Multibit bandpass delta-sigma modulators using N-path structures

Simulation results on two switched-capacitor circuits suitable for use in a multi bit bandpass sigma-delta modulator are presented. The circuits use the N-path technique to guarantee accurate placement of the noise-transfer-function zeros. A 6th-order multibit bandpass analog-to-digital converter with a sampling frequency which is four times the center frequency and with a 5.5% bandwidth relative to the center frequency (the oversampling ratio equals 36) is described as a design example. System-level simulations, reported earlier, showed that this modulator promises a 94-dB SNR as long as the DAC (digital-to-analog converter) linearity is at least 8 b. The simulation results presented show that, in order to achieve this level of performance, the integrator circuits require high op-amp gain, and so a gain-compensated version of one of them is presented. The improved circuit requires an op-amp gain of 54 dB to achieve a modulator SNR of 90 dB