JPH0652194B2 - Photocurrent measurement circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a photocurrent measuring circuit that solves the problem of unstable operation at the initial stage of operation of a photocurrent amplifier circuit used for automatic exposure of a camera and the like.

(Prior Art) It takes some time from when the photocurrent amplifier circuit using a photodiode is turned on until the circuit reaches a steady state.

Japanese Examined Patent Publication No. 60-34848 (title of the invention: photocurrent amplifier circuit) shows a circuit configuration for shortening this time.

This photocurrent amplifier circuit starts discharging or neutralizing the surplus charges accumulated in the photodiode at the time of power-on at the initial stage of power-on, and suspends the charging or neutralization when there is still some surplus charge remaining. Is configured.

With such a structure, the time to reach a stable state can be shortened.

However, the time until reaching the stable state still depends on the amount of incident light, and when the amount of incident light is small, the time until stabilization is longer than when the amount of incident light is large.

(Problems to be Solved by the Invention) Although the time until reaching a stable state when the amount of incident light is small is set in advance and the data after that time is used, accurate exposure control is possible, There is a risk of missing a photo opportunity.

An object of the present invention is to provide a photocurrent measurement circuit that can quickly capture data that has reached a stable state.

(Means for Solving the Problems) In order to achieve the above object, the photocurrent measuring circuit according to the present invention amplifies the photocurrent of a photodiode, and responds to the magnitude of the amount of light incident on the photodiode when the power is turned on. In the photocurrent measuring circuit using a photocurrent amplifier circuit, the response of which is slower than that when the amount of incident light is large and the output of the photocurrent amplifier circuit requires time to stabilize its output. The photometric output value obtained by A / D converting the output is compared with the reference level one or more times, and as the number of comparisons increases, the level value corresponding to the case where the incident light amount is large is changed to the level corresponding to the case where the incident light amount is small. The reference level is changed so that it becomes a value, and at the comparison stage when the photometric output value exceeds the reference level, the photometric output value is validated and used as control data. It is configured as

(Example) Hereinafter, the present invention will be described in more detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a circuit of a camera using a photocurrent measuring circuit according to the present invention. The photometric circuit 1 is connected to the CPU 3 via the digital IC circuit 2.

The controlled camera 4 supplies data of each unit to the CPU 3 and is controlled by the control data from the CPU 3.

FIG. 2 is a circuit diagram mainly showing the configuration of the photometric circuit portion.

A photo diode Pd is connected to the input terminal of the amplifier A ₁ for photocurrent amplification of the photometric circuit 1, and a logarithmic compression diode D is provided between the input and output terminals of the amplifier A ₁ for photocurrent amplification.
Are connected.

A bias circuit B for inserting information such as film sensitivity is connected to _one of the input terminals of the amplifier A ₁ for photocurrent amplification.

In the photocurrent amplifier circuit A ₁ , the surplus charges accumulated in the photodiode Pd when the power is turned on are discharged or neutralized at the initial stage of turning on the power, and when the surplus charges still remain, the charge or There is no circuit to stop neutralization.

In any case, there is no difference in that it depends on the amount of input light until the circuit becomes stable, so this circuit will be described as an example.

The output of the photocurrent amplifier circuit A ₁ is connected to the input terminal of the integrating amplifier A ₂ via the switch SW. This switch SW is operated by the sampling signal SP connected via the digital IC 2, and initially connects the photocurrent amplifier circuit A ₁ to the input terminal of the amplifier A ₂ for integration,
Then, a reference voltage source (for double integration) Vr is connected, and this is repeated thereafter.

An integrating capacitor C is connected to the input / output terminal of the integrating amplifier A _2.
Is connected, and the output terminal of the integrating amplifier A ₂ is
It is connected to the input terminal of the comparator A ₃ .

As a result, a pulse having a length corresponding to the output level of the photocurrent amplifier circuit A ₁ is obtained for each cycle of the sampling signal.

Information on the incident light is obtained by A / D conversion of this pulse and used as control information such as exposure of the camera.

As described above, the time until the output level of the photocurrent amplifier circuit A ₁ stabilizes depends on the amount of light incident on the photodiode Pd.

FIG. 4 is a graph showing changes in the output voltage of the amplifier for photocurrent amplification.

The curves L ₃ , L ₂ , L ₁ and L ₀ are incident light amounts (L ₃ <L ₂
The response characteristics to <L ₁ <L ₀ ) are shown.

As described above, the output voltage of the amplifier for photocurrent amplification using the photodiode Pd is initially unstable, and becomes stable quickly when the amount of incident light is large, but when the amount of incident light is small, the time to reach stability becomes long. . Ideally, the photometric data should be obtained in a short time after the power is turned on. However, as shown in FIG. 4, at t ₀ , the outputs corresponding to L ₁ , L ₂ , and L ₃ still correspond to the brightness. The circuit output has not reached a certain level (steady state).

Further, at the timing of t ₁ , L ₀ and L ₁ have reached the stable level, but L ₂ and L ₃ have not yet reached the stable level.

Next, referring to FIG. 5, C related to the incorporation of the photometric signal
The operation of the PU will be described.

(Step) Initialize the number of photometry (the number of comparisons) N and n.

The photometric number N is the number of times the output level of the photocurrent amplifier circuit A ₁ is checked, and even when the output voltage of the amplifier for photocurrent amplification does not reach a stable level at the initial output evaluation time, Check the number of times. N = 3
An example will be described in which the setting is made to.

(Step) Read the photometric data.

The output voltage Vou of the amplifier A ₁ for photocurrent amplification shown in FIG.
The t is read via the digital IC 2 and A / D converted to obtain comparison data.

(Step) In this step, the digital data LV corresponding to the output voltage Vout is compared with the reference level LV ₀ .

The reference level LV ₀ is digital data corresponding to VT ₀ shown in FIG.

Here, when it is larger than the reference level (L ₀ shown in FIG.
(In the case of the above), the process jumps to the step of Yes, and the data obtained by the double integration immediately after that time is validated and A / D converted by the digital IC circuit 2 to be used as the control data.

If the reference level is not reached in this step (corresponding to L _{1 to} L ₃ shown in FIG. 4), the process proceeds to the next step.

(Step) In this step, the number of photometry n is incremented by 1,
Next, the predetermined number N of photometry is decreased by -1.

(Step) Check the value of N minus -1 in the previous step. When the value becomes 0 (when the number of photometry has been determined in advance), the process proceeds to step.

If the value of N is not zero, go to step.

(Step) After waiting for a certain period of time, proceed to the next step, perform photometry again, and proceed to the step for comparison.

(Step) If the level is lower than the reference level, NO is selected and the operation proceeds to the next step.

If the standard level is reached, proceed to the step.

(Step) Even if the predetermined number of photometers have elapsed and the levels are compared each time, the standard level will not be reached.In this case, the photometric value is out of the photometric range and the photometric value at that point is invalid. It is determined to be out of the photometric range, that is, under, and an under warning is displayed on the display unit of the camera to warn the photographer.

(Step) The data obtained by the double integration immediately after that time is validated and A / D converted by the digital IC circuit 2 and used as control data.

As can be easily understood from the above description, Vout ₀ > VT ₀ (LV> LV ₀ ) when the photocurrent amplifier circuit A ₁ generates an output corresponding to the curve L ₀ shown in FIG. The data is taken out from the first comparison.

When the photocurrent amplifier circuit A ₁ is producing an output corresponding to the curve L ₁ , Vout ₁ <VT ₀ (L
V <LV ₀ ), and Vout ₁ > VT in the second comparison.
_{Since 1} (LV> LV ₀ −ΔLV), the data is extracted by the second comparison.

When the photocurrent amplifier circuit A ₁ is generating an output corresponding to the curve L ₂ , Vout ₂ <VT ₀ (L
V <LV ₀ ), and Vout ₂ <VT in the _second comparison.
₁ (LV <LV ₀ −ΔLV) and Vout ₂ > VT ₂ (LV> LV ₀ −2ΔLV) in the third comparison, so data is extracted by the third comparison.

When the photocurrent amplifier circuit A ₁ is generating an output corresponding to the curve L ₃ , Vout ₃ <VT ₂ (L
Since V <LV ₀ −2ΔLV), the data is not valid even by the third comparison, and it is determined that the photometric value is out of the photometric range, the photometric value at that point is invalid, and the photometric range is out of range, that is, under. An under warning is displayed on the display section to warn the photographer.

(Effects of the Invention) As described above, the photocurrent measurement circuit according to the present invention is
It amplifies the photocurrent of the photodiode.When the power is turned on, the response depends on the amount of light incident on the photodiode.When the amount of light incident on the photodiode is small, the response is slower than when the amount of incident light is large, and its output In a photocurrent measuring circuit using a photocurrent amplifier circuit that requires time to stabilize, the photometric output value obtained by A / D converting the output of the photocurrent amplifier circuit is compared with a reference level one or more times, and the number of comparisons increases. The reference level is changed from the level value corresponding to the case where the incident light amount is large to the level value corresponding to the case where the incident light amount is small, and the comparison step in which the photometric output value exceeds the reference level. Is configured to validate the photometric output value as control data.

Therefore, data can be taken in when the output of the photocurrent amplifier circuit becomes stable.

Further, the number of times of comparison can be set to a plurality of times, and the reference level can be made to gradually decrease, so that data can be captured quickly when there is sufficient incident light.

In this way, when the light is very bright when it is measured, it is unnecessary to wait for taking into account the dark case for the light measurement, even though the output level of the circuit is already stable.

As described above, in the first photometry, the photometric value and VT are compared and calculation is performed. If the level is smaller than that level, photometry is performed again. However, the number of photometry and comparison computations is not limited to once. If the comparison value is changed once (three times in this case), the photometric value in a bright case can be quickly obtained.

This is not limited to when the power is turned on, and the same phenomenon occurs when the photometric elements are switched by switching the photometric modes such as average photometry and partial photometry, so this method is very effective.

As an example, in the case of average metering (AV) and partial metering (SPOT), if the number of times of light metering is set in advance and the number of times is overlaid, the setting value at that time is made effective, or the light metering range is set. It can be judged and displayed as outside.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a circuit of a camera using a photocurrent measuring circuit according to the present invention. FIG. 2 is a circuit diagram mainly showing the configuration of the photometric circuit portion. FIG. 3 is a graph for explaining the operation of the photometric circuit. FIG. 4 is a graph showing changes in the output voltage of the amplifier for photocurrent amplification. FIG. 5 is a flow chart defining the functions of the CPU. 1 ...... metering circuit A ₁ ...... photocurrent amplifier A ₃ ...... comparator amplifier A ₂ ...... integration for amplification Pd ...... photodiode D ...... logarithmic compression diode C ...... integrating capacitor B ...... bias power supply Vr ...... Reference voltage source (for double integration) 2 ...... Digital IC circuit 3 ...... CPU 4 ...... Controlled camera

Claims (2)

[Claims] 1. A method for amplifying a photocurrent of a photodiode, wherein when the power is turned on, the responsivity depends on the amount of light incident on the photodiode. When the amount of light incident on the photodiode is small, the response is greater than when the amount of incident light is large. In a photocurrent measuring circuit using a photocurrent amplifying circuit whose output is slow and requires time for its output to stabilize, the photometric output value obtained by A / D converting the output of the photocurrent amplifying circuit is compared with a reference level one or more times. As the number of comparisons increases, the reference level is changed from a level value corresponding to a large incident light amount to a level value corresponding to a small incident light amount, and the photometric output value is changed to the reference level. A photocurrent measuring circuit configured so that the photometric output value is validated and used as control data in a comparison stage that exceeds the limit. 2. The photocurrent measuring circuit according to claim 1, wherein the reference level to be changed is reduced as the number of comparisons is increased.