Fully integrated triple-mode sigma-delta modulator for speech codec

Mixed-signal speech processing system has been used in microphone-embedded portable electronics. Speech codec is the core of such a system. In speech codecs, sigma-delta modulator is one key block which converts analog voice signal into pulse density modulation (PDM) output for further digital signal processing. This paper shows the design of a triple-mode switched-capacitor sigma-delta modulator for speech codec. High mode (20 kHz signal bandwidth), low mode (4 kHz signal bandwidth), and sleep mode are implemented by addressing a flexible operation current. In order to realize a fully integrated solution, power management circuits dedicated to the modulator are also implemented, which includes frequency detector, LDO, bandgap and reference voltage buffers. The ASIC is fabricated in a 0.18-um CMOS process. In high mode measurement, an A-weighted signal-to-noise ratio (SNR) of 78 dB with -2.5 dBFS input is achieved under 138 uA. In low mode measurement, an SNR of 78 dB with -2 dBFS is achieved under 100 uA. In the standby condition, a sleep mode with current less than 10 uA is realized, so that the battery life is extended.