Design of a 5bit 1GSps VCO Quantizer for a CT Delta Sigma Modulator

This paper presents the design of a 5 bit VCO quantizer with an alternative flipped cross-coupled delay element providing low KV nonlinearities compared to the commonly used current starved inverter delay element. The VCO is demonstrated within a third order continuous time (CT) delta sigma (?Sigma) modulator operating at an fS of 1 GHz with an OSR of only 10 . In using the frequency output of the VCO, fourth order noise shaping is present but with the stability of a third order multi-bit loop-filter. The schematic based VCO achieves a 50 dB linearity and 44.5 dB SNDR within a 50MHz bandwidth (BW) which provides 73.5 dB SNDR performance of the third order modulator with a frequency output VCO quantizer.