A random number generator comprising a sigma-delta modulator, the sigma-delta modulator having a modulation unit and a feedback loop arranged to receive a digital output signal from the modulator and form an adjustment signal in dependence on the digital output signal such that at any given time the absolute difference between an amplitude of a summation output signal of the modulator and a quantizer threshold of the modulator is less than the first voltage range. The random number generator is thus capable of forming a digital output signal that has a high degree of entropy.