Excess-loop-delay compensation technique for CT ΔΣ modulator with hybrid active–passive loop-filters
Analog Integrated Circuits and Signal Processing
The design and optimization methodology for CT RD modulators with hybrid Active-Passive (AP) loopfilters is indicated in this work. From the discussion, by appropriately scaling the passive filter gain and cooperating with a single-bit quantizer, the hybrid AP loop filtering can achieve an approximated noise-shaping function as a fully active DR modulator with the same order. The ELD effect in the hybrid AP CT DR modulator which influences the poles and zeros locations of the Noise Transfer
... tion (NTF) in the modulator is depicted. This paper also investigates the feasibility of applying the ELD compensation techniques that were used to be implemented in the active integrator's case to the hybrid AP CT DR modulator; however, some of them cannot be practically applied since the passive loop-filter cannot perform proportional feedback signal summation. After the discussion and analysis, the technique similar to Vadipour et al. (In: Symposium on VLSI circuits digest of technical papers, 2008) can be easily implemented at circuit-level and after applying it, there is one additional zero to compensate the peak in the NTF. With the help of this technique, the maximum quantizer delay tolerance can be a full clock period. The mentioned ELD compensation technique was applied in a 2nd order CT DR modulator with an active-RC integrator as the 1st stage and a passive RC filter as the 2nd stage, which was verified by transistor-level simulations in 65 nm CMOS. The circuit exhibits either 67.3 dB or 65.3 of SNDR, under the effect of half clock period or one clock period ELD, respectively; by contrast, without compensation, the system is unstable with both half or one clock period ELD effect. The designed hybrid CT DR modulator achieves 2 MHz signal bandwidth and consumes 2.54 mW of power. Keywords CT DR modulator Á Hybrid active-passive loop-filter Á Excess-loop-delay for hybrid active-passive loop-filter Á Excess-loop-delay compensation techniques for hybrid active-passive loop-filter Rui P. Martins on leave from Instituto Superior Técnico/TU of Lisbon, Portugal.