JPH01163632A - Photometry circuit - Google Patents

Abstract

PURPOSE:To eliminate the adverse influence of a dark current accompanied by a temperature rise by providing a photodiode which has the same characteristics in a light shield state successively to the constant-quantity feed cable run of a capacitor which is discharged according to the amount of light incident on the photodiode. CONSTITUTION:A photocurrent is discharged from the integrating capacitor 7 which is charged to a constant amount through the analog switch 2 of a feeding circuit according to the quantity of light incident on the photodiode 8 for measurement and this is detected and measured. This feeding circuit is provided with the photodiode 16 which has the same characteristics with the diode 8 in the light shield state successively nearby the diode 8 and the apparent dark current of the diode 8 is eliminated with a leak current from the diode 16. Consequently, the adverse influence of the dark current due to a temperature rise is eliminated and the in-use temperature range is expanded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an improvement in dark current in a photometric circuit, specifically, a photometric circuit that employs direct photometry in a video camera or an electronic camera.

[Prior Art] The direct photometry method is conventionally well known. As shown in FIG. 3, an example of a conventionally used photometric circuit, this photometric circuit includes a photo diode for fltll light. That is, the reference voltage (2) is applied to the non-inverting input terminal T1 of the operational amplifier 1 forming a voltage follower.

Further, a capacitor 13 is connected to the same terminal for bypassing noise input superimposed on the reference voltage V.

The output terminal of the operational amplifier 1 is connected to a non-inverting input terminal of an operational amplifier 3 having a high input impedance via an analog switch 2 formed of an FET. The gate of the analog switch 2 is connected to the output terminal and the inverting input terminal of the operational amplifier 1 via resistors 4 and 5, and the connection point between the resistors 4 and 5 is F, which forms a high impedance buffer. , ET is connected to a terminal T2, which is connected to a gate array (not shown). An integrating capacitor 7 is connected between the non-inverting input terminal of the operational amplifier 3 and ground. This integrating capacitor 7 is charged to a terminal voltage (2) by the output voltage of the operational amplifier 1 when the analog switch 4 is turned on.

On the other hand, the anode of the photodiode 8 measures the light transmitted through the photographic lens (not shown).

The cathode is the inverting input terminal of operational amplifier 3.

The output terminal of the operational amplifier 3 is connected to the inverting input terminal of the operational amplifier 3 and the non-inverting input terminal of the comparator 9, and the terminal T4 outputs the photocurrent of the photodiode 8. It is connected to the. This terminal T4 is a check terminal for waveform observation, etc. Further, a resistor 10 is connected to this terminal T4 and the output terminal of the comparator 9, and the output terminal of the comparator 9 is connected to the output terminal T5, and the output terminal is connected to the power supply voltage V via a resistor 11. . or is applied.

The comparator output output to the output terminal T5 is used as a close signal or a photometry end signal input to a gate array (not shown).

The inverting input terminal of the comparator 9 is connected to a terminal T6 to which the D/A converted input signal VTR is applied, and a capacitor for bypassing noise superimposed on the input signal VTR is connected to the terminal T6. ]2 is connected. Note that the terminal T3 is a ground terminal.

In the photometry circuit configured as described above, in the normal photometry standby state, the signal supplied to the terminal T2 is at the "L" level, so the analog switch 6 is off. Therefore, since the analog switch 2 is in the on state, the reference voltage V applied to the terminal T is the same as that of the operational amplifier 1. The integrating capacitor 7 is charged to the reference voltage V via the analog switch 2.

Here, when the photometry start signal is output from the gate array to the terminal T2, the terminal T2 becomes "H" level, so the analog switch 6 is turned on, the analog switch 2 is turned off, and the integrating capacitor 7 is turned off. The charging path is cut off. At the same time, the photodiode 8 starts photometry, and the electric charge stored in the integrating capacitor 7 passes through the photodiode 8 and becomes a photocurrent i according to the amount of light incident on the photodiode 8, as shown in FIG. Then, the voltage is discharged toward the terminal T4. This discharge current i is also supplied to the comparator 9, so the comparator 9 compares the voltage due to this current i with the input signal VTR supplied to the terminal T6.

When the signal becomes lower than the VTR, the comparator 9
Since the output of , that is, the comparator output output from the terminal T5 is inverted to the "L" level, this is inputted to the gate array as a shutter close signal or a photometry end signal, and photometry ends here. Also, when the photometry is finished, the terminal T
Since the signal applied to V.2 becomes "L" level and the analog switch 6 is turned off, the analog switch 2 is turned on and the integrating capacitor 7 is charged to the reference voltage V again.

In the photometric circuit operating in this manner, the charge MQ discharged from the integrating capacitor 7 is expressed as Q=CV=it.

Here, C is the capacitance of the integrating capacitor 7, V is the terminal voltage 1 of the integrating capacitor before the discharge starts, and discharge current t is the discharge time.

If the above equation is modified, 1=CV/l, so the discharge current i and the discharge time t are inversely proportional. Therefore, by knowing the time t during which the voltage due to the photocurrent 1 is falling linearly, the amount of photometric light can be determined. That is, when the amount of light incident on the photodiode 8 is large, the discharge time t becomes short, and when the amount of incident light is small,
This discharge time t becomes longer.

The above is the operating principle of the conventionally used photometric circuit.

[Problems to be Solved by the Invention] However, in the conventional photometric circuit described above, there is a problem in that the dark current of the photodiode has a large influence, making it impossible to widen the operating temperature range. In general, dark current is a problem not only in photodiodes but also in operational amplifiers.In particular, in the photometering circuit mentioned above, even though no object light is being received, as the temperature rises, the photodiode has leakage current in the opposite direction. It flows greatly. The dark current of a photodiode when the temperature rises approximately doubles for every 10°C rise in temperature. Therefore, when the operating temperature range of the photometric circuit is set to -10°C to -60°C, the conventional photometric circuit described above has +
The results showed that it could only be used up to 35°C.

In addition, in the conventional photometry circuit described above, as shown in FIG. 4, a spike-like voltage occurs in the output voltage of the photoelectrically converted operational amplifier 3 at the start of photometry, reducing the photometry accuracy. be.

That is, at the start of photometry when the analog switch 2 is turned off from the initial state in which the integrating capacitor 7 is charged to the reference voltage V, normally, as shown by the broken line Ω1 in FIG. The voltage V should decrease with a slope according to the amount of incident light, but at the moment when photometry starts, the voltage increases by V in a spike shape, and then decreases as shown by the solid line β2 from the potential point of 10ΔV. Therefore, the photometry accuracy deteriorates due to this voltage ΔV. This is the result of measurement, power supply voltage ■. 5■, and when the reference voltage ■ is 2±0.4V, Δ■ becomes as much as 140mV, resulting in an error of about 20% on the low illuminance side.

Furthermore, in the conventional photometry circuit described above, exposure correction based on the photometry value is performed by changing the voltage applied to the inverting input terminal of the comparator 9, that is, the voltage of the input signal VTR applied to the terminal T6. However, if it is desired to increase the photometric accuracy and the number of correction stages, this cannot be done because the dynamic range of the output signal to the terminal T4 is small. In the above photometric circuit, the power supply voltage (2) is set as low as 5 (5), so the dynamic range of the signal output to the terminal T4 is only about 2V. If the dynamic range of this signal is widened, the number of exposure correction steps will also increase, making it possible to improve optical precision.

As described above, the conventional photometry circuit described above has many problems.

Accordingly, an object of the present invention is to provide a photometric circuit which can extend the usable temperature range by eliminating the adverse effects of dark current caused by temperature rise, and which also eliminates other drawbacks.

[Means and Effects for Solving the Problems] The photometric circuit of the present invention adjusts the discharge rate of the integrating capacitor charged by the power supply means to a certain amount according to the amount of light incident on the photodiode. In the photometry device, a photodiode having substantially the same characteristics as the photodiode is connected to a switching means inserted in a power supply path from the power supply means to the integrating capacitor. It is characterized by being placed side by side in a state where

[Example] The present invention will be explained below with reference to an illustrated embodiment.

In the photometric circuit of this embodiment, the same components as those of the conventional photometric circuit shown in FIG. 3 are designated by the same reference numerals to avoid duplication of explanation.

FIG. 1 is an electrical circuit diagram of a photometric circuit showing one embodiment of the present invention.

In this embodiment, a compensation photodiode 16 having substantially the same characteristics as the direct photometry photodiode 8 is connected in parallel with the analog switch 2 in a light-shielded state. There is. That is, the photodiode 16 is arranged with its anode and cathode connected to the drain and source of the analog switch 2 formed by an FET, respectively. Also,
In this embodiment, the power supply voltage is V. is set to 7 V, for example, and the reference voltage applied to the operational amplifier 1 can be adjusted with a variable resistor 4, and is set to 4 V, for example. The configuration is such that the applied voltage can be adjusted and applied by a variable resistor 15.

In this embodiment configured as described above, since the analog switch 2 is turned on in the normal photometry standby state, the integrating capacitor 7 is charged to the reference voltage V. When the photometry start signal is applied to the terminal T2, the analog switch 2 is turned off and photometry is started. At this time, if the subject light incident on the photometric photodiode 8 is so weak that almost no photocurrent 1 flows, the charge charged in the integrating capacitor 7 should be retained as it is. In reality, the dark current (1) of the photodiode 8 discharges due to the influence of temperature, and the potential of the capacitor 7 gradually decreases.

However, in the present invention, a temperature compensation photodiode 16 is provided near the photometric photodiode 8 and inserted in parallel with the analog switch 2, so that the integration capacitor 7 and the photodiode 16 are connected in parallel to the analog switch 2. A dark current (2) due to the same temperature increase as that of the photodiode 8 for illll light flows from this photodiode 16. Therefore, since the dark current 2 flowing in from the light-shielded photodiode 16 and the dark current 11 flowing out from the photometric photodiode 8 are approximately equal, the dark current can be regarded as zero, and the point P There is no current flowing in or out at.

In other words, if there is no photodiode 6 for temperature compensation, the dark current of the photodiode 8 will discharge the charging voltage of the integrating capacitor 7 to zero when viewed over a very long time interval. Due to the function of the compensation diode] 6, the dark current can be almost ignored.

2(A) and 2(B) are the signal waveforms applied to the terminal T2 and the voltage waveforms of the integrating capacitor 7 in the photometric circuit shown in FIG. 1, observed using an oscilloscope. A) is the first diode for temperature compensation]6
FIG. 2(B) shows the case where there is no compensating photodiode 16, and the case where there is a compensating photodiode 16, respectively. As shown in FIG. 2A, even if no object light is incident on the photodiode 8, the potential of the integrating capacitor 7 decreases due to the dark current of the photodiode 8. In contrast, Fig. 2 (B
), it can be seen that when the photodiode 16 is provided, the potential of the integrating capacitor 7 does not change because it can be assumed that almost no dark current flows.

Generally, as described above, there is an electrically exponential relationship between temperature and dark current, in which the dark current doubles for every 10° C. rise in temperature. Therefore, when the photodiodes 8 and 16 are arranged close to each other as in this embodiment, the temperatures of both diodes 8 and 16 rise in the same way.

Therefore, dark current can be efficiently prevented not only at room temperature but also at high temperature as well as at room temperature.

Moreover, when the photodiode 16 for compensation is not provided as in the present invention, the spike-like voltage Δ which was about 140 mV
It was confirmed through experiments that (2) was also reduced to about 40 mV by providing the photodiode 16 to prevent dark current. Furthermore, in the first embodiment, the power supply voltage was increased to, for example, 7V, thereby expanding the dynamic range, increasing the number of exposure steps, and improving photometry accuracy.

[Effects of the Invention] As described above, according to the present invention, when the photodiode having substantially the same characteristics as the photometric photodiode is shielded from light, the analog switch near the photometric photodiode is By arranging them in parallel, the dark current of the photometric photodiode can be made to appear to be zero due to the leakage current of the photodiodes arranged in parallel, which eliminates the drawbacks of conventional photometry circuits. A photometric circuit having temperature compensation characteristics that is effective over a wide range can be provided.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram of a photometric circuit showing an embodiment of the present invention, and FIGS. 2(A) and 2(B) are diagrams showing voltage waveforms of the terminal T2 and the integrating capacitor in FIG. 1, respectively. 3 is an electric circuit diagram of a conventional photometric circuit, and FIG. 4 is a waveform diagram of signal voltages input and output to each terminal in FIG. 3. 1......Operative amplifier (power supply means) 2...
・・・・・・・・・Analog switch (switching means)

Claims (1)

[Scope of Claims] A photometric device configured to detect the amount of incident light based on the discharge rate of the capacitor when the capacitor charged to a certain amount by the power feeding means is discharged according to the amount of light incident on the photodiode, A photometric circuit characterized in that a photodiode having substantially the same characteristics as the photodiode is arranged in parallel in a light-shielded state in a switching means inserted in a power supply path from the power supply means to the capacitor.