomes large, which hinders the charge-voltage conversion function of the capacitor.

The present invention has been made in view of the above-mentioned problems of the prior art, and the charge-voltage conversion is performed so that the optimum S / N ratio can be obtained according to the amount of incident light without hindering the charge-voltage conversion function. It is an object of the present invention to provide a charge detection method for a CCD image pickup device whose efficiency is variable.

[0010]

In order to achieve the above-mentioned object, a charge detecting method for a CCD image pickup device according to the present invention comprises:
By providing a plurality of horizontal transfer registers of the CD image pickup device, connecting a charge detection circuit having a different charge-voltage conversion efficiency to each horizontal transfer register, and selectively using the horizontal transfer register according to the amount of received light, The feature is that the detection circuit is switched.

In a preferred embodiment, the charge detection circuit comprises a capacitor for storing photoelectrically converted charges,
It comprises a reset switch and an output extraction amplifier, and is characterized in that the charge-voltage conversion efficiency is changed by making the capacitances of the capacitors of the charge detection circuits different.

Further, in order to achieve the above object, the CCD image pickup device according to the present invention comprises a plurality of light receiving pixel portions for performing photoelectric conversion, and a plurality of vertically transferring electric charges photoelectrically converted in each pixel portion. In the CCD image pickup device having a vertical transfer register arranged in parallel, a horizontal transfer register provided at an end of the plurality of vertical transfer registers, and a charge detection capacitor connected to the horizontal transfer register, the horizontal transfer A plurality of registers are provided in parallel, a switching circuit for selectively connecting one of the plurality of horizontal transfer registers to the vertical transfer register is provided, and the capacitance of the charge detection capacitor is provided for each horizontal transfer register. It is characterized by different.

[0013]

The horizontal transfer register is selected according to the amount of incident light, whereby the charge detection circuits having different fixed capacitances connected to each horizontal transfer register are switched to change the charge-voltage conversion efficiency.

[0014]

1 is a block diagram of a CCD image pickup device according to an embodiment of the present invention. The CCD image pickup device 10 has a large number of light receiving pixel portions 1 arranged in a matrix in the vertical and horizontal directions, and each pixel portion 1 is connected to a vertical transfer register 2 provided for each column of the matrix. The light incident on each pixel portion 1 is accumulated in the form of electric charge according to the intensity of light (light amount) and is transferred to the vertical transfer register 2. The vertical transfer register 2 is formed by arranging a large number of electrodes side by side in the vertical direction, and by sequentially applying a voltage to each electrode, the charges photoelectrically converted are transferred in the vertical direction as shown by arrows.

At the end of each vertical transfer register 2, two horizontal transfer registers 3 and 4 are provided in parallel. The two horizontal transfer registers 3 and 4 are connected to each vertical transfer register 2 via a switching gate circuit 5. The horizontal transfer registers 3 and 4 are driven by driving the switching gate circuit 5.
One of the two is connected to the vertical transfer register 2. As a result, the signal charges sent through the vertical transfer register 2 are selectively sent out sequentially to the horizontal transfer register 3 or 4 in a time series.

Alternatively, all the signal charges sent through the vertical transfer register 2 are once sent out to the horizontal transfer register 3, and a pulse is applied to the gate circuit 5 as necessary to transfer the signal charges in the horizontal transfer register 3 to the horizontal transfer register. 4 may be configured to be transferred.

Separate charge detection circuits are connected to the horizontal transfer registers 3 and 4, respectively. The horizontal transfer circuit 3 includes a capacitor 6 and a reset switch 11 as a charge detection circuit.
And the output extracting amplifier 8 is connected. Similarly, the horizontal transfer register 4 includes a capacitor 7, a reset switch 9, and an output extraction amplifier 9 as a charge detection circuit.
Are connected. The capacitors 6 and 7 accumulate the signal charges sent through the horizontal transfer registers 3 and 4, respectively, and convert them into a voltage. This voltage is output amplifiers 8 and 9
Is output to the outside via.

The capacitance of the capacitor 6 connected to the horizontal transfer register 3 and the capacitance of the capacitor 7 connected to the horizontal transfer register 4 are different from each other. Therefore, by selecting and using either of the two horizontal transfer registers 3 and 4, the output voltage can be obtained through the charge detection circuits having different charge-voltage conversion efficiencies.

In this case, when the amount of incident light is large, a horizontal transfer register connected to a capacitor having a large capacitance is selected and the amount of signal charge input to the capacitor is increased as much as possible to reduce the generation ratio of shot noise with respect to the amount of signal charge. Thus, the S / N ratio can be improved.

When the amount of incident light is small, a horizontal transfer register connected to a capacitor having a small capacitance is selected,
Increase the output voltage with respect to the signal charge amount. As a result, the generation ratio of reset amplifier noise with respect to the output voltage is reduced, and the S / N ratio is improved. In this case, since the charge-voltage conversion rate with respect to the signal charge amount is increased, the detection sensitivity is improved.

Although two horizontal transfer registers are provided in the above-described embodiment, three or more horizontal transfer registers may be provided and connected to capacitors of different capacities so as to be appropriately switched and used.

[0022]

As described above, according to the present invention, a plurality of horizontal transfer registers connected to capacitors having different capacities are provided, and the horizontal transfer register to be used is selected according to the amount of incident light. The charge amount can be detected by changing the charge-voltage conversion efficiency using a capacitor having a capacitance. Therefore, when the signal charge of the incident light is large, a capacitor having a large capacitance is used, and when the signal charge of the incident light is small, a capacitor having a small capacitance is used, and the optimum charge-voltage conversion efficiency with respect to the incident light amount is S.
The / N ratio can be improved. Further, when the amount of incident light is small, the charge-voltage conversion rate with respect to the amount of signal charges is increased, so that the detection sensitivity is improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a CCD image pickup device according to an embodiment of the present invention.

FIG. 2 is a graph of signal and noise output with respect to incident light amount.

[Explanation of symbols]

 1 ... Pixel part 2 ... Vertical transfer register 3, 4 ... Horizontal transfer register 5 ... Switching gate circuit 6, 7 ... Capacitor 8, 9 ... Output extraction amplifier 10 ... CCD image pickup device 11, 12 ... Reset switch

Claims (3)

[Claims] 1. A plurality of horizontal transfer registers of a CCD image pickup device are provided, charge detection circuits having different charge-voltage conversion efficiencies are connected to each horizontal transfer register, and the horizontal transfer register is selectively used according to the amount of received light. A charge detection method for a CCD image pickup device, characterized in that the charge detection circuit is switched by performing the above. 2. The charge detection circuit includes a capacitor that stores photoelectrically converted charges, a reset switch, and an output extraction amplifier. The charge detection method for a CCD image pickup device according to claim 1, wherein the conversion efficiency is changed. 3. A plurality of light receiving pixel portions for performing photoelectric conversion,
A plurality of vertical transfer registers arranged in parallel for transferring charges photoelectrically converted in each pixel section in the vertical direction, a horizontal transfer register provided at an end of the plurality of vertical transfer registers, and connected to the horizontal transfer register In a CCD image pickup device having a charge detecting capacitor, a plurality of the horizontal transfer registers are provided in parallel, and one of the plurality of horizontal transfer registers is selectively connected to the vertical transfer register. A CCD image pickup device characterized in that a circuit is provided and the capacitance of the charge detecting capacitor is made different for each horizontal transfer register.