JPH0758588A - Filter time constant switching circuit - Google Patents

Abstract

PURPOSE:To provide a filter time constant switching circuit in which filter characteristic can be prevented from being affected by the parasitic capacitance of a capacitor to form a time constant on an IC chip when it is open circuited. CONSTITUTION:The filter time constant switching circuit which switches a filter time constant by connecting a capacitance element C2 selectively between the connecting point of an element which comprises the time constant of a filter circuit consisting of a resistor R1 and a capacitor C1 formed on the chip of an integrated circuit device directly or via a resistor R2 is equipped with a buffer amplifier Ap whose input terminal is connected to the connecting point of the filter circuit, and a switch element SW2 which connects the reference potential line connection side terminal of the capacitance element C2 selectively to either the output terminal of the buffer amplifier Ap or a reference potential line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter time constant switching circuit incorporated in an IC (integrated circuit device) for switching the filter time constant of a filter circuit formed on an IC chip.

[0002]

2. Description of the Related Art Conventionally, when a filter circuit is built into an IC, a capacitor having a filter time constant is often formed on an IC chip due to the progress of mounting technology. In this case, for example, as shown in FIG.
An insulating film such as silicon dioxide (SiO ₂ ) is used as a dielectric, and an N-type conductive silicon layer (N ⁺ ) and aluminum (Al) are formed on the P-type silicon wafer (P-sub).
A capacitor is formed by sandwiching it with a metal film such as.

By the way, when switching the time constant of a filter circuit in an IC, generally, a resistor is connected directly or between a connection point of elements constituting the time constant of the filter circuit and a ground line (reference potential) of the IC chip. The capacitors are selectively connected via. An example thereof is shown in FIG.

In FIG. 3, one end of a resistor R1 is connected to the signal input end, and the other end is grounded via a capacitor C1, whereby an RC filter circuit using R1 and C1 as time constant elements. Are configured. By selectively grounding a connection circuit between R1 and C1 of this filter circuit with a series circuit of a resistor R2 and a capacitor C2 by controlling ON / OFF of a switch element SW1, a filter time constant can be switched and a filter characteristic can be varied. it can.

However, when a capacitor is actually formed on the IC chip, as shown in FIG. 4, the electrode on the N-type silicon layer (N ⁺ ) side always has a P-type silicon wafer (P-sub). A parasitic capacitance (Cp) is formed between them. From this, P-type silicon wafer (P-
Since the sub) is normally grounded, for example, in the case of the filter time constant switching circuit shown in FIG.
Parasitic capacitances C _1p and C _2p are attached to 2 in almost the same proportion.

However, as can be seen from the circuit configuration, since the parasitic capacitance C _1p side of the capacitor C1 has a low impedance, its influence is almost negligible. Further, also in the capacitor C2, when the switch element SW is on, there is no problem.

However, when the switch element SW is off,
The parasitic capacitance C _2p is still attached to the capacitor C2 to be opened. The parasitic capacitance C _2p is about 20% of C 2 and varies, which causes deterioration of filter characteristics. This problem becomes remarkable especially when the capacitance of the capacitor C2 is larger than that of the capacitor C1.

[0008]

As described above, in the conventional filter time constant switching circuit, when the capacitor built in the IC is opened, the influence of its parasitic capacitance remains, and the filter characteristic deteriorates. There was a problem of doing.

The present invention has been made in order to solve the above problems, and when a capacitor which is a filter time constant is opened on an IC chip, the parasitic capacitance does not affect the filter characteristics. An object is to provide a constant switching circuit.

[0010]

In order to achieve the above object, the present invention is directed to a connection point between elements forming a time constant of a filter circuit formed on a chip of an integrated circuit device and a reference potential line of the chip. In the filter time constant switching circuit that switches the filter time constant by selectively connecting a capacitive element directly to or through a resistor, a buffer amplifier whose input end is connected to the output end of the filter circuit, and the capacitive element And a switch element for selectively connecting the reference potential line connection side end to either one of the output end of the buffer amplifier and the reference potential line.

[0011]

In the filter time constant switching circuit having the above structure,
When the switch element is connected to the output end of the buffer amplifier, the capacitance element is ideally opened by preventing a potential difference from occurring between both ends of the capacitance element.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. Note that, here, description will be made assuming that the same filter characteristics as those of the circuit shown in FIG. 3 are obtained. FIG. 1 shows the configuration of a filter time constant switching circuit according to the present invention, in which a signal input terminal is grounded via a resistor R1 and a capacitor C1. This constitutes an RC filter circuit. The connection point between R1 and C1 of this filter circuit is a resistor R
2 and the capacitor C2 to be connected to the movable end a of the switch element SW2 and also to the input end of the buffer amplifier Ap. An output is derived from the connection point of R1 and C1 of the filter circuit as the output end of the circuit.

One fixed end b of the switch element SW2 is grounded (turns on), and the other fixed end c is connected to the output end of the buffer amplifier Ap (turns off). Incidentally, C _2p is a parasitic capacitance associated with the capacitor C2.

The operation of the above arrangement will be described below. First, when the switch element SW2 is on, exactly the same as in the case of the switch on of the conventional circuit shown in FIG. 3, the series circuit of the resistor R2 and the capacitor C2 is connected in parallel to the capacitor C1, whereby the filter time constant Will change.

Next, considering the case where the switch element SW2 is off, if the gain of the buffer amplifier Ap is 1,
The input voltage (output voltage of the RC filter circuit) V1 of the buffer amplifier Ap becomes the output as it is. Therefore, since the potential difference across the series circuit of the resistor R2 and the capacitor C2 is always 0, no current flows there. Therefore, the capacitor C2 at this time becomes the same as in the completely open state.

Further, even if the gain of the buffer amplifier Ap is slightly lowered from 1 or the output impedance is slightly increased, the influence of the parasitic capacitance C _2p can be made very small as compared with the conventional circuit without the buffer amplifier Ap. Further, since the capacitor C2 is DC-cut, the buffer amplifier Ap need not care about the DC shift at all. When the transient response during the switch operation becomes a problem, the ground side may be connected to the DC potential of the output of the buffer amplifier Ap.

FIG. 2 shows the circuit configuration of FIG. 1 more specifically. In FIG. 2, the transistor Q1 and the current source I1 form a buffer amplifier Ap of the emitter follower, the base of the transistor Q1 is connected to the filter circuit output terminal, and the collector is connected to the VCC power supply line.
The emitter is grounded via the current source I1. The ground side of the capacitor C2 is connected to the emitter of the transistor Q1 and the current source I.
It is connected to the connection point with 1.

On the other hand, the transistors Q2 and Q3 and the resistor 3
Is a switch element SW2. The base of the transistor Q2 is connected to the VCC power supply line via the resistor R3, the collector is directly connected to the VCC power supply line, and the emitter is connected to the current source I1. A switching control signal is supplied to the base of the transistor Q3, the collector is connected to the base of Q2, and the emitter is grounded. That is, the transistor Q2 can be switching-controlled by turning on / off the transistor Q3 with the switching control signal.

Here, when the transistor Q3 turns off, the transistor Q2 turns on and Q1 turns off, and the emitter potential of the transistor Q1 rises to near the power supply potential, so that the switch element SW2 in FIG. 1 is turned on. Similarly, the resistor R1 and the capacitor C2 are equivalently connected in parallel to the capacitor C1. When the transistor Q1 is on and the transistor Q2 is off, the transistor Q1 comes to function and the potential difference between both ends of the resistor R2 and the capacitor C2 becomes zero. That is, it is the same as when the switch element SW2 in FIG. 1 is turned off, and the transmission signal is not affected by the stray capacitance C2p.

Therefore, in the filter time constant switching circuit having the above structure, when the switch element is connected to the output end side of the buffer amplifier, voltage is not applied to both ends of the capacitor element, so that the capacitor element is ideal. The parasitic capacitance does not affect the filter characteristics.

It goes without saying that the resistor R2 in series with the capacitor may be short-circuited if necessary. Further, the filter circuit including the resistor R1 and the capacitor C1 is merely an example, and the present invention is not limited to this, and this node may be a part of the feedback filter. Needless to say, the present invention can also be implemented in the same manner even if various modifications are made without departing from the scope of the present invention.

[0022]

As described above, according to the present invention, there is provided a filter time constant switching circuit in which the parasitic capacitance does not affect the filter characteristics when the capacitor having the filter time constant is opened on the IC chip. Can be provided.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a filter time constant switching circuit according to the present invention.

FIG. 2 is a circuit diagram showing a more specific configuration of the same embodiment.

FIG. 3 is a circuit diagram showing a configuration of a conventional filter time constant switching circuit.

FIG. 4 is a chip cross-sectional view showing a structure in the case where a capacitor having the above filter time constant is formed on an IC chip.

[Explanation of symbols]

R1, R2, R3 ... Resistor, C1, C2 ... Capacitor, C
p, C1p, C2p ... Parasitic capacitance, Ap ... Buffer amplifier, Q
1, Q2, Q3 ... Transistor, I1 ... Current source.

Claims (2)

[Claims] 1. A capacitive element is selectively connected directly or through a resistor between a connection point of elements forming a time constant of a filter circuit formed on a chip of an integrated circuit device and a reference potential line of the chip. In the filter time constant switching circuit for switching the filter time constant, a buffer amplifier whose input end is connected to the output end of the filter circuit, and a reference potential line connection side end of the capacitive element are connected to the output end of the buffer amplifier and the output end of the buffer amplifier. A filter time constant switching circuit comprising: a switch element selectively connected to either one of the reference potential lines. 2. The filter time constant switching circuit according to claim 1, wherein the capacitance element has a terminal having a larger parasitic capacitance connected to the switch element.