Abstract
We combine a first-order single-bit CTΔΣM employing finite impulse-response (FIR) feedback with a 1-bit second-order ΔΣ back end to achieve a modulator with maximum stable amplitude (MSA) that is close to full scale, and a third-order overall noise transfer function (NTF). FIR feedback is used in the input stage to reduce clock-jitter sensitivity, improve linearity, and reduce chopping artifacts. We show that in a MASH ADC, FIR feedback has the additional benefit of filtering the error waveform of the first stage that is fed into the second stage. We apply the principle to an audio continuous-time delta-sigma modulator. A prototype chip, fabricated in 180-nm CMOS to demonstrate the principle, achieves 100.9-dB SNDR in a 24-kHz bandwidth and dissipates 265μW. The resulting Schreier figure of merit is 180.5 dB.
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