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A new single-chip monolithic compressed/expanded (companded) pulse-code modulation (PCM) coder/decoder (codec) is described. The associated switched-capacitor filters and reference voltage are also implemented on the chip, using a silicon-gate CMOS process. The DAC and ADC used incorporate a binary-weighted capacitor array and a string of equal-valued resistors. The circuit operators from a/spl plusmn/5 V supply and it consumes 65 mW in normal operation and 5 mW in the power-down condition. The implementation of the critical circuits in CMOS technology is discussed in detail.