JPH0720049B2 - Offset compensation circuit for switched capacitor filter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switch capacitor filter, and more particularly to an offset automatic compensating circuit for automatically compensating an offset voltage generated in the switch capacitor filter.

[Background of the Invention]

2. Description of the Related Art In recent years, due to advances in microfabrication technology and MOS analog technology, a switchable capacitor filter that can realize a monolithic IC has attracted attention. The switch capacitor filter has a problem that a large offset voltage is generated due to spike noise and the like accompanying the switching operation of the switching element, and a wide input range cannot be secured.

As a method for solving the above problems, a subtraction circuit, a determination circuit, an attenuation circuit, an integration circuit, etc. are provided in the subsequent stage of the filter,
The offset amount is estimated by extracting the difference (error voltage) between the input and output voltages of the decision circuit, integrating it through the attenuation circuit, and then integrating it with the integrating circuit. By subtracting the offset amount from the output signal of the filter. There is a method for obtaining a signal without offset (Japanese Patent Laid-Open No. 58-3308).

In this method, the algorithm for offset compensation is complicated, and the determination circuit, the subtraction circuit, and the attenuation circuit for extracting the error voltage need to be highly accurate.
Therefore, there is a drawback that each element must be highly accurate. That is, there is a drawback that it is affected by the deviation and fluctuation of the initial value of the element, and it is not suitable for LSI implementation. Further, when the offset erasing circuit is integrated, the resistors and capacitors are formed with high accuracy on the semiconductor chip, so that 1) it is difficult to set the resistance value, and 2) a large chip area is required.
Furthermore, a large number of circuits having subtraction, judgment and attenuation functions for offset removal are required, which is not sufficient as an LSI circuit.

[Object of the Invention]

An object of the present invention is to reduce the offset voltage in a switch capacitor filter, which is not easily affected by the initial value deviation, fluctuation and variation of the element, and which can automatically reduce the offset voltage with a simple circuit configuration and a rough offset detection circuit. It is to propose a suitable offset voltage automatic compensation circuit.

[Outline of Invention]

In order to achieve the above object, the automatic compensation circuit of the switch capacitor filter according to the present invention is provided with a low-pass filter (offset voltage detection circuit) whose summing frequency is sufficiently lower than the signal component and an adding circuit, By combining the offset voltage and the output of the filter in the forward or backward direction, the offset voltage is less affected by the initial value deviation and fluctuation of the device, and the offset voltage can be reduced.

The following means can be mentioned as an embodiment for embodying such an invention.

(1) The output of the switch capacitor filter is connected to an adder circuit, the output of this adder circuit is connected to an offset voltage detection circuit with a gain of n times, and the output of the offset voltage detection circuit is connected to the adder circuit (feedback). )
By doing so, it is combined with the output of the switch capacitor filter to obtain an output voltage whose offset voltage is automatically reduced.

(2) Connect the output of the switch capacitor filter to the offset voltage detection circuit with a gain of 1.
By connecting the output of the offset voltage detecting circuit and the switch capacitor filter to the adder circuit, the offset voltage is automatically subtracted, and the output voltage with the offset voltage reduced is obtained.

(3) Connect the output of the switch capacitor filter to the offset voltage detection circuit whose gain is n times.
By connecting the output of the offset voltage detection circuit to the + side terminal of the operational amplifier of the switch capacitor, it is combined with the output of the switch capacitor filter so that the offset voltage is automatically reduced. To do. In the methods (1) and (3), the offset voltage detection circuit is provided backward and the method (2) is provided forward.

Example of Invention

An embodiment of an automatic offset voltage compensation circuit for a switch capacitor filter according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. As shown in FIG. 1, a well-known switch switch 1, adder circuit is known.
2, composed of an offset voltage detection circuit 3.

The adder circuit 2 shown in FIG. 1 is composed of switch equivalent capacitors 4,5 and 6, and an operational amplifier 7, and the offset voltage detection circuit 3 is equivalent to the switch capacitor equivalent resistors 8 and 9.
It is composed of a capacitor 10 and an operational amplifier 11.

Next, in FIG. 1, detailed circuits of the switch capacitor equivalent resistors 4, 5 and 6 which constitute the adder circuit 2 are shown in FIGS. 2 (a) and 2 (b). In FIGS. 2A and 2B, the analog switches 12, 13, 19 and 20 are
ON when the clock signal in Fig. 3 (a) is at "H" level
Then, it turns off at the “L” level. Further, the analog switches 14, 15, 17 and 18 are turned on when the clock signal of FIG. 3 (b) is at "H" level and turned off when it is at "L" level. By repeating these series of operations at the clock frequency s, the equivalent resistance R shown in the following equation is realized.

The equivalent switch resistance in Fig. 2 (a) is the terminal
Be sure to connect the capacitor C ₁ or C ₁ ′ between A and B.
Further, the switch equivalent capacitor of FIG. 2B has a characteristic of inverting the polarity of the voltage of the input terminal A and transmitting it to the terminal B, that is, a characteristic as an inverter.
Therefore, the switch equivalent capacitor of FIGS. 2A and 2B is used. In the adder circuit 2, since the − input terminal and the output terminal are not opened, the output is not saturated, and
One of the inputs is an adder circuit having a subtraction function because the polarity is inverted.

Next, FIG. 2 (c) shows a detailed circuit of the switch capacitor equivalent resistors 8 and 9 constituting the offset voltage detecting circuit 3 in FIG. In FIG. 2 (c), the analog switches 22 and 23 are turned on when the clock signal of FIG. 3 (a) is at "H" level, and turned off when it is at "L" level. The analog switches 24 and 25 are shown in Fig. 3 (b).
Turns on when the clock signal of is at "H" level, and turns off when it is at "L" level. By repeating a series of these operations at the clock frequency s, the equivalent resistance R on the input side shown by the following equation is obtained.
And the feedback side equivalent resistance R ₁ can be realized.

In addition, the switch equivalent resistances R and R ',
The time constant τ, the gain coefficient K and the transfer function of the offset voltage detection circuit (3 in FIG. 1) formed by the integrating capacitor C ₃ (10 in FIG. 1) are shown in the following equations.

FIG. 4 (a) shows an example of 1-gain-frequency characteristics of the switch capacitor transfer filter shown in FIG. 1 in which the center frequency is o.

FIG. 4B shows an example of gain-frequency characteristics of the offset voltage detection circuit shown in FIG.

The offset voltage detection circuit detects a direct current component, which is the offset voltage, and inverts and adds it to the adder circuit. Therefore, as shown in FIG. 4 (b), the gain and the phase error are made extremely small. Must be sufficiently attenuated. Therefore, the offset voltage detection circuit is
From equation (6), a large time constant is required.

Next, the operation of the present invention will be described.

In FIG. 1, a switch 1
When the sine wave power source Vi shown in FIG. 5 (a) is input, the stepped waveform Vo ₁ shown in FIG. 5 (b) including the offset voltage Ve ₁ due to the offset and switching noise of the operational amplifier is included.
Is output. By inputting the output voltage Vo ₁ and the output voltage Vo _F of the offset voltage detection circuit 3 having a gain of 1 to the adder circuit, the offset voltage is halved and Ve ₂ is reduced as shown in FIG. 5 (c). Then the stepped waveform Vo ₂ is output.

Further, in FIG. 1, when the gain of the offset voltage detection circuit 3 is increased by a factor of 10, for example, the output of the adder circuit 2 has a sufficiently small offset voltage as shown in FIG. 5 (d).
The reduced staircase waveform Vo ₂ ′ is output to Ve ₂ ′.

That is, in FIG. 1, the offset voltage Ve is
As shown in the figure, it can be reduced as the gain of the offset voltage detection circuit 3 is increased.

As described above, in the circuit shown in FIG. 1, by sufficiently increasing the time constant and the gain of the offset voltage detection circuit 3, the offset automatic compensation of the switch capacitor filter 1 becomes possible. Further, it goes without saying that the offset voltage generated in the adder circuit 2 and the offset voltage detection circuit 3 can be combined and compensated.

In FIG. 1, it goes without saying that the adder circuit 2 and the offset voltage detection circuit 3 can be constituted by a conventional resistance element as a switch capacitor equivalent resistance. Further, in the case of a monolithic IC, the adder circuit 2 can realize the calculation with high accuracy because the calculation can be realized by the capacitance ratio of the switch equivalent capacitors of the input side and the feedback side. The offset voltage detection circuit 3 does not need to be highly accurate, and the time constant τ
Also, as long as the gain is large, it is possible to provide a large capacitance integrating capacitor 10 outside the IC and configure a switch capacitor equivalent resistance with a conventional resistance element (diffusion resistance, etc.) in a sufficiently small area. realizable.

Next, FIG. 7 shows a second embodiment of the present invention.

As shown in FIG. 7, switch switch 1,
It is composed of an adder circuit 2 and an offset voltage detection circuit 30.

The adder circuit 2 shown in FIG. 7 is the adder circuit described in FIG. 1, and is equivalent to the switch capacitor equivalent resistances 4,5 and 6,
It is composed of an operational amplifier 7. The offset voltage detection circuit 30 shown in FIG. 7 comprises a switch capacitor equivalent resistance 27, a capacitor 29, and an operational amplifier 28.

Next, in FIG. 7, the detailed circuit of the switch capacitor equivalent resistance 27 which constitutes the offset voltage detection circuit 30 is shown in FIG.
It is shown in FIG. Analog switch in FIG. 2 (c)
22 and 23 are turned on when the clock signal of FIG. 3 (a) is at "H" level, and turned off when it is at "L" level. Further, the analog switches 24 and 25 are turned on when the clock signal of FIG. 3 (b) is at "H" level, and turned off when it is at "L" level. By repeating these series of operations at the clock frequency s, the equivalent switch resistance of the switch capacitor shown in the following equation is realized.

In addition, in FIG.
The time constant τ and transfer function of the offset voltage detection circuit composed of R and the integrating capacitor C ₄ are shown in the following equation.

FIG. 4 (b) shows an offset voltage detection circuit shown in FIG.
An example of gain-frequency characteristics of 30 is shown. In this case the operational amplifier
Since 28 is a voltage follower type, the gain coefficient K is 1. In FIG. 7, an offset voltage detection circuit
30 detects the direct current component, which is the offset voltage, and directly
Since the polarity is inverted and added to the adder circuit, it is necessary to sufficiently attenuate the signal frequency region in order to make the gain and the phase error extremely small as shown in FIG. 4 (b). Therefore, the offset voltage detection circuit is
The need for a large time constant is exactly the same as in the embodiment of FIG.

Next, the operation of the present invention will be described.

As shown in FIG. 7, the switch 1
When the sine wave voltage Vi shown in FIG. 8 (a) is input, the staircase waveform Vo ₁ shown in FIG. 8 (b) containing the offset voltage Ve ₁ due to the offset and switching noise of the operational amplifier is output. By inputting this output voltage Vo ₁ to the offset detection circuit 30, the offset voltage Ve ₁ can be detected as shown in FIG. 8 (c). Further, the output voltage Vo ₁ of the switch capacitor filter ₁ and the output voltage e ₁ of the offset voltage detection circuit are input to the adder circuit 2. Slave connexion addition circuit 2, as shown in FIG. 8 (d), including a slight offset voltage Ve ₃ by itself occurs, the output voltage Vo ₃ obtained by subtracting the offset voltage from the signal components obtained, offset voltage automatic compensation it can.

The present invention is characterized in that the offset voltage detection circuit 30 has a gain coefficient of 1 in order to faithfully compensate for the offset voltage generated in the switch capacitor 1.

In FIG. 7, it goes without saying that the adder circuit 2 and the offset voltage detection circuit 30 can be formed by a conventional resistance element as a switch capacitor equivalent resistance.

Further, in the case of a monolithic IC, as in the case of FIG. 1, the adder circuit 2 can realize a high precision because the calculation can be realized by increasing the capacitance of the switch equivalent capacitor. The offset voltage detection circuit does not need to be highly accurate, as long as the gain is 1 and the time constant τ is sufficiently large, a large-capacity integrating capacitor is provided outside the IC, and a switch capacitor equivalent resistance is conventionally used. It is also possible to use a resistance element (diffusion resistance or the like) of (3), which can be realized in a sufficiently small area.

FIG. 9 shows a third embodiment of the present invention. As shown in FIG. 9, it is composed of a switch capacitor band pass filter 39 and an offset voltage detecting circuit 3.

Switch pass filter shown in Fig. 9
39 is equivalent resistance of switch switch, 32, 33 and 34,
It is composed of capacitors 35 and 36 and operational amplifiers 37 and 38.

Next, in FIG. 9, detailed circuits of the switch capacitor equivalent resistances 31, 32, 33 and 34 which constitute the switch capacitor bandpass filter 39 and the offset voltage detecting circuit 3 are shown in FIGS. 2 (c) and (d). Shown in. 2 (c) and 2 (d), the analog switches 22 and 23 are turned on when the clock signal in FIG. 3 (a) is at "H" level, and "L".
Turns off at the level. Also, analog switches 24 and 25 are turned on when the clock in Fig. 3 (b) is at "H" level.
Then, it turns off at the “L” level. By repeating this series of operations at the clock frequency s, the equivalent resistance R shown in the equation (2) is realized.

In FIG. 9, the transfer function of the switch capacitor passband filter is shown in the following equation.

ωo: Shutter frequency Q: Selectivity H: Gain coefficient Next, the operation of the present invention will be described.

In FIG. 9, when the sine wave voltage Vi shown in FIG. 1 (a) is input to the switch capacitor band pass filter 39,
The staircase waveform Vo ₁ shown in FIG. 10B is output, which includes the offset voltage Ve ₁ due to the operational amplifier offset and switching noise. This output voltage is input to the offset voltage detection circuit 3 having a gain of 1, and the output voltage of the offset voltage detection circuit 3 is input to the + side input terminal of the operational amplifier 38 of the bandpass filter 39, whereby
As shown in Figure (c), the offset voltage is halved and Ve ₂
The staircase waveform Vo ₂ is output.

Further, in FIG. 9, when the gain of the offset voltage detection circuit 3 is increased by a factor of 10, for example, the output Vo of the switch capacitor bandpass filter 39 is sufficiently small as shown in FIG. 10 (d). The reduced staircase waveform Vo ₂ ′ is output to VVe ₂ ′.

That is, in FIG. 9, the offset voltage Ve is
As shown in the figure, it can be reduced as the gain of the offset voltage detection circuit 3 is increased.

As described above, in the circuit shown in FIG. 9, the offset automatic compensation of the switch capacitor band pass filter 39 is possible by making the time constant and the gain of the offset voltage detection circuit 3 sufficiently large. Further, since the offset voltage and the signal component are added by the operational amplifier 38 in the switch capacitor pass band filter 39, the adder circuit can be omitted.

In FIG. 9, the offset voltage detection circuit 3 uses a switch capacitor equivalent resistance as a conventional resistance element (diffusion resistance, etc.).
It goes without saying that it can be configured with. Further, in the case of a monolithic IC, the offset voltage detection circuit 3 does not need to be highly accurate, as long as the time constant τ and the gain are large, the integration capacitor 10 having a large capacitance is provided outside the ID, and further, This can be realized in a small area by configuring the equivalent switch resistance by a conventional resistance element (diffusion resistance or the like).

Further, not only the band-pass filter but also the low-pass filter can be automatically compensated for offset. First
Figure 12 shows a configuration example of the automatic offset voltage compensation circuit for the switch capacitor low-pass filter. In the figure, the low-pass filter is an output terminal of the second stage operational amplifier.
By connecting the output of 38 to the offset voltage detection circuit 3, and further connecting the output of the offset voltage detection circuit 3 to the + side terminal of the operational amplifier 37 of the first stage of the filter,
It can be easily understood that the offset voltage can be reduced as in the bandpass filter.

〔The invention's effect〕

As described above, according to the present invention, in the switch capacitor filter, the offset voltage of the operational amplifier and the offset voltage generated by the switching operation can be automatically compensated, and the offset voltage extraction unit can be provided with a rough accuracy. Therefore, it is possible to sufficiently deal with the deviation and fluctuation of the initial value of the element, and it is possible to greatly improve the characteristics of the switch capacitor filter. Therefore, the merit in practical use is very large.

[Brief description of drawings]

FIG. 1 is a first embodiment of an offset voltage automatic compensation circuit for a switch capacitor filter according to the present invention, and FIG. 2 is a circuit diagram showing an example of a switch capacitor equivalent resistance.
FIG. 4 is a waveform diagram showing the drive waveform of the switching element, and FIG. 4 is a gain-frequency characteristic example of the switch capacitor filter and the offset voltage detection circuit in the first embodiment.
FIG. 5 is a waveform diagram showing the waveform of each part in the first embodiment, FIG. 6 is the offset voltage with respect to the gain of the offset voltage detection circuit in the first embodiment, and FIG. 7 is the switch capacitor filter according to the present invention. The second embodiment of the offset voltage automatic compensating circuit, FIG. 8 is a waveform diagram showing the waveform of each part in the second embodiment, and FIG. 9 is the third embodiment of the offset voltage automatic compensating circuit of the switch capacitor filter according to the present invention. FIG. 10 is a waveform diagram showing the waveform of each part in the third embodiment, FIG. 11 is the offset voltage with respect to the gain of the offset detection circuit in the third embodiment, and FIG. 12 is the third embodiment. A modification of is shown.

Claims (2)

[Claims] 1. An offset compensating circuit for a switched capacitor filter for compensating an offset superimposed on an output of a switched capacitor filter, wherein an adder circuit composed of an operational amplifier and a switched capacitor equivalent resistance is connected to an output section of the switched capacitor filter. An offset voltage detecting circuit for detecting a direct current component of the adding circuit and inputting the detected value to an operational amplifier of the adding circuit, wherein the offset voltage detecting circuit outputs the output from the adding circuit to a first switched capacitor. Input to the inverting input section of the operational amplifier via the data equivalent resistance, the second switched capacitor equivalent resistance and the capacitor are connected in parallel, negative feedback is applied, and by the first switched capacitor equivalent resistance and the second switched capacitor equivalent resistance, Gain n times Offset compensation circuit of the switched capacitor filter, characterized in that. 2. An offset compensating circuit for a switched capacitor filter, comprising a bandpass filter having an operational amplifier in the switched capacitor filter and compensating an offset superimposed on an output of the switched capacitor filter, wherein a direct current component of an output part of the switched capacitor filter is provided. And an offset voltage detection circuit for inputting the detected value to a non-inverting input section of an operational amplifier of the bandpass filter, wherein the offset voltage detection circuit is equivalent to a first switched capacitor equivalent output of the switched capacitor filter. Input to the inverting input section of the operational amplifier via a resistor, the second switched capacitor equivalent resistor and the capacitor are connected in parallel, and negative feedback is applied to the first switched capacitor equivalent resistor and the second switched capacitor. Offset compensation circuit of the switched capacitor filter, characterized in that the gain n times by the valence resistance.