A Dual-Channel Sigma-Delta Voiceband PCM Codec

The use of oversampling and decimation for analog-to-digital converters has the advantage of requiring no precision analog components, and since most of the circuitry is digital, it benefits from the scaling of digital technology. Oversampled converters based on single-loop modulation have been reported [1], but a disadvantage of the single-loop modulator is that it generates large noise peaks for certain input values [2]. Double-loop modulation avoids this problem, and it also provides noise shaping which results in an improved S/N ratio for a given sampling rate [3].