Excess loop delay compensation using RZ DAC for the third order CT ΔΣ modulator with CRFB-FF structures

Abstract

Continuous-time Delta-Sigma Modulators (CT-DSM) suffer from one of the system-level non-idealities called an excess loop delay. This excess loop delay changes the NTF (Noise Transfer Function) and affects the stability of the modulator. Return-To-Zero DAC (RZ-DAC) solves this problem if the delay value is ranged between 0 and 0.5. Adding an additional path around the quantizer compensate the excess loop delay for the range 0.1 to 1. But this requires the delay value to be known before. A simple technique for compensating excess loop delay for any unknown delay values up-to 1 is suggested in this paper. The compensation technique uses RZ-DAC with an additional compensation loop with delay $\mathrm{z}^{-1/2}$ and a simple modification around the quantizer. To verify the compensation technique the third order CT-DSM with CRFBFF structure has been chosen. The modulator achieves constant S(Q)NR of 87. 4dB with loop delay up-to 1 in the signal chain.