Integrated active low-pass filter of the first order

Abstract

An integrated, low-pass filter of the first order made using the switched capacitors technique utilizes advantageously a single switched capacitor and only two switches in contrast to the filters of the prior art which utilize two switched capacitors and four switches. The filter of the invention requires a smaller integration area and moreover exhibits a greater precision of its DC gain.

Description

ting input of the operational amplifier is connected to ground. A single switched capacitor Cx and the two switches driven at the frequency f_s are connected so that a first armature side of the switched capacitor is switched by a first one of said two switches between the inverting input of the operational amplifier and the circuit's ground node. The second armature side of the switched capacitor is switched by the other of said two switches between an input terminal of the filter and the output terminal thereof.

An analysis of the operation of the circuit of the active low-pass filter of the first order of the invention is herein shown, by utilizing the time scale indicated in the diagram of FIG. 3. ##EQU5## by introducing the z- transform: ##EQU6##

As it is well known to the expert technician, the frequency response of the system is obtained by substituting e^j2πfT s in place of z; where Ts is the sampling period of the circuit and f is the current frequency.

It may be immediately observed that the DC gain (i.e. f=0 and therefore z=1) is 1, i.e. 0 db, the cut-off frequency of the filter, as already indicated before, is given by: ##EQU7##

In respect to the known solutions, the integrated low-pass, active filter of the first order of the invention offers the advantage of requiring a reduced number of components thus permitting to save integration area. Furthermore, the DC gain of the filter has an infinite precision because it is no longer dependent from the precision of a ratio between integrated capacitors, as in the filters of the prior art.

Claims

1. An integrated, low-pass, active first order filter of the first order operable for producing an output signal at an output terminal thereof in as a function of a signal applied to an input terminal thereof and comprising an operational amplifier having an inverting input and, a noninverting input, and a single an output coinciding conterminous with said output terminal of the filter and, said filter being provided with a negative feedback network which comprises a continuous integration capacitor connected between said output terminal of the filter and said inverting input of the operational amplifier, the noninverting input of which said operational amplifier being connected to ground, and characterized by comprising:

a single switched capacitor and two switches operating at a preset frequency;

a first armature side of said switched capacitor being switched by a first one of said two switches between said inverting input of the operational amplifier and ground;

a second armature side of said switched capacitor being switched by the other of said two switches between said input terminal and said output terminal of the filter. 2. The active filter of claim 1 wherein said preset frequency and the values of said first and second capacitors determines the cut-off frequency of the filter. 3. The active filter of claim 1 wherein said active

filter has a D.C. gain of exactly 1. 4. The filter of claim 1, wherein said first and second switches are jointly connected so that

said first side of said capacitor is connected to ground while said second side thereof is connected to said input terminal, and

said first side of said capacitor is connected to said inverting input of said amplifier while said second side of said capacitor is connected to

said output terminal of the filter. 5. An integrated circuit filter, comprising:

an amplifier having inverting and noninverting inputs and an output, said noninverting input being connected to a reference potential;

a capacitor having first and second terminals;

a first switch operable to connect said first terminal of said capacitor either to said inverting input of said amplifier or to said reference potential; and

a second switch operable to connect said second terminal of said capacitor either to said output of said amplifier or to an input voltage;

whereby, when said first and second switches are switched synchronously and periodically said output carries a signal which is filtered with respect

to said input voltage. 6. The filter of claim 5, further comprising an additional capacitor connected from said inverting input of said amplifier to said output thereof. 7. The filter of claim 5, configured to have a DC gain of exactly 1. 8. The filter of claim 5, wherein said first and second switches are each connected to switch at a common preset frequency. 9. The filter of claim 5, wherein said first and second switches are jointly connected so that

said first terminal of said capacitor is connected to said reference potential while said second terminal thereof is connected to the input voltage, and

said first terminal of said capacitor is connected to said inverting input of said amplifier while said second terminal of said capacitor is

connected to said output of said amplifier. 10. An integrated circuit filter, comprising:

an amplifier having inverting and noninverting inputs and an output, said noninverting input being connected to a reference potential;

a capacitor having first and second terminals;

a first switch operable to connect said first terminal of said capacitor either to said inverting input of said amplifier or to said reference potential;

a second switch operable to connect said second terminal of said capacitor either to said output of said amplifier or to an input voltage; and

an additional capacitor connected between said inverting input of said amplifier and said output of said amplifier;

said first and second switches being jointly connected so that

said first terminal of said capacitor is connected to said reference potential while said second terminal thereof is connected to the input voltage, and

said first terminal of said capacitor is connected to said inverting input of said amplifier while said second terminal of said capacitor is connected to said output of said amplifier;

whereby, when said first and second switches are switched synchronously and periodically, said output carries a signal which is filtered with respect

to said input voltage. 11. The filter of claim 10, configured to have a DC gain of exactly 1. 12. The filter of claim 10, wherein said first and second switches are each connected to switch at a common preset frequency.