JPH07113659A - Current-voltage conversion circuit and photoelectric encoder using current-voltage conversion circuit - Google Patents

Abstract

PURPOSE:To provide a current-voltage conversion circuit having a gain adjusting function, which can be formed into an IC without using a high-performance resistor or a special process. CONSTITUTION:A first analog switch S1 driven by a clock CK having specified frequency is mounted between the non-inverting input-terminal of an operational amplifier OP, in which a non-inverting input terminal is grounded, and a signal input terminal, first and second capacitors C1, C2 are connected in series between the non-inverting input terminal of the operational amplifier OP and an output terminal, and a third capacitor C3, in which capacitance can be varied by a control signal, is installed between the connecting node of these first second capacitors C1, C2 and a grounding terminal. A second analog switch S2 driven by a clock having antiphase to the first analog switch S1 is set up in parallel with the series circuit of the first and second capacitors C1, C2, thus constituting a current-voltage conversion circuit capable of adjusting gains.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current-voltage conversion circuit and a photoelectric encoder using the same.

[0002]

2. Description of the Related Art A photoelectric encoder includes a main scale and an index scale, each of which is provided with a graduation made of an optical grating and is arranged so as to be relatively movable, a means for irradiating these scales with parallel light, and a main scale and an index. It has a detection means for detecting light and darkness caused by repetition of overlapping scales. The detection means includes two light receiving elements for detecting two-phase signals having different phases that change sinusoidally with respect to the displacement, and two currents for converting the output current of each light receiving element into a voltage. -A voltage conversion circuit and a differential amplifier circuit that takes the difference between these outputs. Usually, the index scale, the parallel light irradiating means, and the light receiving element are combined to form a detection head that moves relative to the main scale.

In such a photoelectric encoder, a phototransistor has been conventionally used as a light receiving element. Since the phototransistor has an amplification effect,
Since it is relatively resistant to external noise, a large distance can be provided between the detection head and a signal processing circuit including a current-voltage conversion circuit that processes the output signal of the detection head. However, the response frequency of the phototransistor depends on the amount of light applied to the phototransistor, and normally only about 100 kHz can be obtained due to the limitation of the light emitting source.

Therefore, in order to improve the response frequency,
A photodiode, whose response does not depend so much on the amount of light as a single element, is used as a light receiving element. However, also in this case, the actual response frequency is often determined by the relationship with the subsequent current-voltage conversion circuit, and it is not preferable that the capacitive load increases due to the connection between the photodiode and the current-voltage conversion circuit. Further, since the photodiode has a smaller output current than the phototransistor, it is also vulnerable to external noise. For these reasons, it is desirable to arrange the photodiode and the current-voltage conversion circuit as close to each other as possible, and preferably to mount the current-voltage conversion circuit on the detection head unit. Furthermore, in order to reduce the number of connections between the detection head and the counting display, it is desirable to mount a differential amplifier that takes the difference between the outputs of the two current-voltage conversion circuits on the detection head.

[0005]

However, if the signal processing circuit is mounted on the detection head, it becomes difficult to adjust the DC level, which has been performed by the conventional volume control. That is, it is difficult to put a volume in a detection head, such as a gauge, which is desired to be miniaturized.

Therefore, a method of utilizing laser trimming of a resistor instead of the DC level adjustment by a volume, or a method of controlling a switch by an external signal by incorporating a combination of a plurality of resistors and a switch by IC technology can be considered. However, each of these has its own problems. The former method is technically very difficult since it involves fine trimming while monitoring the signal after the detector is assembled. In the latter method, a high resistance excellent in voltage characteristics and temperature characteristics cannot be obtained by a normal IC process, and the resistance of the switch becomes a problem. A combination of high-performance thin-film resistors requires a special process and is expensive.
It becomes C.

It is an object of the present invention to provide a current-voltage conversion circuit incorporating a gain adjusting function which can be integrated into an IC without using a high performance resistor or a special process. Another object of the present invention is to provide a photoelectric encoder in which a current-voltage conversion circuit having a gain adjusting function is integrated without using a high-performance resistor or a special process in a small detection head.

[0008]

A current-voltage conversion circuit according to the present invention is provided between an operational amplifier whose non-inverting input terminal is set to a reference potential and between the inverting input terminal and the signal input terminal of this operational amplifier. A first analog switch driven by a clock having a predetermined frequency, first and second capacitors connected in series between the inverting input terminal and the output terminal of the operational amplifier, and the first and second capacitors. A third capacitor, which is provided between a connection node of the capacitor and a reference potential terminal and whose capacitance can be varied by a control signal, and the first analog switch, which is connected between the inverting input terminal and the output terminal of the operational amplifier. And a second analog switch driven by a clock of opposite phase.

In the photoelectric encoder according to the present invention, graduations made of optical gratings are respectively formed and are arranged to face each other so as to be movable relative to each other, a first scale and a second scale, means for irradiating these scale portions with parallel light, and The detection means has a detection means for detecting light and darkness caused by repetition of the overlapping of the first scale and the second scale, and the detection means includes a means for irradiating one of the first scale or the second scale and the parallel light. A photodiode in the head and a current that converts the output current of this photodiode into a voltage −
The current-voltage conversion circuit is integrated with a voltage conversion circuit, and the current-voltage conversion circuit is provided with an operational amplifier whose non-inverting input terminal is set to a reference potential, and is provided between the inverting input terminal and the signal input terminal of the operational amplifier. A first analog switch driven by a frequency clock; first and second capacitors connected in series between an inverting input terminal and an output terminal of the operational amplifier; a connection node of these capacitors and a reference potential terminal; Driven by a clock having a phase opposite to that of the first analog switch, which is connected between the third capacitor and is connected between the inverting input terminal and the output terminal of the operational amplifier. And a second analog switch that is configured to operate.

[Action]

The current-voltage conversion circuit according to the present invention constitutes an equivalent feedback resistance by using a so-called switched capacitor. At this time, the feedback resistance value depends on the capacitance value of the capacitor and the clock frequency for driving the analog switch, and a high resistance value can be easily obtained. In addition, the third capacitor of the switched capacitor is a variable capacitor, and by controlling this with a control signal, the current-voltage conversion rate can be changed and the gain can be adjusted (therefore, a DC level can be adjusted by forming a differential circuit). ). The current-voltage change circuit according to the present invention can be easily realized as a monolithic IC by using, for example, the process of CMOS IC.

The photoelectric encoder according to the present invention is integrated with the photodiode having excellent high-speed response and the above-described current-voltage conversion circuit in the detection head to form a small detection head and a direct current It can have a level adjustment function.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a current-voltage conversion circuit according to an embodiment of the present invention. The non-inverting input terminal of the operational amplifier OP is grounded,
A signal to be converted is input to the inverting input terminal via the first analog switch S1. The feedback circuit provided between the inverting input terminal and the output terminal of the operational amplifier OP is composed of an equivalent resistance that is a combination of a capacitor and an analog switch. That is, the first capacitor C1 and the second capacitor C2 are connected in series between the inverting input terminal and the output terminal, and the capacitance is variable between the connection node and the ground terminal under the control of the control signal. Is connected to the capacitor C3. A second analog switch S2 is provided in parallel with the series circuit of the first capacitor C1 and the second capacitor C2. The first analog switch S1 is a clock CK having a predetermined frequency.
ON / OFF control by the second analog switch S
Reference numeral 2 is a clock obtained by inverting the clock CK by an inverter I, and is on / off controlled.

The resistance value Rv of the equivalent feedback resistor of the current-voltage conversion circuit thus constructed is the clock C
The frequency of K is F, which is expressed by the following equation.

[0014]

[Formula 1] Rv = (C1 + C2 + C3) / (C1 · C2 · F)

Therefore, the resistance value of the equivalent feedback resistor is set by selecting the value of each capacitor and the clock frequency. In the case of this embodiment, three types of capacitors are used to attenuate the feedback signal,
For example, a feedback resistor having a high resistance can be obtained by using a capacitor having a relatively large capacitance of about several pF. Further, the feedback resistance value can be variably controlled by controlling the third capacitor C3, which is a variable capacitor, and thus the current-voltage conversion rate can be changed.

The variable third capacitor C3 is, for example,
For example, as shown in FIG.
Passita C31, C32, C33, C34 and select these
Select analog switch S31, S32, S33, S3
4 and. Analog switch S31, S3
2, S33, S34 to control signals b1, b2, b3, b4
To selectively turn on the capacitors C31, C32,
By selecting the combination of C33 and C34,
The capacitance value of the capacitor C3 of 3 is determined. In Figure 2
If four capacitors are used, 2 4 = 16 stages
The capacitance value can be selected with, and the current-voltage
The exchange rate can be obtained.

The current-voltage conversion circuit according to this embodiment is
It can be easily integrated into an IC using the CMOS / IC technology. As shown in FIG. 3, the analog switches S1 and S2 are composed of an n-channel MOS transistor QN and a p-channel MOS transistor QP and an inverter I0 connected in parallel. Can be formed into a silicon chip by the process of. If you use a resistor instead of a capacitor,
It is possible to realize the same adjustment function. However, if a resistor is used, when the operational amplifier has an offset, a direct current flows through the resistor connected to the ground terminal corresponding to the third capacitor C3, and the output voltage when there is no input current depends on the resistance value. Occurs. In this embodiment using a capacitor, such a wasteful direct current does not flow.

Next, an embodiment of a photoelectric encoder in which the above-mentioned current-voltage conversion circuit is mounted on a detection head together with a photodiode will be described. FIG. 4 shows the overall configuration of the photoelectric encoder. A main scale 1 and an index scale 2 each having an optical grating formed thereon are arranged to face each other so as to be relatively movable, as indicated by arrows. A light emitting element 3 for radiating parallel light and a concave mirror 4 for collimation are arranged on one side of these scale parts, and a photodiode 5 and a signal processing IC 6 for processing the output signal thereof are arranged on the other side. The light emitting element 3, the concave mirror 4, the photodiode 5 and the signal processing IC 6 are integrated with the index scale 2 to form a detection head 7, as indicated by the alternate long and short dash line. The external view of the relationship between the detection head 7 and the main scale 1 is as shown in FIG.

FIG. 5 shows a photodiode 5 (PDa, P
Db) and an equivalent circuit showing a specific configuration of the signal processing IC 6 portion. The detection head has signals φ A, / A having opposite phases which repeatedly change sinusoidally according to the displacement of the scale.
Two photodiodes PDa for detecting φA,
A signal processing circuit is mounted together with PDb. Actually, a similar signal detection circuit is provided for signals φB and / φB whose phases are different from those of signals φA and / φA by 90 °. The current-voltage conversion circuits 11a and 11b that convert the output currents of the photodiodes PDa and PDb into voltages are the same as those described above with reference to FIG. Therefore, the same reference numerals as those in FIG. 1 are used for the portions corresponding to those in FIG. 1, and the suffixes a and b are attached to the two systems. At the output terminals of the current-voltage conversion circuits 11a and 11b, analog switches,
Sample and hold circuits 12a and 12b configured using capacitors and operational amplifiers are provided. A differential amplifier 13 including an operational amplifier and a resistor is provided to take the difference between the signals passing through the sample hold circuits 12a and 12b. Current-voltage conversion circuit 11
The adjustment logic circuit 14 for digitally variably controlling the third capacitors C3a and C3b of a and 11b is also mounted on the signal processing IC 6.

As described above, according to this embodiment, it is possible to obtain a photoelectric encoder having a small detection head in which a photodiode having excellent high-speed response and a signal processing circuit including a current-voltage conversion circuit are integrated. Unlike the conventional one in which the signal processing circuit is provided outside the detection head, in this embodiment, the connection between the photodiode and the current-voltage conversion circuit is short, so that it has excellent resistance to external noise.
Further, the detection head is also equipped with a level adjusting function corresponding to a conventional volume, and the balance can be easily adjusted by an external digital signal.

In the photoelectric encoder of the embodiment,
A specific circuit method of the portion shown as the adjustment logic circuit 14, that is, the third of the current-voltage conversion circuits 11a and 11b.
As a specific balance adjusting method for fixing the control level of the internal switches of the capacitors C3a and C3b, the wiring of the substrate on which the IC chip of the detection head is mounted is selectively cut, and the data is added to this by adding an EEPROM. It is conceivable to write in, or perform an automatic level adjustment sequence before using the measuring instrument.

The method of (1) uses a laser trimming technique which is often used in a thin film circuit using a ceramic substrate.
The H and L levels of the control signals b1, b2, b3, b4 described in 1 above are determined, and the wiring is trimmed based on these.
Of the control signals b1, b2, b3, b4.
In this method, the level is given from the EEPROM. EEPR
In the OM, the adjustment value calculated in the same manner as is written as data. In this method, readjustment can be performed when the level shifts due to changes over time. Method incorporates a current-voltage conversion circuit and a feedback circuit that monitors the direct current level and brings it close to 0. By utilizing this circuit before using the measuring instrument, H of control signals b1, b2, b3, b4, L
Determine the level. If the above method is used together, the adjustment value is retained even when the power is turned off, so there is no need to worry about turning the power on or off.

Although the transmissive photoelectric encoder is shown in the embodiment of FIG. 4, the present invention is not limited to this, and the present invention can be similarly applied to a reflective photoelectric encoder. . Further, as the collimating means for the irradiation light, a lens can be used instead of the concave mirror.

[0024]

As described above, according to the present invention, an equivalent feedback resistance is realized by using a switched capacitor, and a current-voltage conversion circuit capable of adjusting a DC level by a control signal can be obtained. it can. In addition to the above-mentioned current-
By integrating the voltage conversion circuit in the detection head,
It is possible to obtain a photoelectric encoder as a small-sized detection head, which has a DC level adjusting function.

[Brief description of drawings]

FIG. 1 shows a current-voltage conversion circuit according to an embodiment of the present invention.

FIG. 2 shows a configuration example of a third capacitor of the same embodiment.

FIG. 3 shows a configuration of an analog switch of the same embodiment.

FIG. 4 shows an overall configuration of a photoelectric encoder according to an embodiment of the present invention.

FIG. 5 is a photodiode and signal processing I of the same embodiment.
The structure of C is shown.

FIG. 6 shows an appearance of the photoelectric encoder of the same embodiment.

[Explanation of symbols]

OP ... Operational amplifier, C1 ... 1st capacitor, C2 ... 2nd capacitor, C3 ... 3rd capacitor, S1, S2
... Analog switch, I ... Inverter, 1 ... Main scale, 2 ... Index scale, 3 ... Light emitting element, 4 ...
Concave mirror, 5 ... photodiode, 6 ... signal processing IC, 7
... Detection heads, 11a, 11b ... Current-voltage conversion circuit,
12a, 12b ... Sample and hold circuits, 13 ... Differential amplifiers, 14 ... Adjustment logic circuits.

Claims (2)

[Claims] 1. An operational amplifier having a non-inverting input terminal set to a reference potential, and a first analog switch provided between the inverting input terminal and the signal input terminal of the operational amplifier and driven by a clock of a predetermined frequency. And first and second capacitors connected in series between the inverting input terminal and the output terminal of the operational amplifier, and provided between the connection node of the first and second capacitors and the reference potential terminal for control. A third capacitor whose capacitance can be varied by a signal; and a second analog switch connected between the inverting input terminal and the output terminal of the operational amplifier and driven by a clock having a phase opposite to that of the first analog switch. And a current-voltage conversion circuit. 2. A first scale and a second scale, each of which has a graduation made of an optical grating and is arranged so as to be movable relative to each other, a means for irradiating these scale portions with parallel light, and the first scale and the second scale. In a photoelectric encoder having a detection means for detecting light and darkness caused by repetition of overlapping of two scales, the detection means is provided in a detection head including one of the first scale or the second scale and a means for irradiating the parallel light. A photodiode and a current-voltage conversion circuit that converts an output current of the photodiode into a voltage are integrated, and the current-voltage conversion circuit includes an operational amplifier whose non-inverting input terminal is set to a reference potential, and A first analog switch provided between the inverting input terminal and the signal input terminal of the operational amplifier and driven by a clock of a predetermined frequency; A first and a second capacitor connected in series between the inverting input terminal and the output terminal of the operational amplifier, and a first capacitor provided between a connection node of these capacitors and a reference potential end, the capacitance of which can be varied by a control signal. The first capacitor connected between the capacitor 3 and the inverting input terminal and the output terminal of the operational amplifier.
And a second analog switch driven by a clock having an opposite phase to the analog switch of FIG.