Low-voltage fully-differential switched-opamp bandpass ΣΔ modulator

The paper deals with the design of a low-voltage bandpass ΣΔ modulator implemented with the switched-capacitor technique. To use standard technology (no low-threshold devices) and without an on-chip voltage multiplier, the switched-opamp technique has been adopted. The basic building blocks for the construction of a ΣΔ modulator (SC integrator, quantiser, and feedback DAC) are proposed in a fully differential version. In addition, they have been improved with respect to those previously reported to give a larger output swing at a higher sampling frequency and without any voltage reference. The validity of the proposed circuits is demonstrated by the realisation of a second-order bandpass ΣΔ modulator operating with a single 1 V supply within a standard 0.5 μm CMOS technology (VTHn=0.65 V, VTHp=0.7 V). The modulator operates at a sampling frequency of 1.8 MHz with a full-scale input range of 2 Vpk-pk and achieves a 45 dB dynamic range in a 20 kHz bandwidth