Adaptive Sigma–Delta Modulation With One-Bit Quantization

A method for improving the signal-to-noise ratio (SNR) of sigma–delta modulators with one-bit quantization is presented. The two-level feedback signal of a standard sigma–delta modulator is replaced by a multilevel signal, which is a superposition of two parts. One part ( ) represents a rough estimate of the instantaneous amplitude of the input signal (prediction signal), and the other ( ) is the sign of the quantizer output, multiplied with constant . Compared to a nonadaptive modulator, the amplitude of ( ) is reduced. Therefore, less noise power is introduced in the quantizer, and the SNR is considerably enhanced. Signal ( ) is derived numerically from the quantizer output 0( ) according to a particular adaptation algorithm. Except for the dc-level of ( ), sequence 0( ) contains the full digital information of the modulator input signal. From 0( ), a digital multilevel sequence 0( ) can be calculated, which represents the digital modulator output. The price paid for the improved SNR is a moderate slew rate limitation of the input signal. The approach is basically suited for a wide class of sigma–delta modulators. Here, simulation results and an example for a practical implementation of an adaptive sigma–delta modulator of first order are presented.