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Sigma delta modulation is a popular method for converting signals from analog to digital and vice-versa. However, sigma delta modulators (SDMs) may suffer from limit cycles, where the output bits may enter a repeating pattern. Current methods of preventing this phenomenon introduce unwanted noise, do not always succeed, and are often implemented when not needed. We present a more effective method for detecting and removing unwanted limit cycles. This method includes adding a small disturbance to the input, which destroys the periodicity of sigma-delta analog-to-digital conversion (ADC) modulator's output sequence and thereby removes the limit cycles. Compared with conventional methods this method is simpler to implement, and the SDM has less signal-to-noise ratio (SNR) penalty and a higher allowed input dynamic range. Various implementations of the limit cycle detection and removal schemes are described for feedforward SDMs. Results are reported which demonstrate the success of these methods.
Circuits and Systems I: Regular Papers, IEEE Transactions on (Volume:55 , Issue: 10 )
Date of Publication: Nov. 2008