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Delta-sigma converters used in synchronous demodulation sensing applications like tuning fork gyroscopes or accelerometers typically contain zeroes in the noise transfer function for aggressive noise shaping to get higher resolution; however, analog component errors typically result in misplacement of the notch frequency resulting in lower SNDR and reduced sensor sensitivity. The error detection technique proposed here corrects for the analog errors in the digital domain by injecting a single tone directly at the input of the quantizer inside the loop and monitoring the RMS output in the digital domain. Expecting the tone to vanish due to the presence of the notch, a closed loop least mean squares (LMS) algorithm adjusts either the component values (capacitor tuning), or the sampling frequency of the modulator until the tone strength is reduced to a minimum, which corresponds to the accurate placement of the notch. Simulation results have demonstrated an improvement of 20 dB for a Single-Loop, Third-Order CIFB (Cascade of Integrators, Feedback Form), single-bit quantizer, delta-sigma modulator with an oversampling ratio (OSR) of 128 and a notch filter coefficient error of 50%.
Date of Publication: Jan. 2011