Switched-opamp: an approach to realize full CMOS switched-capacitor circuits at very low power supply voltages

The implementation of analog CMOS circuits that operate in the very low power supply voltage range (1 V to 2 V) becomes more important nowadays. Most accurate filter circuits are designed in the switched-capacitor technique. The existing design techniques require, however, the on-chip generation of a higher voltage by means of a voltage multiplier. In this paper, a novel technique, derived from the standard switched-capacitor technique, is presented. It is called switched-opamp because it is based on the replacement of the critical switches with opamps which are turned on and off. This technique results in a true, very low voltage operation without the need for voltage multipliers. As an example, a second order lowpass switched-capacitor filter is implemented in the switched-opamp technique. This filter operates with only a 1.5 V power supply. It is realized in a 2.4-μm CMOS process with V_T=±0.9 V. It has a measured total harmonic distortion of -60 dB for a signal swing of 600 mV_ptp and a powerdrain of only 110 μW