JPS6333003A - Offset correction circuit - 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 [Field of Industrial Application] The present invention relates to an offset correction circuit for an operational amplifier, and particularly to an offset correction circuit that can use inexpensive capacitors.

[Conventional technology]

Figure 2 is a circuit diagram of a conventional offset correction circuit. In the figure, (1) is a first operational amplifier (hereinafter referred to as an operational amplifier) with negative feedback amplification, and (2) is one end connected to an external input. and (6) has one end connected to the inverting input terminal of the first operational amplifier (6) and the other end connected to the inverting input terminal of the first operational amplifier (1). A second resistor (feedback resistor) connected to the output terminal of the operational amplifier 1, (4) is the second operational amplifier, (5)
is a third resistor (integrating resistor) whose one end is connected to the output terminal of the first operational amplifier (1) and the other end is connected to the inverting input terminal of the second operational amplifier (4); a capacitor (connected to the inverting input terminal of the second operational amplifier (4) and the other end connected to the output terminal of the second operational amplifier (4) and the non-inverting input terminal of the first operational amplifier (1);
(integrating capacitor), (7) has one end connected to the second operational amplifier (
A fourth resistor is connected to the non-inverting input terminal of 4) and has the other end connected to ground.

The conventional offset correction circuit is configured as described above, and the resistance values of the first resistor (2) to the fourth resistor (7) are respectively expressed as "2p R5p R5p R7", and the resistance value of the capacitor (6) is The capacity is expressed as C.

First, consider the case where the second operational amplifier (4), the third resistor (5), the capacitor (6), and the non-inverting input terminal of the first operational amplifier (1) are connected to ground. We will focus only on the magnitude of the offset voltage at the output terminal of the first operational amplifier (1), and consider the external input voltage Vi=,0. The offset voltage between the inverting input terminal and non-inverting input terminal of the first operational amplifier (1) is Vos+
Then, the output voltage ■0 of the first operational amplifier (1) is expressed by the following equation (1).

Vo = (j+shi) Vos+...song (1)
Next, consider the case where there is a second operational amplifier (4), a sixth resistor (5), and a capacitor (6). At this time as well, we will focus on the magnitude of the offset voltage at the output terminal of the first operational amplifier (1), and consider that the external input voltage Vi=0. The offset voltage of the first operational amplifier (1) is Vosl, the offset voltage of the second operational amplifier (4) is vos2, and the output voltage of the second operational amplifier (4) is e.
shall be. Output voltage V of the first operational amplifier (1).

Regarding, vO=(θ+Vast) (1+odd)e@・・・・・・(
2) holds, and regarding the output voltage θ of the second operational amplifier (4), e = −−(7o−Vosz) −t−1−Voez
-----・(5) C@R5 holds true. In equations (2) and (3), if sufficient time has elapsed, equations (4) and (5) hold, respectively.

When comparing equations (1) and (4), the offset voltage v
Assuming that 081 and v082 are equal, it can be seen that the offset voltage appearing at the output is compressed to R2/R3-) - R2 times, and the second operational amplifier (4) and the third resistor (5)
It is understood that the integrator circuit including the capacitor (6) and the capacitor (6) operate as an offset correction circuit. In addition, in the figure,
A fourth resistor (7) connected to the non-inverting input terminal of the second operational amplifier (4) is used for offset current correction.

[Problem that the invention seeks to solve]

In conventional offset correction circuits, an integrating capacitor (
According to equation (5), the voltage applied between both ends of 6) may cause a loss of capacity if an aluminum electrolytic capacitor or the like is used here. Therefore, there is usually a problem in that an expensive tantalum capacitor or the like must be used here.

The present invention was made to solve these problems, and an object of the present invention is to provide an offset correction circuit that can use an inexpensive aluminum electrolytic capacitor as an integrating capacitor.

[Means to solve the problem]

The offset correction circuit according to the present invention supplies the output of the second operational amplifier and the integrating circuit composed of the sixth resistor and the capacitor to the inverting input terminal of the first operational amplifier through the fifth resistor (feedback resistor). A positive or negative power supply is connected to the inverting input terminal of the first operational amplifier via a bias resistor.

[For production]

In this invention, a bias resistor connected to a positive or negative power supply and an output terminal of the second operational amplifier and the first operational amplifier are connected to each other.
A predetermined bias voltage is generated at the output terminal of the first operational amplifier by a resistor connected between the non-inverting input terminals of the first operational amplifier.

〔Example〕

FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG.
Unlike the conventional example shown in the figure, instead of directly connecting the connection point between the other end of the capacitor (6) and the output terminal of the second operational amplifier (4) to the non-inverting input terminal of the first operational amplifier (1), A fifth resistor (8) is connected between the above connection point and the non-inverting input terminal of the first operational amplifier (1), and a positive or negative power supply is connected to the non-inverting input terminal of the first operational amplifier (1). A sixth resistor (9) is connected between the two. In addition,
The resistance values of the fifth and sixth resistors (8) and (9) are respectively R
6, represented by R9. In addition, a second operational amplifier (4), a third resistor (5), a capacitor (6) and a fifth resistor (
8) constitutes a low-pass filter.

In the offset correction circuit configured as described above, we will focus only on the offset voltage generated at the output terminal of the first operational amplifier (1).
It is assumed that the voltage supplied to the resistor (2) is OV. The offset voltage between the input terminals of the first operational amplifier (1) is Vos+, the offset voltage between the input terminals of the second operational amplifier (4) is Vos2, and the output voltage of the first operational amplifier (1) is VO1O2O3 operational amplifier (4). The output voltage of e
, the voltage generated at the non-inverting input terminal of the first operational amplifier (1) is 01, and the power supply voltage is Vcc, then θ1. Vo,
Regarding e, the following equations (6) to 8) hold true.

RgVCe 10R9e ” ”' R8+R9°−−−−−− (6) R3・
・・・・・・(7) Vo = (e, +Vos+) (1+ possible) e =
VO32-(Vo-VO82)ffi-・・
...a (8) Here, t is the elapsed time.

(6) From equations (9) and (10), To and e after sufficient time have elapsed are vO = voe2, respectively, as t-+ω.
......(11) According to the result of equation (11), the offset voltage generated at the output terminal of the first operational amplifier (1) is
) is the only offset voltage. In addition, the voltage generated at the output terminal of the second operational amplifier (4) is Vcc
By taking a sufficiently large value for l and Voθ2 and appropriately combining each resistance value, it is possible to obtain an arbitrary magnitude of I, and therefore it is connected to the feedback circuit of the second operational amplifier (4). It can be seen that a sufficient bias voltage can be generated across the capacitor (6).

In the above explanation, the output of the second operational amplifier (4) is
Although we have described the case where the feedback is fed back to the non-inverting input terminal of the operational amplifier (1), it goes without saying that the same effect can be obtained when the feedback is fed back to the inverting input terminal due to the polarity of the feedback loop.

Also, although we have described the case where the sixth bias resistor (9) is connected to the non-inverting input terminal of the first operational amplifier (1), the same effect can also be obtained by connecting it to the inverting input terminal. Needless to say.

〔Effect of the invention〕

As described above, according to the present invention, two resistors are connected in series between the output terminal of the second operational amplifier and the power supply, and the connection point of these resistors is connected to the non-inverting input terminal of the first operational amplifier. By connecting a constant voltage to both ends of the capacitor that constitutes the integrating circuit, the aluminum 'I! Since the capacitance loss mode during no-load, which is characteristic of electrolytic capacitors, does not occur, an inexpensive aluminum electrolytic capacitor can be used instead of an expensive tantalum capacitor, and the offset correction circuit can be manufactured at a low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional offset correction circuit. In the figure, (1) is the first operational amplifier, and (2) is the first operational amplifier.
resistor, (5) is the second resistor, (4) is the second operational amplifier, (5) is the third resistor, (6) is the capacitor, (7)
is the fourth resistor, (8) is the fifth resistor, (9) is the sixth resistor, and (Vcc) is the power supply. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1) Consisting of a first operational amplifier with negative feedback amplification, a second operational amplifier, a resistor, and a capacitor, the output of the first operational amplifier is connected to one of the input terminals of the first operational amplifier. An offset correction circuit comprising: a low-pass filter for resistive feedback; and a bias resistor connected between the input terminal of one of the first operational amplifiers and a positive or negative power supply.