On the Use of FIR Feedback in Bandpass Delta-Sigma Modulators

This paper presents a new architecture for bandpass delta-sigma modulators (BP Delta Sigma Ms) featuring finite impulse response (FIR) filters in the feedback path. The effectiveness of FIR feedback in lowpass delta-sigma modulators (LP Delta Sigma Ms) has been well-established in improving loop-filter linearity and robustness to clock jitter. Building upon these findings, we explore the application of bandpass FIR filters in single-bit BP Delta Sigma Ms. By contrast to conventional BP Delta Sigma Ms, the proposed technique significantly reduces out-of-band quantization error contents in the feedback signal. This approach is applicable to both discrete time and continuous-time implementations. Further, we show that performance does not improve by increasing the number of FIR taps beyond a certain point. However, we can enhance filtering performance by employing non-equal coefficients within the filter. To validate the efficacy of the presented approach, the paper includes electrical simulation of a 4th-order active-RC BP Delta Sigma M.