JPH0284819A - Double integral type a/d converter - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a double integral type A/D converter, and more specifically, the present invention relates to a double integral type A/D converter, and more specifically, the present invention relates to a double integral type A/D converter. This invention relates to an integral type A/D converter.

<Prior Art> In recent years, compact type autofocus cameras, which have become rapidly popular, generally employ a position detection element known by the abbreviation PSD as a distance measuring element. This element is a light receiving element whose resistance ratio between its two sets of output terminals changes depending on the receiving position of infrared rays projected from the camera body and reflected by the subject.

On the other hand, the intensity of the infrared rays reflected by the object and reaching the element is proportional to the reciprocal of the square of the distance between the object and the camera, so the change in the absolute value of the resistance of the element corresponding to the intensity is It has the property of becoming extremely large.

In order to remove the change in the absolute value of the two resistances and extract only the ratio of the two sets of resistances, we perform logarithmic compression of the current corresponding to the resistance value, and then guide the signals of those two sets to a differential circuit. , the configuration is such that a signal corresponding to the ratio is extracted.

Since the signal extracted in this way is a signal indicating the distance to the subject, this signal is converted into a digital value by leading it to a double integration type A/D converter,
Based on the obtained digital values, the photographic lens was moved to focus on the subject.

<Problem to be solved by the invention> The position of the infrared rays incident on the position detection element changes depending on the distance to the subject, but the ratio between the length of the baseline, which is the reference for distance measurement, and the distance from the camera to the subject is an extremely large value, so even if there is a large change in the distance to the subject, only a slight change will occur in the light receiving position of the element. This means that when installing a position detection element, a slight error in position has a large effect on the accuracy of distance measurement. , mounting of the position detection element means that the position needs to be precisely adjusted. This means that it is necessary to incorporate a mechanism that can precisely move the position detection element into the camera, resulting in a problem that the mechanical part for distance measurement becomes a complicated mechanism.

The present invention was devised to solve the above problems, and its purpose is to provide a double-integration type A/D converter that can improve distance measurement accuracy without moving the position detection element. It is in.

Means for Solving the Problems> In order to solve the above problems, the double integration type A/D converter of the present invention charges or discharges the integration capacitor for a preset period with a current value corresponding to the input. At the same time, the amount of charge in the integrating capacitor that has changed due to charging or discharging of the current value corresponding to the input is returned to the amount before the change by discharging or charging the integrating capacitor with a preset current value. The configuration is such that input is detected based on the required time.When the current corresponding to the input charges the integrating capacitor, the integrating capacitor is discharged in parallel with this charging, and the current corresponding to the input charges the integrating capacitor. When discharging, a constant current circuit is provided that charges the integrating capacitor in parallel with this discharging, and a configuration is used in which the current value of this constant current circuit is made variable.

Since the time for charging or discharging the integral capacitor with the current value corresponding to the input is a fixed period set in advance, the current of the constant current circuit is used to charge or discharge the integral capacitor. The time taken is also constant.

Therefore, the amount of charge charged to or discharged from the integrating capacitor by this constant current circuit is also constant.

Since this change in the amount of charge will always be added to the change in the amount of charge on the integrating capacitor corresponding to the input, it is subsequently performed and the change in the amount of charge on the integrating capacitor is The integration time to return to the amount always decreases by a constant amount of time, depending on the value of the current in the constant current circuit.

Therefore, the value of the output of the double integration type A/D converter becomes the value given the shift value corresponding to the current value of the constant current circuit.

<Embodiment> FIG. 1 is a circuit and block diagram showing the electrical configuration of an embodiment of the present invention.

In the figure, the outputs of current detection units 11a and llb that detect the current flowing between two sets of terminals of the position detection element 11 are connected to diodes DiSD2 provided correspondingly.
are connected to the anodes of these diodes Di
, D2 are connected in parallel and connected to a first reference voltage Vref1. And current detection section 11
The outputs of a and llb are sent to the respective amplifiers 12a
, 12b, transistors 13a, 13b
is guided by the base of

The emitters of these two transistors 13a and 13b
The terminals are connected to each other and grounded via a constant current circuit 15. The collector of one transistor 13b is connected to the positive power supply VcC, and the collector of the other transistor 13a is connected to the positive power supply via a constant current circuit 14 whose current value can be varied by a variable resistor 142. Connected to Vcc.

The other terminal of the integrating capacitor 17, one terminal of which is grounded, is connected to the terminals of three switches Sl, S2, and S3 constituted by analog switches, and the comparator 1.
The other terminal of the switch S1 is connected to the collector of the transistor 13a, the other terminal of the switch S2 is connected to the positive power supply Vcc via the constant current circuit 16, and the other terminal of the switch S3 is connected to the positive power supply Vcc via the constant current circuit 16. The terminal is connected to the output of comparator 18. The other terminal of the comparator 18 is led to a second reference voltage Vref2.

The counter circuit 20 has three inputs, one of which is the clock input, which receives the output of the clock generation circuit 21, and the input for instructions to reset and start counting is the timing control circuit. The output of comparator 18 is led to the count end input. The output 201 of the counter circuit 20 is sent to a lens driving device (not shown) as a distance output that indicates a value corresponding to the distance to the subject as a digital value.

The operation of one embodiment of the zero-starting system having the above configuration will be described below.

The outputs of the current detecting sections 11a and 11b of the position detecting element 11 are determined while keeping the receiving position of the reflected light from the subject constant.
When the intensity is changed, the current ratio does not change, but the current value itself changes. On the other hand, the intensity of the reflected light is the reciprocal of the square of the change in distance to the subject, so the outputs of the current detection units 11a and llb are as follows:
It changes extremely depending on the change in distance to the subject.

In order to convert this extremely large change in current into a change within a manageable range, the cathode is connected to a first reference voltage Vrefl.
By guiding the outputs of the current detection units 11a and llb to the anodes of the diodes D1 and D2 connected to the
Compress the output changes of 1a and llb. The compressed output is passed through amplifiers 12a and 12b to transistors 13a and 13b forming a differential amplification circuit.
lead to the base of

Furthermore, since the relationship between the base voltage and the base current of the transistors 13a and 13b is an antilogarithmic characteristic, and since the transistors 13a and 13b form a differential amplifier, the outputs of the current detection units 11a and llb are expressed as Ia, If the collector current of the transistor 13a is indicated by Ic, and the current value of the constant current circuit 15 is indicated by 115, then Ic=115X Ha/(Ia+Ib)).

FIG. 2 is an explanatory diagram showing temporal changes in the voltage across the integrating capacitor 17, and the explanation will be continued with reference to FIG. 2 as necessary.

At time T1, a signal for resetting the counter in the counter circuit 20 is sent from the timing control circuit 19 to the counter circuit 20. Also switch S
By closing 3, the voltage of the integrating capacitor 17 is set to the second reference voltage Vref2, and at the same time, infrared light is emitted from a light projector (not shown) to the subject, and the reflected light is guided to the position detection element 11. .

The reflected light reaches the position detection element 11, and a current corresponding to the distance to the subject flows through the current detection units 11a and 11b. Then, by opening the switch S3 and closing the switch St, the charge in the integrating capacitor 17 starts to be discharged.

Discharging at this time is performed by the collector current 1c of the transistor 13a described above, and in parallel with this discharging, the integral capacitor 17 is charged by the constant current circuit 14.

The period during which this operation is performed is tl, which is a preset period, and its length is always fixed.

A broken line 31 in FIG. 2 shows a change in the voltage of the integrating capacitor 17 when no current is supplied from the constant current circuit 14 and the integrating capacitor 17 is discharged by the collector current 1c. Since current is supplied from the constant current circuit 14 to the integrating capacitor 17 in parallel with the discharge of the collector current 1c, the actual change in the voltage of the integrating capacitor 17 becomes the change shown by the solid line 32. .

Further, a broken line 33 indicates a change in the voltage of the integral capacitor 17 when the distance to the subject is different and only discharge is performed by the collector current 1c, and the constant current circuit 14
When there is a current supply from , the change will be as shown by the solid line 34 .

Then, at time T2 after the period tl, the timing control circuit 19 opens the switch S1 and opens the switch S.
Close 2. It also sends a signal to the counter circuit 20 to instruct it to start counting.

After this time, the integral capacitor 17 is charged by the constant current circuit 16, and the voltage across it increases.

Then, the voltage of the integrating capacitor 17 becomes the second reference voltage V
When reaching ref2 (times T3 and T4), the output level of the comparator 18 is inverted, and the output level of the counter circuit 2 is inverted.
0 stops the counting operation.

The count value at this time is a value indicating the period t2 and t3 from time T2 until the voltage of the integrating capacitor 17 reaches the second reference voltage Vref2, and this value is
A signal 201 indicating the distance is sent to the lens driving device, and the lens is moved to a position corresponding to the value.

　　.

The voltage of the integrating capacitor 17 at time T2 when the constant current circuit 16 starts charging the integrating capacitor 17 is:
Although the values of the voltages v1 and v2 of the difference differ depending on whether or not current is supplied from the constant current circuit 14, the value of the current from the constant current circuit 14 is constant, and the current flowing Since the period t1 is fixed, if the current value of the constant current circuit 14 is 114 and the capacity of the integrating capacitor 17 is F, then V1 = 114xtl/F V2-114Xt, 1/F, and 1-V2. .

On the other hand, since the integration capacitor 17 is charged after time T2 by a constant current from the constant current circuit 16, the slopes of voltage changes due to charging are all the same. Therefore, the periods t4 and t5, which are the differences between when the current is supplied from the constant current circuit 14 and when it is not, have the same length. In other words, from the constant current circuit 14 to the integrating capacitor 1
7 always causes the output value of the counter circuit 20 to decrease by a constant value, no matter what the distance to the subject.

Further, adjusting the variable resistor 142 is performed by the constant current circuit 1
Since the charging current from constant current circuit 14 is changed, the difference between integration periods t4 and t5 between when charging from constant current circuit 14 is performed and when there is no charging is changed.

On the other hand, if the position detection element 11 is installed out of position, the receiving position of the reflected light from the subject will always be shifted by a certain distance, regardless of the distance to the subject. will be done.

Therefore, if it is possible to always assign a constant value to the output value of the counter circuit 20, regardless of the distance to the subject, the installation of the position detection element 11 means that the positional deviation can be corrected. do.

This means that the mounting position of the position detection element 11 is corrected by adjusting the variable resistor 142.

From the above, in the actual machine, when the rotational position of the variable resistor 142 that changes the current value of the constant current circuit 14 is set to the center, when the position detection element 11 is in the normal mounting position, the counter circuit Since the value outputted from 20 is configured to indicate the correct distance to the subject, when the position detection element 11 shifts in either the left or right direction, by adjusting the variable resistor 142,
The deviation is corrected.

FIG. 3 is a partial circuit diagram showing an embodiment in which the order of charging and discharging the integrating capacitor 17 is reversed.

PNP transistors are used as the transistors 32a and 32b in the same embodiment, and the base of the transistor 32a receives a signal corresponding to the output of the current detection section 11a, and the base of the transistor 32b receives a signal corresponding to the output of llb. being guided. Further, the collector of the transistor 32a is grounded via a constant current circuit 14a. Further, a constant current circuit 33 whose one terminal is grounded is connected to the integrating capacitor 17 via a switch S2.

In the figure, during the period t1 shown in FIG. 2, the transistor 32a charges the integrating capacitor 17, and the constant current circuit 14a performs discharging in parallel with this charging, and after time T2, The integral capacitor 17 is discharged by the constant current circuit 33.

The operation of the constant current circuit 14a in these operations is the same as the operation already described, so the explanation thereof will be omitted.

Note that the present invention is not limited to the above embodiments, and if the change in the current of the position detection element is not extremely large, the diode D
1 and D2 can be omitted.

Furthermore, in this embodiment, a case has been described in which the double integration type A/D converter is applied to a distance measuring device of an autofocus camera, but other devices that require a fixed shift value to perform measurements have been described. For example, it is possible to apply the present invention to an ammeter that measures current by canceling the bias current.

<Effects of the Invention> The double-integrating A/D converter according to the present invention uses a constant current circuit in which the current value is variable, so that when the current corresponding to the input charges the integrating capacitor, this charging When the current corresponding to the input discharges the integrating capacitor,
By charging the integral capacitor in parallel with this discharging, the configuration provides a constant shift value to the input, so the mounting of the position detection element is is corrected by assigning a shift value, so there is an effect that distance measurement accuracy can be improved without moving the position detection element.

[Brief explanation of the drawing]

Fig. 1 is a circuit and block diagram showing the electrical configuration of an embodiment of the present invention, Fig. 2 is an explanatory diagram showing temporal changes in voltage across an integrating capacitor, and Fig. 3 is similar to Fig. 1. FIG. 3 is a partial circuit diagram showing an embodiment in which the order of discharging and charging of the illustrated embodiment is reversed. 14... Constant current circuit 17... Integrating capacitor 142... Variable resistor. Patent Applicant Sharp Corporation Agent Patent Attorney Koji Onishi Figure 3

Claims (1)

[Claims] (1) Charging or discharging an integrating capacitor for a preset period with a current value corresponding to the input, and measuring the amount of charge on the integrating capacitor that has changed due to charging or discharging the current value corresponding to the input, A double integration type A/D converter that detects an input based on the time required for discharging or charging the integrating capacitor to restore the amount before the change by discharging or charging with a preset current value, When the current corresponding to said input charges the integrating capacitor, the integrating capacitor is discharged in parallel with this charging, and when the current corresponding to said input causes the discharging of the integrating capacitor, the integrating capacitor is discharged in parallel with this discharging. A double integration type A/D converter comprising a constant current circuit for charging, and a variable current value of the constant current circuit.