Analog Workshop 2023

An Active Time Constant Tuning Technique for Continuous-Time Delta-Sigma Modulators

We present a novel technique to compensate the effects of R-C / gm-C time-constant (TC) errors due to process variation in continuous-time delta-sigma modulators. Local TC error compensation factors are shifted around in the modulator loop to positions where they can be implemented efficiently with tunable circuit structures, such as current-steering digital-to-analog converters (DAC). This approach constitutes an alternative or supplement to existing compensation techniques, including capacitor or gm tuning. We apply the proposed technique to a third-order, single-bit, low-pass continuous-time delta-sigma modulator in cascaded integrator feedback structure. A feedback path tuning scheme is derived analytically and confirmed numerically using simulations. The modulator circuit was implemented in a 0.35-µm CMOS process using an active feedback coefficient tuning structure based on current-steering DACs. Measurements on the fabricated circuit show that with this tuning structure, constant performance and stable operation can be obtained over a wide range of TC variation.