In an apparatus and method for improving performance of a third order, double-sampled, sigma-delta modulator (SDM), a first one of three feedback elements included in a feedback loop of the SDM is selected to complete the feedback loop during a first half-cycle of the clock used for the double-sampling. The first one is restricted from being reselected during a subsequent half-cycle of the clock until the first one is reset. A second one of the three feedback elements is selected during a second half-cycle of the clock that is consecutive to the first half-cycle, the second one being different than the first one. A third one of the three feedback elements is selected during a third half-cycle of the clock that is consecutive to the second half-cycle, the third one being different than the second one.
With a delta sigma modulator of this invention, a plurality of clocks required to control a switching circuit can be easily generated and correlation among phases of the plurality of clocks can be automatically maintained while a frequency of the clocks is modified. A ring oscillator is formed of three delay circuits provided with differential amplifiers in the delta sigma modulator. A clock producing circuit produces the plurality of clocks to control the switching circuit by delaying three-phase clocks outputted from the ring oscillator. All the tail currents Ic in the differential amplifiers in the delay circuits in the ring oscillator and the tail currents Ic in the differential amplifiers in the delay circuits in the clock producing circuit are proportional to each other.
