Effective Filtering of Requantization Error in Dual Quantized CTDSM using FIR DAC

Dual quantization is an effective method to combine the benefit of single bit quantization, which results in inherently linearized DAC, and multibit quantization, which results in lower sampling rate. However, extra noise inserted in the loop due to requantization needs to be shaped out of the signal band to ensure the inband characteristics to remain as without the dual quantization. However, this additional shaping may result in extra noise injected into out of the band, which, if not filtered properly, may affect the loop dynamics and stability. Prior arts have used FIR filtering to filter out this additional digital quantization noise but in an ineffective way. In this paper, we show that the cutoff of the FIR filter can be reduced to much lower than Nyquist bandwidth(f
_s
/2) and can be brought close to the signal bandwidth of the modulator resulting in a further improved performance.