A Programmable Impedance Matching Circuit for Voiceband Modems

A line impedance-matching circuit for a voiceband modem is described. The circuit can be programmed to realize four different impedances. A switched-capacitor filter is used to generate the required transfer functions. The impedance-matching concept and circuits are described. Measured results are presented that show that the four impedances are realized with sufficient accuracy for voiceband modems. The circuit was fabricated in a standard 0.35-mum mixed-signal CMOS process.