We describe an in-situ analog technique for estimating time constant shifts in continuous-time single bit delta-sigma modulators. We show that the variance of the first integrator output of the modulator's loop filter is a good indicator of RC time constants. The nominal values of the time constants are restored by digitally controlling the resistance and capacitance values (realized as switched banks). Simulation results that demonstrate the efficacy of the time-constant tuning system are given for a third order CIFF modulator. ©2009 IEEE.