Switched capacitor filter circuit and method of fabricating the same

#This application is based on and incorporates herein by reference Japanese patent application No. 2002-117603 filed on Apr. 19, 2002.

#The present invention relates to a switched capacitor filter circuit and a method of fabricating the circuit. Specifically it relates to the switched capacitor filter circuit that can reduce feedthrough noise and the method of fabricating the circuit.

#Various switched capacitor circuits are proposed.

#The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

#FIG. 1 is a circuit diagram of a switched capacitor filter circuit forming a first-order filter according to the first embodiment of the present invention;

#FIGS. 2A and 2B are schematic diagrams showing a combination of a switching transistor and a noise compensation transistor, and a process of fabricating the switched capacitor filter circuit;

#(First Embodiment)

#Referring first to FIG. 1, a switched capacitor filter circuit is formed as a first-order filter. It is provided with first and second noise compensation transistors 108, 109 in addition to the switched capacitor filter circuit shown in FIG. 11. Drains and sources of the first and second noise compensation transistors 108, 109 are respectively connected between the switching transistors 101, 102 and the inverting input terminal of the operational amplifier 113. When a control signal /φ inverted from the control signal φ inputted to the gates of switching transistors 101, 102 is inputted to the gates, these noise compensation transistors 108, 109 cancel feedthrough noise generated by the switching transistors 101, 102.

#Cancellation of this feedthrough noise will be described below. FIG. 2A illustrates a circuit where the switching transistor 101 and the first noise compensation transistor 108 are connected in series. The switching transistor 101 generates feedthrough noise between the gate and source thereof when the control signal φ changes. Since the first compensation transistor 108 performs switching operation when a control signal /φ inverted from the control signal φ is inputted to the gate thereof, this transistor 108 generates both feedthrough noise and feedthrough noise of inverted polarity generated by the switching transistor 101.