Bitwise ELD Compensation in ΔΣ Modulators

Abstract

Excess loop delay (ELD) in high speed continuous-time (CT) Delta-Sigma-Modulators (DSMs) imposes design challenges and limits the use of high resolution, e.g. successive-approximation-register (SAR) based internal quantizers, as usual compensation techniques like the use of a direct path around the quantizer come with increased swings and reduced maximum stable amplitude (MSA). In this paper, two bitwise ELD compensation approaches applicable to cascade-of-integrators with distributed feedback (CIFB) loop filters are proposed, which alleviate this problem for SAR and other multi-step quantizers by sequentially feeding bits into the feedback loop when they are available (MSB first, LSB last). Loop-filter equivalence for such bitwise ELD compensation is analytically derived. System-level simulations using Matlab & Simulink for exemplary 4-bit 2nd, 3rd and 4th-order modulators show 40% reduction of the last integrator output swing compared to the conventional direct path compensation. This potentially allows to avoid the swing, quantizer scaling and noise trade-offs due to ELD in state of the art designs.