Switched-Opamp, a Technique for Realising full CMOS Switched-Capacitor Filters at Very Low Voltages

Existing design techniques for very low voltage (1.5 V) switched-capacitor filters need the on-chip generation of clock-signals with a higher voltage to drive the MOS-switches. Here a novel technique for the realisation of low voltage switched-capacitor filters in CMOS technologies is introduced which uses clock signals with voltages equal to the power supply. This new technique is called switched-opamp because it replaces some critical switches in a classic biquad structure by opamps which are turned on and off. A 2nd order lowpass switched-capacitor filter has been implemented and is discussed. This filter operates at only 1.5 V power supply and has a total harmonic distortion of ¿60 dB with a signal swing of 550 mVptp. It is realised in a 2.4 ¿m CMOS process with VT = ±0.9 V.