Low-voltage fully differential switched-opamp bandpass ΣΔ modulator

The paper deals with the design of a low-voltage bandpass Sigma Delta 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 Sigma Delta 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 Sigma Delta modulator operating with a single 1V supply within a standard 0.5 mu m CMOS technology (V-THn = 0.65V, V-THp = 0.7V) The modulator operates at a sampling frequency of 1.8MHz with a full-scale input range of 2V(pk-pk) and achieves a 45dB dynamic range in a 20kHz bandwidth.