Dual tone and modem frequency generator with on-chip filters and voltage reference

A single-chip CMOS circuit is described that contains a dual-tone multifrequency and modem frequency generator. For optimum performance and economy, switched-capacitor techniques are used for the on-chip bandgap reference voltage, digital-to-analog converters, and filter. CEPT recommendations on output level stability and distortion are met without recourse to external filtering and without a stabilized supply or external reference voltage. A self-aligned contact CMOS process with 4-μm design rules and with 500-Å-thick gate oxide is used to manufacture the circuit.