JP2000151358A - Filter device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set the cutoff frequency of a filter device with high precision. SOLUTION: Switches S1 and S2 at both the ends of a capacitor Cr of an adjusting circuit 100 are switched at a frequency fck and the capacitor functions as a dummy resistance. An operational amplifier 2 and a drive part 3 are so controlled that the voltage divided by resistances R3 and R4 becomes equal to the voltage divided by a resistance Rv2 and the dummy resistance. Consequently, the time constant obtained by a resistance Rv2 and a capacitor Cr ends up having precision based upon the precision of the frequency fck of a clock signal. The resistance Rv2 is associated with the resistance Rv, so that the time constant obtained by the resistance Rv and capacitor C of a filter circuit part 200 can be set with high precision.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter, and more particularly, to a filter having a frequency characteristic determined by a time constant of a resistor and a capacitor.

[0002]

2. Description of the Related Art Filters that pass or block electric signals according to their frequencies are used in various electric and electronic circuits and the like, and their fields of use are very wide.

In general, in a filter using an active element called an active filter, the size of the circuit is reduced and the S / N ratio is reduced.
In order to improve the ratio (Signal Noise Ratio), a circuit is configured only with a resistor, a capacitor, and an active element (for example, an operational amplifier) without using a coil.

[0004]

However, when such an active filter is formed as a monolithic IC on a semiconductor substrate, the precision (absolute precision) of elements of resistors and capacitors formed on the substrate is not so high. However, there is a problem that the cutoff frequency cannot be set accurately.

For example, in the case of the primary filter shown in FIG. 7, the cutoff frequency fc can be expressed by the following equation.

[0006]

Fc = 1 / (2π · R2 · C) (1) Here, an element value of a resistor or a capacitor formed on the semiconductor substrate has an error of about 10% with respect to a set value. It is common to have Therefore, the cutoff frequency fc determined by the value of the product of the resistance and the element value of the capacitor is also
There will be a large error depending on the variation of the element.

Incidentally, although the accuracy (absolute accuracy) of the element formed on the semiconductor substrate is not so high as described above, the relative accuracy of the element is higher than the absolute accuracy (for example,
About 1%).

In view of the above, there has been conventionally proposed a switched capacitor filter in which the transfer function of the filter is represented by the ratio of elements (mainly capacitors) using such characteristics.

In this switched capacitor filter,
By switching the charge flowing into the capacitor at a predetermined frequency fs by the semiconductor switch, a pseudo resistor is formed, and a filter circuit is formed by the pseudo resistor and a capacitor (a capacitor that is not switched).

However, since the switched capacitor filter performs a switching operation, it can be regarded as a filter discrete in the time axis direction (time discrete filter). Therefore, the maximum frequency included in the signal is fm
Then, the switching frequency fs must be set to satisfy Nyquist's theorem (to satisfy fs> 2 · fm). Therefore, there is a problem that it is difficult to handle a high-frequency signal.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a filtering device which can respond to a high frequency with high accuracy without being affected by variations in elements. I do.

[0012]

According to the present invention, in order to solve the above-mentioned problems, a filtering circuit having a frequency characteristic determined by a time constant formed by a first resistor and a first capacitor is provided. A second capacitor that functions as a pseudo-resistor by being switched in a cycle, and a second capacitor whose resistance value changes in conjunction with the first resistor.
And a control unit that controls the resistance value of the second resistor so that the ratio of the resistance value of the pseudo resistor by the second capacitor to the resistance value of the second resistor becomes constant. And a regulating circuit section having the following.

Here, the filtering circuit has a frequency characteristic determined by a time constant formed by the first resistor and the first capacitor. The adjustment circuit section includes a second capacitor that functions as a pseudo resistor by being switched at a predetermined cycle, a second resistor whose resistance value changes in conjunction with the first resistor, and a second capacitor. And a control unit that controls the resistance value of the second resistor so that the ratio of the resistance value of the pseudo resistor to the resistance value of the second resistor becomes constant.

A filtering circuit having a frequency characteristic determined by a time constant formed by the first resistor and the first capacitor, and a switching circuit which is switched at a predetermined cycle to function as a pseudo resistor. ,
A second capacitor whose capacitance value changes in conjunction with the first capacitor, a second resistor, a resistance value of a pseudo resistor by the second capacitor, and a resistance value of the second resistor And a control circuit for controlling a capacitance value of the second capacitor so that the ratio of the first and second capacitors becomes constant.

Here, the filtering circuit has a frequency characteristic determined by a time constant formed by the first resistor and the first capacitor. The adjustment circuit unit functions as a resistor in a pseudo manner by being switched at a predetermined cycle, and has a second capacitor whose capacitance value changes in conjunction with the first capacitor; a second resistor; A control unit that controls a capacitance value of the second capacitor so that a ratio between a resistance value of the pseudo resistor formed by the second capacitor and a resistance value of the second resistor is constant.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram illustrating a configuration example of an embodiment of the present invention.

In FIG. 1, a circuit block 100 and a circuit block 200 surrounded by broken lines are an adjusting circuit section and a filtering circuit section, respectively (hereinafter, adjusting circuit section 1).
00 and the filtering circuit section 200).

The filtering circuit section 200 is a primary active filter similar to the case shown in FIG. 7, and includes a resistor R1, a resistor Rv, a capacitor C, and the operational amplifier 1.

The resistor Rv is a variable resistor.
This resistance Rv is, for example, an FET (Field Effect Transformer).
istor), and the resistance value changes according to the applied voltage. Alternatively, as shown in FIG. 2, a resistor Rs having an arbitrary resistance value
1 to Rsn in series, and a switch SW1 is connected between each resistor.
To SWn, and the switches SW1 to SWn may be appropriately controlled by the driving unit 3.

The input signal Vin input to the filtering circuit section 200 is filtered as a signal having a frequency corresponding to a time constant formed by the resistor Rv and the capacitor C, and is output as an output signal Vout.

The adjustment circuit section 100 includes resistors R3, R4, R
v2, switches S1 and S2, capacitors Cr and Ca, comparator 2, and drive unit 3. Note that the resistor Rv2 is a variable resistor and has the same configuration as that of the resistor Rv.

The resistors R3 and R4 are connected to the power supply voltage Vc.
c is divided and input to the inverting input terminal of the comparator 2. Similarly, the resistor Rv2 and the capacitors Ca and Cr divide the power supply voltage Vcc and input it to the non-inverting input terminal of the comparator 2.

The capacitor Cr is switched by a clock signal of a predetermined frequency fck by switches S1 and S2 provided at both ends thereof. For example, when the clock signal is in an "H" state, the capacitor Cr is connected to the right side. (Shown by a solid line in the figure), and the capacitor Ca
And are connected in parallel.

When the clock signal is in the "L" state, the capacitor Cr is connected to the left side (the state shown by the broken line in the figure).
r is short-circuited.

The drive section 3 controls the resistors Rv2 and Rv according to the output signal of the comparator 2. Next, the operation of the above embodiment will be described.

FIG. 3 is a diagram for explaining the operation of the capacitor Cr and the switches S1 and S2 shown in FIG. Assuming that the switches S1 and S2 are connected to the left side as shown in FIG. 3A, the capacitor Cr is in a short-circuit state, and the accumulated charge is transferred in the direction indicated by the arrow in the figure. It will be discharged.

Next, assuming that the phase of the clock signal changes and the switches S1 and S2 are connected to the right side, all the charges of the capacitor Cr have been discharged in the previous phase. Charge Δq corresponding to the voltage Vcr applied to the capacitor Ca flows into the capacitor Ca. Note that the capacitor Ca is a smoothing capacitor.

Since the above operation is repeated according to the phase of the clock signal, the electric charge flowing into the capacitor Cr is as shown in FIG. Here, assuming that the frequency of the clock signal is fck, the above operation is repeated at a constant period of 1 / fck. Since the inflowing electric charge Δq is proportional to the voltage Vcr applied to both ends of the capacitor Cr, the capacitor C
The voltage Vcr applied to r and the charge (ie, current) flowing within a predetermined time have a directly proportional relationship.

For example, when the voltage Vcr applied to the capacitor Cr is constant, a constant current Iav flows through the capacitor Cr as shown in FIG. Here, Iav can be represented by the following equation.

[0030]

Iav = Δq / (1 / fck) = Δq · fck (2) Further, Δq can be expressed by the following equation.

[0031]

Δq = Cr · Vcr (3) Therefore, the circuit constituted by the capacitor Cr and the switches S1 and S2 functions as a pseudo resistor Rcr as shown in FIG. Become. Where Rcr
Can be represented by the following equation from the equations (2) and (3).

[0032]

Rcr = 1 / (Cr · fck) (4) Note that the capacitor Ca is a smoothing capacitor as described above, and has an infinite impedance with respect to a DC voltage. Therefore, this part can be ignored.

Therefore, the voltage Vb applied to the non-inverting input terminal of the comparator 2 can be expressed by the following equation.

[0034]

Vb = Vcc · Rcr / (Rcr + Rv2) (5) Further, the voltage Va applied to the inverting input terminal of the comparator 2
Can be represented by the following equation:

[0035]

Va = Vcc · R4 / (R3 + R4) (6) Considering that the comparator 2 and the drive unit 3 control the resistor Rv2 so that Va = Vb, the equation (5) and The following equation is obtained from the equation (6).

[0036]

[Mathematical formula-see original document] Rcr / (Rcr + Rv2) = R4 / (R3 + R4) (7) Here, for simplicity of description, R3 = R4, and substituting equation (4) into equation (7), To get the formula.

[0037]

1 / (1 + Rv2 · Cr · fck) = 1/2 (8) Here, when Rv2 · Cr = τ and Equation (8) is solved for τ, the following equation is obtained.

[0038]

Τ = 1 / fck (9) As can be seen from this equation, τ is determined by the reciprocal of the frequency of the clock signal. Since the clock signal is generated by a crystal oscillator or the like, its accuracy is very high. Therefore, it can be considered that the time constant τ always has a constant value regardless of circuit variations.

By the way, the time constant by the capacitor C and the resistor Rv for determining the cutoff frequency of the filter circuit 200 is τ.
If 2 (= Rv · C), as shown in the following equation, τ
2 can be expressed as a constant multiple of τ.

[0040]

Τ2 = k · τ (10) where the constant k is the value of the capacitor C and the capacitor C
r and the ratio of the resistances Rv and Rv2.
As described above, since the relative accuracy of the element formed on the semiconductor substrate is high, the constant k can be set with high accuracy.

Accordingly, the time constant τ2 always has a constant value irrespective of circuit variations. As a result, the cut-off frequency determined by this time constant τ2 also becomes
The setting can be performed with high accuracy without being affected by variations in individual circuits or fluctuations in the power supply voltage.

In the above embodiment, the time constant of the circuit is controlled by the variable resistor.
The time constant of the circuit can be controlled by a variable capacitor.

FIG. 5 shows a configuration example in that case. In this figure, the same parts as those in FIG. 1 are denoted by the same reference numerals, and their description is omitted. In the embodiment of FIG. 5, the resistor Rv2 of the adjustment circuit unit 100 is replaced by a fixed resistor R6, and the capacitor Cr is replaced by a variable capacitor Crv, as compared with the case of FIG. Further, the variable resistor Rv of the filtering circuit unit 200 is replaced with a fixed resistor R5,
The capacitor C is replaced with a variable capacitor Cv.
Other configurations are the same as those in FIG.

Here, the variable capacitors Crv and Cv are:
For example, as shown in FIG. 6, both ends of capacitors Cs1 to Csn having an arbitrary capacitance value are connected in parallel via SW1a to SWna and SW1b to SWnb, and switches SW1a to SWna and switches SW1b to SWnb are driven. An equivalent circuit can be obtained by appropriately controlling according to (3). By the way, in the above circuit, two switches are provided for one capacitor, but it is also possible to control with only one switch.

In the above embodiment, there is no substantial difference in the operation, only the variable capacitors Crv and Cv are controlled by the drive unit 3. Therefore, description of the operation is omitted.

According to the above embodiment, the filtering circuit unit 2
This makes it possible to set the cutoff frequency of 00 with high accuracy and reduce the variation in the cutoff frequency of each circuit.

According to the above embodiment, the cut-off frequency of the filter circuit 200 can be arbitrarily set by appropriately setting the frequency fck of the clock signal for driving the switch.

In the above embodiment, the first-order filtering circuit has been described as an example. However, the present invention is not limited to such a case. Needless to say, this is also applicable.

Further, in the above embodiment, the switches S1 and S2 are provided at both ends of the capacitors Cr and Crv. For example, the lower ends of the capacitors Cr and Crv are connected to the ground, and the switch S1 is connected to the ground. You may make it connect alternately to an inversion input terminal.

According to such a configuration, the circuit can be further simplified.

[0051]

As described above, according to the present invention, in a filtering device, a filtering circuit portion having a frequency characteristic determined by a time constant formed by a first resistor and a first capacitor is provided at a predetermined period. A second capacitor that functions as a pseudo resistor by being switched, a second resistor whose resistance value changes in conjunction with the first resistor, and a resistance value of the pseudo resistor by the second capacitor. ,
An adjusting circuit section having a control section for controlling the resistance value of the second resistor so that the ratio with the resistance value of the second resistor becomes constant; Can be set.

Further, according to the present invention, in the filtering device, a filtering circuit portion having a frequency characteristic determined by a time constant formed by the first resistor and the first capacitor is switched at a predetermined cycle. A second capacitor which functions as a resistor in a pseudo manner and whose capacitance value changes in conjunction with the first capacitor;
And a control unit that controls the capacitance value of the second capacitor so that the ratio between the resistance value of the pseudo resistor formed by the second capacitor and the resistance value of the second resistor becomes constant. With the provision of the circuit section, the cutoff frequency of the filtering circuit can be freely set.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a resistor Rv shown in FIG.

3A and 3B are diagrams illustrating the operation of a switch and a capacitor. FIG. 3A illustrates a case where a capacitor is discharged, FIG. 3B illustrates a case where a capacitor is charged, and FIG. FIG. 3C shows an equivalent circuit diagram of this part.

FIG. 4A is a diagram illustrating electric charges flowing into a capacitor, and FIG. 4B is a diagram illustrating a state in which electric charges are averaged over time.

FIG. 5 is a circuit diagram showing a configuration example of another embodiment of the present invention.

FIG. 6 is a circuit diagram showing a configuration example of capacitors Crv and Cv shown in FIG. 5;

FIG. 7 is a circuit diagram showing a configuration example of a conventional filtering circuit.

[Explanation of symbols]

1 ... operational amplifier, 2 ... comparator, 100 ... adjustment circuit section, 200 ... filtering circuit section

Claims (4)

[Claims] 1. A filter circuit section having a frequency characteristic determined by a time constant of a first resistor and a first capacitor, and a second capacitor which functions as a pseudo resistor by being switched at a predetermined cycle. And a ratio of the resistance of the second resistor, the resistance of the pseudo resistor by the second capacitor, and the resistance of the second resistor, the resistance of which varies in conjunction with the first resistance, is constant. And a control circuit section having a control section for controlling the resistance value of the second resistor so as to satisfy the following condition. 2. The control section according to claim 1, wherein the control section includes a first voltage generated by dividing a power supply voltage by a third resistor and a fourth resistor, and a pseudo resistance generated by the second capacitor. The filtering device according to claim 1, wherein the second resistor is controlled so that a second voltage generated by dividing the voltage by the second resistor and the second resistor becomes equal. 3. A filter circuit section having a frequency characteristic determined by a time constant of a first resistor and a first capacitor; and a switching circuit that is switched at a predetermined cycle to function as a pseudo-resistor. A second capacitor whose capacitance value changes in conjunction with the first capacitor;
And a control unit that controls the capacitance value of the second capacitor so that the ratio of the resistance value of the pseudo resistor by the second capacitor to the resistance value of the second resistor is constant. An adjustment circuit unit having: and a filter device. 4. The control unit includes: a first voltage generated by dividing a power supply voltage by a third resistor and a fourth resistor; and a pseudo resistance by the second capacitor, the first voltage being generated by dividing the power supply voltage by the second capacitor. The filtering device according to claim 3, wherein the second capacitor is controlled so that a second voltage generated by dividing the voltage by the second resistor and the second resistor is equal.