An 18-MS/s 76-dB SNDR Continuous-Time 16 Modulator Incorporating an Input V oltage Tracking GmC

Abstract

This article presents a highly linear continuous- time (CT) 16 modulator incorporating energy-efficient open- loop GmC integrators with 1-V ppd full-scale (FS) voltage. The second-order loop filter is inherently linearized using an input voltage tracking technique. Here, the modulator input and digital to analog converter (DAC) feedback signal have the same linear and non-linear voltage to current conversion. Subtracted in the current domain in the Gm-stage, the non-linear contributors are eliminated and a highly linear wide-swing open-loop filter architecture is obtained. Linearity impairments due to tracking errors between the feedback DAC signal and the modulator input are analytically discussed and optimizations are introduced. The proposed 16 architecture takes advantage of a single feedback DAC which is shared among both the GmC integrators within the loop filter. A first-order finite impulse response (FIR) filter incorporated in the single feedback DAC is implemented by two 5-bit time-interleaved source-series terminated (SST) DACs comprising a data-dependent supply impedance compensation. Fabricated in a 22-nm FD SOI process, the 18-MS/s prototype consumes 3.21 mW combined from a 0.9- and 1.3-V power supply. Revealing 93-dB spurious free dynamic range (SFDR) at 76.5-dB peak signal to noise and distortion ratio (SNDR) performance, the prototype design achieves a 171-dB figure of merit (FoM) S with only 41-fF input capacitance.