JPH03181212A - Offset compensating circuit - Google Patents

Abstract

PURPOSE:To control the DC offset voltage of output with an area that is economically realizable by providing a DC reference power supply to the input terminal of an operational amplifier to supply the DC voltage for offset control. CONSTITUTION:When a circuit consisting of the operational amplifier 8 has an offset, the input voltage value to be applied to the amplifier 8 from a DC reference power supply 1 is controlled in accordance with the offset output value. In such as case, the power supply 1 consists of a voltage dividing circuit consisting of a trimming controllable resistance, etc., and the output of the power supply 1 is changed in accordance with the offset. Thus the offset is controlled. Thus it is possible to control the DC offset voltage being an output regardless of the type of factor with an area that is economically realizable.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an operational amplifier (hereinafter referred to as an operational amplifier).
The present invention relates to an offset compensation circuit for a circuit using a rational amplifier, and particularly to an offset compensation circuit suitable for a switched capacitor (hereinafter referred to as SC) filter circuit.

[Conventional Wiring Technique] Since the characteristic t' of many active elements such as transistors is sensitive to temperature, the DC level of the output fluctuates as the element temperature fluctuates. The DC component that appears at the output terminal of the amplification ω path during unmanned operation is called offset.

For example, in an ideal operational amplifier, if the input voltage is zero, the output will also be zero. However, in an actual operational amplifier, even if the three forces are zero, the output does not completely become zero, and a DC voltage may be generated at the output terminal. In such a case, in order to make the output DC zero, it is necessary to apply a DC voltage to the input terminal f in advance. The DC voltage to be applied to this input is called the scar offset voltage.

Operational amplifier was originally the name for a high-gain amplifier for analog computer calculations, but with the development of monolithic integrated circuit operational amplifiers, its range of applications rapidly expanded as an extremely flexible and powerful circuit element, and it became the most popular. This has led to the development of linear integrated circuits.

FIG. 5 is a circuit diagram showing a method for adjusting the offset of an operational amplifier.

Resistors R,,R,, variable resistor R0 and capacitor C1
This is a practical and simple adjustment method.

If there is an offset at the output terminal (○tJT), the output offset voltage is suppressed by changing the resistance value of the variable resistor R and applying an appropriate input offset voltage.

Such operational amplifiers are described in Nobuo Fujii, "Design of Operational Amplifier Circuits" (published by Domori Publishing Co., Ltd.), pages 60 and PP, 66-72.

Currently, there are various types of circuits that utilize this operational amplifier. For example, it is an active filter or an SC filter circuit.

A filter is a transmission circuit network having frequency selectivity that allows only power in a desired frequency band to pass and does not allow power in other frequency bands to pass.

Depending on the required characteristics, filters can be wide-area filters,
It can be divided into low-pass filters, bandpass filters, bandstop filters, etc.

A filter usually consists of a coil (L), a capacitor (C),
It is composed of a resistor (R), but the coil is large in shape and uses an iron core etc. especially at low frequencies, so
It is difficult to realize a coil with good characteristics. Another major drawback of LCR filters is that coils are difficult to integrate into integrated circuits (ICs).It is possible to configure a filter using only resistors and capacitors without using coils, but compared to LCR filters, Due to its poor characteristics, it is not used much except in special cases.

The so-called active filter, in which active elements (transistors, operational amplifiers, etc.) are added to this RC filter, is
Not only can characteristics identical to those of the R filter be obtained, but also characteristics that are difficult to achieve with an LCR filter can be constructed with an active filter.

On the other hand, there is an SC filter that uses a capacitor and an operational amplifier to achieve desired filter characteristics. Regarding SC filter circuits, please refer to the PP of the "Electronic Information and Communication Handbook" edited by the Institute of Electronics, Information and Communication Engineers.

271-274.

The SC circuit is composed of a capacitor, a switch that opens and closes periodically, and a voltage-controlled voltage source.

Voltage controlled voltage sources are typically implemented with operational amplifiers. Although the response of an actual SC circuit is a transient response, by imposing a condition that the switch is opened and closed only after the circuit response reaches a steady state, analysis focusing only on the steady state is possible. This is equivalent to the resistance being O and the charge moving instantaneously.

FIG. 6 is a circuit diagram showing the configuration of a CMOS type analog switch in an SC integrated circuit.

Terminals A and B are connected by nMOS and pMOS every half cycle according to the phase of the control clock.

The information transferred to the next state by opening and closing the switch is:
It is the charge, or equivalent voltage, that was stored on the capacitor just before the switch opened.

The signal that controls the opening and closing of a switch is called a clock. The switch opens and closes at high level (1) and low level (0) of the clock. The state of the clock is (φ,,φ,)(1,O),
When there are two types (o, +), it is called a two-phase clock or a two-phase SC circuit. The clock waveform is provided with an interval of (φ1.φ,)=(0,0) in consideration of the response time of the switch.

In addition, the SC integrated circuit, which is an integrated circuit of the SC filter, is
An analog filter circuit is constructed using ideal switches and capacitors (
capacitance) and an operational amplifier, the filter constant is determined only by the relative ratio of the capacitors and the period of the switch drive pulse, and the filter constant is several hundred nanometers.
Since it is possible to realize a time constant over a wide range from seconds to several milliseconds, it is a circuit system suitable for integrating filter functions.

In such a general SC circuit, a DC offset occurs at the output terminal even when there is no input at the input terminal.

As a device that compensates for such an offset, there is an automatic offset compensation circuit J based on a switched capacitor filter, which is described in Japanese Patent Laid-Open No. 61-19-6612. By providing a low-pass filter (offset voltage detection circuit) and an adder circuit, the extracted offset voltage and the output of the filter are combined in a forward or backward direction, thereby reducing the initial value deviation and fluctuation of the element. This is to reduce the offset voltage due to the influence of the offset voltage.

Also, rMicroelectron J,,
13. + G described in PP37-40 of 982 J
regorian R, author rAn 0fTset
-free 5w1tced Capacitor
BiquadJ describes a method for reducing the output offset of an SC filter due to the input offset of an operational amplifier. .

In addition, in Nobuo Fujii's "Design of Operational Amplifier Circuits" (published by Domori Publishing Co., Ltd.), P. 60 and P. P. 66-72, there is a method for adjusting the offset of a single operational amplifier as shown in Figure 5. is listed.

[Problems to be Solved by the Invention] However, in the SC filter circuit, even when there is no DC offset at the input terminal, a DC offset occurs at the output terminal. This is mainly due to the fact that the input offset voltage of the operational amplifier and the clock field-through component via the switch used in the SC circuit are on the input side of the operational amplifier. The magnitude of the DC offset voltage generated at the output terminal at this time varies greatly depending on the size of the switch and the size of the SC.

As a conventional offset compensation circuit for an SC filter, there is one described in, for example, Japanese Patent Laid-Open No. 196612/1983. This is done by passing the output signal through a low-pass filter with a very large time constant to extract the DC offset component, and then feeding it back and synthesizing it.
This reduces the DC offset voltage of the output. However, in order to create such a low-pass filter with a very large time constant, a large area is required and integration is difficult.

In addition, the circuit configurations shown in rM 1croelectron J and Nobuo Fujii's ``Design of Operational Amplifier Circuits'' are effective in reducing the output offset voltage caused by the input offset of the operational amplifier, but the output of the switched capacitor circuit is Output offset voltages caused by other causes, including clock field-through, which is another major cause of offset, cannot be reduced.

An object of the present invention is to provide an offset compensation circuit that solves these problems of the prior art and makes it possible to adjust the output DC offset voltage regardless of the type of cause in an economically feasible area. That's true.

[Means for Solving the Problems 1] In order to achieve the above object, the offset compensation circuit of the present invention is characterized in that: (1) a DC reference power supply that supplies a DC voltage for offset adjustment to the input terminal of the operational amplifier is provided. shall be.

(2) In the offset compensation circuit described in <1) above, the DC reference power supply is configured with a voltage divider circuit using a trimming-adjustable resistor, and the DC voltage for offset adjustment is supplied by adjusting the voltage divider circuit. It is characterized by

(3) In the offset compensation circuit described in (1) above, the circuit network is a switched capacitor filter circuit, and is a DC based? It is characterized in that an input switched capacitor is provided to the FIffl source, and a DC voltage for offset adjustment is supplied via this input switched capacitor.

(4) In the offset compensation circuit described in (3) above, the switched capacitor filter circuit is constituted by a semiconductor integrated circuit, and the DC reference power supply is constituted by a voltage divider circuit using a trimming-adjustable resistor, and this DC The reference power supply is characterized in that it is provided within the semiconductor integrated circuit or outside the semiconductor integrated circuit.

(5) In the offset compensation circuit described in (3) above, the manually operated switched capacitor is configured based on a computer program or by trimming the capacitive element.

[Effect]

In the present invention, when there is an offset in a circuit consisting of an operational amplifier, the value of the input type 1r from the DC reference power source to the operational amplifier is adjusted in accordance with the offset output value. In this case, the DC reference power source is configured with a voltage dividing circuit including a trimmable and adjustable resistor, and the output of the DC reference power source is changed in accordance with the offset to adjust the offset.

Furthermore, for offset in the switched capacitor filter circuit, an input switched capacitor is added to the DC reference power supply, and a DC voltage for offset adjustment is supplied via this manually operated switched capacitor. In this case, the DC base* can be adjusted by adjusting the output value of the source, or the input switched capacitor can be configured based on a computer program, or a capacitive element can be used. The offset is adjusted by changing the capacitance of this input switched capacitor and changing the input value to the operational amplifier.

[Example 1 Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of an offset compensation circuit according to the present invention.

The base voltage source l (hereinafter referred to as V(rer)1) of the present invention is added to the primary low-pass filter 0 using a general SC circuit.
is input via the input SC5. That is, the first-order low-pass filter 9 connects the operational amplifier 8 with SC.
4, 6, and a capacitor C, and are connected to an external circuit via an input terminal 11 and an output terminal 12.

Then, V for compensating the DC offset voltage of the output terminal 12.
(ref) l is switch S, and this switch S
It is input to the operational amplifier 8 via a switch S8 having a phase opposite to that of the input signal SC5, and an input SC5. Also, SC・1 and SC6
, respectively, are provided with switches S and S, similarly to the human-powered SC5.

An analog signal is input to an input terminal 11 of the primary low-pass filter 9 . This analog signal is passed through a switched capacitor 4 into two phases (φ, .

The switches S1 and Sl controlled by the clock of φ, )
The signal is input to the operational amplifier 8 at the timing of “1” and “O”, is amplified by the operational amplifier 8, and is output to the output terminal 12.

SC6 and capacitor C0 form a negative feedback circuit for operational amplifier 8, and reduce the amplification factor of operational amplifier 8 to stabilize its operation.

This first-order low-pass filter 9 has a relative ratio of capacitors,
and has a filter constant determined only by the period of the switch drive pulse. Based on this filter constant, only frequencies below a certain frequency are amplified and output.

However, in this primary low-pass filter 9, even when there is no input to the input terminal 11, a DC offset normally occurs at the output terminal 12.

This is mainly due to the fact that the manual offset voltage of the operational amplifier 8 and the clock twist-through component via the switches S1 and S are on the inverting input side of the operational amplifier 8. Further, the magnitude of the DC offset voltage generated at the output terminal 12 varies greatly depending on the sizes of the switches S and Sl, and the sizes of SC4 and SC6 and capacitor C0.

V(ref)l can change its output. Then, depending on the magnitude of the DC offset voltage at the output terminal I2 of the first-order low filter 9, the basis of V[ref)l?
By adjusting the F'fi pressure value, the DC offset voltage value of the output terminal 12 can be adjusted without changing the gain-frequency characteristics.

At this time, even if an output offset voltage is required, the reference voltage value of V(ref)I is adjusted according to the magnitude of the DC offset voltage, so that an arbitrary value can be obtained.

FIG. 2 is an explanatory diagram showing the gain-frequency characteristics of the first-order low-pass filter in FIG. 1.

The operational amplifier 8 in FIG. 1 is
It has high gain and also has a feedback circuit added. Therefore, it is very easy to cause oscillation.

The operational amplifier 8 shown in FIG. 1 is required to have frequency characteristics that do not cause such oscillation.

FIG. 3 is a circuit diagram showing the configuration of V(ref) in FIG. 1 using a resistor.

Figure 3(a) shows how V(ref) can be calculated using resistors R and R4.
1. This resistance R3 is
) is obtained from the resistances R, , R, , ~Rn. In other words, prepare unit resistors RIll Ram to Rn in parallel, and set them to match the desired offset value.
It is obtained by appropriately cutting the resistance from R1 to Rn.

At this time, by connecting an offset measuring device and a laser cutter using software that has been programmed to calculate the correspondence between the desired offset value and the cutting location, it is possible to automatically measure the offset value and cut the resistance.

When the SC circuit in Figure 1 is realized as an integrated circuit, it is possible to match the V(ref)I circuit as such a resistive voltage divider circuit either inside or outside the integrated circuit. It is.

FIG. 4 is a circuit diagram showing the configuration of the V(ref) transistor in FIG. 1.

FIG. 4(a) shows transistors T with different threshold voltages.
In order to change the input to the operational amplifier 8 in FIG. 1, the capacitance shown in FIG. 4(b) is trimmed, that is, unit capacitors C, , C ,~ Prepare Cn in parallel, and set the desired offset (to match directly, C4□
It can be obtained by appropriately cutting a capacitor from Cn to Cn.

At this time, the offset value can be measured and adjusted by connecting the offset measuring device and the laser cutter using software programmed with calculations that correspond to the desired offset value and the cutting location, similar to what was explained in Figure 3. can be done automatically. When the SC circuit shown in FIG. 1 is realized as an integrated circuit, it is possible to fit it either inside the integrated circuit or outside the integrated circuit as a capacitor that can be tuned.

In addition, when reducing the DC offset voltage value by changing SC5 without adjusting the value of V(ref)l, it is possible to program from outside the integrated circuit using a computer etc. without using SC5 as a capacitive element. By adopting such a configuration, the value of the capacitance (SC5) may be adjusted according to the DC offset voltage value of the output 5 (out) of the filter 9.

The configuration of V (ref) in FIG. 1 is as shown in FIG.
And, the configuration is not limited to that shown in FIG.
Any method that can obtain similar results is fine.

As described above, according to this embodiment, the offset voltage of the circuit is adjusted by inputting the DC reference power supply, so the SC
A filter can be realized. The offset voltage can then be adjusted without changing the frequency response of the circuit.

Furthermore, if the DC reference power source is configured with a voltage dividing circuit using resistors, it is possible to create a DC reference power source with an arbitrary value that is less susceptible to temperature changes.

Furthermore, when the SC filter circuit is integrated with a semiconductor integrated circuit, the offset compensation circuit can be formed on the same chip, so that the entire system can be miniaturized.

Then, even if the value of the DC base (IT) power source is fixed, the input SC filter can be configured to be trimmable, or the input SC filter can be configured to be programmable from the suburban area. , the offset voltage can be adjusted by adjusting the capacitance ff1 (SC) i1α according to the output offset voltage value.

In addition, an offset compensation circuit is provided in addition to the semi-dedicated integrated circuit.
Trimable resistance for assembly) Els 1
Since the circuit is external, the offset voltage can be adjusted by trimming the resistor even after the semi-integrated circuit is packaged.

[Effects of the Invention] According to the present invention, it is possible to adjust the output DC offset voltage in an economically feasible area, regardless of the cause.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an offset compensation circuit according to the present invention, FIG. 2 is an explanatory diagram showing the gain-frequency characteristics of the first-order low-pass filter in FIG. 1, and FIG. 4 is a circuit diagram showing the configuration of V(ref) in FIG. 1 using a transistor. FIG. FIG. 6 is a circuit diagram showing the configuration of a CMO8 type analog switch in an SC integrated circuit. ]: V (rcf), 4: SC, 5 manual SC, 6
, :SG, 8: Operational amplifier, 9. First-order low-pass filter, 11. Input terminal, 12. Output terminal, C0, C capacitor, C4,, C,, ~Cn: Unit capacitor. Ro: variable resistance, R1 to R9: resistance, R, , R,,
~Rn: 141st resistance, S,, S, near) -0 switch. Trl, Tr2: Transistor. ” Figure Frequency Figure 3 Figure (a) Figure (a)

Claims (5)

[Claims] (1) In an offset compensation circuit that adjusts the offset generated in a circuit network consisting of an operational amplifier connected to an active element and a passive element, a DC voltage corresponding to the offset is supplied to the input terminal of the operational amplifier. An offset compensation circuit characterized by being provided with a DC reference power supply. (2) In the offset compensation circuit according to claim 1,
The offset compensation circuit is characterized in that the DC reference power supply is configured with a voltage dividing circuit using resistors, and the DC voltage corresponding to the offset is supplied by trimming the resistors. (3) In the offset compensation circuit according to claim 1,
The offset compensation circuit is characterized in that the circuit network is a switched capacitor filter circuit, and an input switched capacitor is provided to the DC reference power supply, and a DC voltage corresponding to the offset is supplied via the input switched capacitor. . (4) In the offset compensation circuit according to claim 3,
An offset compensation circuit characterized in that the switched capacitor filter circuit is constituted by a semiconductor integrated circuit, and the DC reference power supply according to claim 2 is provided inside or outside the semiconductor integrated circuit. (5) In the offset compensation circuit according to claim 3,
An offset compensation circuit characterized in that the input switched capacitor is configured based on a computer program or configured by trimming a capacitive element.