A low power 13-bit linear feature codec with programmable frontend

A 13-bit linear codec designed in 1.2 μm CMOS process for interfacing two microphones, earpiece, loudspeaker and tone ringer is presented. Special analog structures have been used to achieve high input sensitivity (46 dB) and high power outputs combined with low voltage, low power (30 mW at 3 V) and high power supply rejection ratio (>50 dB). A sigma delta technique has been chosen for D/A and A/D conversion to minimize analogue circuitry. All filter functions as well as the fine gain setting and side tone generation have been realized in hard wired, bit serial, multiplier-free digital logic. All gain parameters are programmable, the absolute accuracy is achieved by a factory trimmed bandgap reference which minimizes the amount of external components. A digital bandpass filter in the encoder and lowpass filter in the decoder exceed ETSI prETS30085 and CCITT G712 requirements. Additional features are a programmable tone generator, stabilized voltage for electret microphones, a multimode PCM and a serial micro controller interface