"Sigfred" - A Low-Power DTMF and Signalling Frequency Detector

A CMOS-circuit is presented containing an analog switched-capacitor processor for detection of DTMF* and signalling tones on a telephone subscriber-line and a digital detection algorithm for postprocessing. The chip has been designed to work with low current consumption at low supply voltages and is fabricated in a standard CMOS-technology with self-aligned contacts [1] for digital circuits.