Sigma-delta modulator having a plural order loop filter with successive filter sections of successively smaller signal excursion range

Claim 1

A sigma-delta modulator for converting an input signal into a digital output signal, comprising: a difference stage for receiving the input signal and a feedback signal, and generating a difference signal corresponding to the difference therebetween; filter means including: a plurality N of first-order integrating sections of rank K, where K ranges from 1 to N, connected in series in ascending rank order; each section having an input, an output at which it produces a section output signal, and a first-order transfer function from the input to the output with a section gain G K ; the input of the section of rank 1 being connected to said difference stage to receive said difference signal, the input of each remaining section of rank K being coupled to the output of the preceding section of rank K-1 to receive therefrom the output signal of the preceding section; a plurality N of weighting amplifiers of rank K each having an input and an output, the input of each amplifier being coupled to the output of an integrating section of corresponding rank, said amplifiers having respective weighting factors W K so that N weighted section output signals are produced at the outputs of the N amplifiers; and an adder stage connected to the outputs of the N amplifiers for producing a filter signal corresponding to the sum of the N weighted section output signals; a quantizer for quantizing the filter signal, the quantized filter signal constituting said digital output signal; and feedback means for deriving said feedback signal from said digital output signal and supplying it to said difference stage; characterized in that each integrating section of rank K is adapted to limit the amplitude of the output signal produced thereby to a respective limit value L K ; each section of rank K>2 has an associated quotient Q K which corresponds to the section limit value L K divided by the product of the section gains G K , G K-1 , . . . G 1 , of that section and of all preceding sections back to and including the section of rank 1; and the quotient Q K associated with a section of rank K is smaller than the quotient Q K-1 of the preceding section of rank K-1.