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.

What I claim is: 
     
       1. An integrated, low-pass, active .Iadd.first order .Iaddend.filter .[.of the first order operable.]. for producing an output signal at an output terminal thereof .[.in.]. .Iadd.as a .Iaddend.function of a signal applied to an input terminal thereof and comprising an operational amplifier having an inverting input .[.and.]..Iadd., .Iaddend.a noninverting input.Iadd., .Iaddend.and .[.a single.]. .Iadd.an .Iaddend.output .[.coinciding.]. .Iadd.conterminous .Iaddend.with said output terminal of the filter .[.and.]..Iadd., said filter being .Iaddend.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.]. .Iadd.said operational amplifier .Iaddend.being connected to ground, .[.and characterized by.]. comprising.Iadd.: .Iaddend. a .[.single switched.]. capacitor and two switches operating at a preset frequency;   a first .[.armature.]. .Iadd.side .Iaddend.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.]. .Iadd.side .Iaddend.of said .[.switched.]. capacitor being switched by the other of said two switches between said input terminal and said output terminal of the filter. .Iadd.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. .Iaddend..Iadd.3. The active filter of claim 1 wherein said active   
     
     
        filter has a D.C. gain of exactly 1. .Iaddend..Iadd.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. .Iaddend..Iadd.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. .Iaddend..Iadd.6.  The filter of claim 5, further comprising an additional capacitor connected from said inverting input of said amplifier to said output thereof. .Iaddend..Iadd.7. The filter of claim 5, configured to have a DC gain of exactly 1. .Iaddend..Iadd.8. The filter of claim 5, wherein said first and second switches are each connected to switch at a common preset frequency. .Iaddend..Iadd.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. .Iaddend..Iadd.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. .Iaddend..Iadd.11.  The filter of claim 10, configured to have a DC gain of exactly 1. .Iaddend..Iadd.12. The filter of claim 10, wherein said first and second switches are each connected to switch at a common preset frequency. .Iaddend.