JPS59115610A - semiconductor filter circuit - Google Patents

Abstract

PURPOSE:To offer a semiconductor filter circuit reducing the dependency on a manufacture process by forming a resistive element of a filter circuit as a symmetric form of a series combined resistance between a diffusion resistive element and other resistance means respectively. CONSTITUTION:An input terminal IN is taken as an external terminal P1 in a semiconductor integrated circuit and provided with a diffusion resistor R1' for electrostatic destruction prevention, with which a polysilicon resitor R'' is connected in series to constitute one resistor R1 of the filter circuit. The other combined resistor R2 is also made up of a series circuit between a diffusion resistor R2' and the polysilicon resistor R'' in the symmetric form as the combined resistor R1. The filter characteristic Q is obtained by Equation I at K=1. In the ratio of the resistors R1/R2, the respective corresponding resistance ratios R1'/ R2' and R1''/R2'' are formed to a prescribed value not affected by the manufacture process by constituting the resistors R1, R2 with the symmetric diffusion resistors R1', R2' and polysilicon resistors R1'', R2'', and the ratio of the combined resistor ratio R1/R2 is also made nearly constant.

Description

[Detailed description of the invention] The present invention relates to a semiconductor filter circuit.

For example, a second-order active low-pass filter as shown in FIG. 1 is known.

The inventor's research has revealed that when such an active filter is built into a semiconductor integrated circuit, the following problems occur.

When the input terminal IN of the active filter circuit is configured as an external terminal of a semiconductor integrated circuit, it is necessary to provide an electrostatic damage prevention circuit. Usually, this electrostatic breakdown prevention circuit is constituted by a diffused resistance element, and its resistance value and a parasitic diode clamp the high electrostatic voltage at the external terminal.

Therefore, the input resistor R1 constituting the active filter circuit is composed of a series composite resistor of the diffused resistor and another resistive element, such as a polysilicon resistive element. On the other hand, since the other resistor R2 is constituted by the polysilicon resistor, the resistance ratio R1/R2
is affected by manufacturing variations and inputs of semiconductor integrated circuits.

That is, the variation in the resistance value of the polysilicon resistor and the variation in the resistance value of the diffused resistor occur without any relation to each other.

In the active low-pass filter, if there is a variation in the resistance ratio R1'/R2, the filter characteristic Q is affected as shown in the transfer characteristic diagram of FIG. Therefore, this becomes a problem when obtaining a desired Q, or when it is desired to flatten the frequency characteristics of a passing signal by setting Q to 1, such as when using it as a noise removal filter.

An object of the present invention is to provide a semiconductor filter circuit with reduced manufacturing process dependence.

Other objects of the invention will become apparent from the following description and drawings.

Hereinafter, this invention will be explained in detail together with examples.

FIG. 3 shows a circuit diagram of an active low-pass filter according to an embodiment of the present invention.

The input terminal IN is an external terminal P1 in the semiconductor construction circuit.
Therefore, a diffused resistor R1' is provided to prevent electrostatic damage. A resistor R1'' is connected in series with this resistor R1' and constitutes one resistor R1 in the filter circuit. The resistor R1'' is constituted by a polysilicon resistance element. Further, the other composite resistor R2 is connected between the composite resistor R1 and the non-inverting input (''-) of the operational amplifier OP. This composite resistor R2 is also constituted by a series circuit of a diffused resistor R2' and a polysilicon resistor R2'' which are symmetrical to the composite resistor R1. A parasitic diode is constructed.

Note that the inverting input (-) of the operational amplifier op and its output are connected to form a voltage follower configuration.

In other words, the gain K is set to 1. and,
The connection point between the combined resistors R1 and R2 and the operational amplifier O
One capacitor C1 is provided between the output of the operational amplifier OP and the other capacitor C2 is provided between the non-inverting input of the operational amplifier OP and the ground potential of the circuit.

In this example circuit, as is well known, the filter characteristic Q
is determined by the following equation (1) when K=1.

(1) The ratio of the capacitance values of the capacitors C1 and C2 can be set accurately when they are configured between the same semiconductor integrated circuits. Also, the ratio of resistors R1 and R2 is as in this example,
By configuring symmetrical diffused resistors R1' and R2' and polysilicon resistors R1'' and R2'', the corresponding resistance ratios R1''/R2'' and R1'''/R2 are obtained, respectively.
" can be formed to a constant value that is not affected by the manufacturing process. Therefore, the ratio of the combined resistance RI/R2 can also be made constant. In particular, when Q=1, Cl= C2, R1-R2, and R1'
= R2' and R1'' = R2''.

Such a secondary active low-pass filter is not particularly limited, but may be suitable as an aliasing noise prevention filter in a digital transmission device (CODEC) that digitizes and transmits audio signals.

FIG. 4 shows a circuit diagram of another embodiment of the invention.

In this embodiment, in order to set a desired gain to the operational amplifier OP, its inverting input and output are connected to external terminals P2.
P3, and an external resistor R3 for gain setting is provided.

In this case, diffused resistors R2'' and R3'', which constitute the electrostatic damage prevention circuit, are provided in the same manner as above.In this case, by making the resistance values of the diffused resistors R2' and R3'' equal, Since similar resistors R2'' and R3' are provided in both the operational amplifiers OP, the noise components from the semiconductor substrate can be canceled out. Therefore, in the semiconductor filter circuit of this embodiment, the noise components from the semiconductor substrate can be canceled out. It is possible to obtain an output signal that is not affected by the components.

The invention is not limited to the above embodiments.

In the embodiment shown in FIG. 3 as well, a diffused resistor for canceling the noise from the semiconductor substrate may be provided at the inverting input of the operational amplifier OP.

Further, the present invention can be widely used in semiconductor filter circuits whose transfer characteristics are set by resistance ratios.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a known secondary active low-pass filter, FIG. 2 is a characteristic diagram showing its transfer characteristics, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. FIG. 3 is a circuit diagram showing another embodiment of the present invention. Fig. 1 Fig. I Fig. 2 Fig. 3 Fig. I Fig. 4 P4g, ? (

Claims (1)

[Claims] (1) In a semiconductor filter circuit that has a resistor element whose one end is connected to an external terminal, and whose frequency transfer characteristics are set by the ratio of the resistance values of the resistor elements including the resistor element,
A semiconductor filter circuit characterized in that each of the resistance elements is a symmetrical type of series composite resistance of a diffused resistance element and another resistance means. 2. Claim 1, characterized in that the other resistance element is constituted by a polysilicon resistor.
Semiconductor filter circuit described in Section 1. 3. The first or second claim, wherein the diffused resistance is an equivalent resistance in an electrostatic breakdown prevention circuit.
Semiconductor filter circuit described in Section 1. 4. The semiconductor filter circuit according to claim 1, 2, or 3, wherein the semiconductor filter circuit constitutes a secondary active low-pass filter.