A sigma-delta modulator for forming a digital output signal representative of the magnitude of an analog input signal, the modulator comprising a modulation unit comprising: a summation unit for summing the analog input signal with an adjustment signal to form a summation output signal; an integrator arranged to receive the summation output signal and form an integrator output signal dependent thereon; and a quantizer arranged to receive the integrator output signal and form the digital output signal dependent thereon; the sigma-delta modulator further comprising a feedback loop for generating the adjustment signal and comprising a selection circuit arranged to form the adjustment signal by selecting between one of two boundary values for the adjustment signal, the selection being performed in dependence on the digital output signal.