A low-power NMOS transmit/receive IC filter for PCM telephony

A very low-power single chip NMOS implementation of the transmit and receive PCM filters used in telephony applications is described. The circuit utilizes doubly-terminated switched capacitor low-pass and high-pass filters and meets accepted requirements without trimming. The filters have low idle-channel noise, consume only 20 mW of power, and are realized in a small area of silicon. Design of the overall architecture, the individual filter sections, and the operational amplifiers in NMOS technology is described.