Frequency and Q tuning techniques for continuous-time bandpass sigma-delta modulator

For continuous-time (CT) sigma-delta modulator, degradations in SNR can be caused by the non-idealities and variations of circuit components such as integrators, resistors and capacitors since the center frequency and/or the Q-factor of the CT filter in the modulator can vary significantly due to temperature and process variations. Thus tuning is necessary especially for high order modulator. In this paper, a direct background digital frequency technique and a direct background digital Q-factor tuning technique are proposed. Both techniques measure the noise spectral densities of the modulator output digitally during normal operation and adjust the tuning codes according to the measured spectral densities. No extra ADC or DAC are required.