SIGFRED: a low-power DTMF and signaling frequency detector

A CMOS circuit is presented containing an analog switched-capacitor processor for detection of dual-tone multifrequency, (DTMF) and signaling tones on a telephone subscriber line and a digital detection algorithm for postprocessing. The major issues for the design were that it should be compatible with a CMOS fabrication process for microcontrollers to be included as an on-chip component and that its power consumption should be as low as possible. It has been designed to work with a current of less than 0.5 mA at low supply voltages (2.5-3 V). The maximum supply voltage is limited to 6 V by the process.<>