An apparatus for calibrating a differential circuit that includes a differential integrator having an input, a gain, and an output connected to a comparator. The differential integrator output is chargeable to a threshold prior to an integration period. The differential integrator integrates the input during the integration period such that the differential integrator output goes toward zero from the threshold. The comparator detects the output of the differential integrator reaching zero. The apparatus includes a closed-loop gain trim circuit to perform a coarse calibration to adjust and set the gain of the differential integrator and a reference generator that generates the threshold to which the differential integrator output is pre-charged. The reference generator is trimmable during a fine calibration to adjust and set the threshold to correct for residual gain error in the differential circuit remaining after the coarse calibration is performed.