Digital-to-analog converter circuit incorporating hybrid sigma-delta modulator circuit

A circuit comprising:

• a sigma-delta modulator circuit having an input for receiving a digital input word and having an output for generating a first output signal corresponding to the digital input word, wherein said sigma-delta modulator operates as a second order modulator whenever the digital input word has a value within a middle portion of its range, and operates as a first order modulator whenever the digital input word has a value outside the middle portion of its range;
• a filter circuit having an input for receiving the first output signal and having an output for generating thereon an analog voltage signal related to the first output signal;
• and
• a varactor circuit coupled to receive the analog voltage signal, said varactor circuit providing an incremental change in capacitance as a function of incremental changes in the analog voltage signal which is greater in a region of the analog voltage signal corresponding to the middle portion of the digital input word range than in other regions of the analog voltage signal.

A circuit comprising:

• a digtal to-analog converter (DAC) having an input for receiving a digital input word and having an output for generating thereon an analog signal corresponding to the digital input word, said DAC configured to generate a range of voltages at least including a lower reference voltage and an upper reference voltage, said DAC including a sigma-delta modulator circuit configurable to either operate as a second order sigma-delta modulator or operate as a first order sigma-delta modulator;
• a first multiplexer circuit coupled to receive the analog signal, the upper reference voltage, and the lower reference voltage, said first multiplexer circuit for generating a first output signal which is selectable to follow the analog signal, the upper reference voltage, or the lower reference voltage;
• a control circuit configured to switch the first output signal between the analog signal and the upper reference voltage when the analog signal has a value substantially equal to the upper reference voltage, and configured to switch the first output signal between the analog signal and the lower reference voltage when the analog signal has a value substantially equal to the lower reference voltage, thereby seamlessly switching the first output signal without substantial perturbation;
• wherein the sigma-delta modulator circuit is configured to operate as a second order sigma-delta modulator when the digital input word has a value corresponding to an analog voltage within a middle portion of a range between the lower and upper reference voltages, and to operate as a first order sigma-delta modulator otherwise.

A circuit comprising:

• a digital-to-analog converter (DAC) circuit having an input for receiving a digital input word and having an output for generating thereon an analog signal corresponding to the digital input word, said DAC configured to generate a range of voltages at least including a lower reference voltage and an upper reference voltage, said DAC including a sigma-delta modulator circuit configurable to either operate as a second order sigma-delta modulator or operate as a first order sigma-delta modulator;
• a plurality of multiplexer circuits, each coupled to receive the analog signal, the upper reference voltage, and the lower reference voltage, each of said plurality of multiplexer circuits for generating a respective output signal which is selectable to follow the analog signal, the upper reference voltage, or the lower reference voltage;
• and
• a control circuit configured to connect at most one of the multiplexer circuit output signals to the analog signal, and remaining ones of the multiplexer circuit output signals to either the upper reference voltage or the lower reference voltage;
• wherein the sigma-delta modulator circuit is configured to operate as a second order sigma-delta modulator when the digital input word has a value corresponding to an analog voltage within a middle portion of a range between the lower and upper reference voltages, and to operate as a first order sigma-delta modulator otherwise.

A method for generating an analog varactor control signal corresponding to a digital input value, said method comprising the steps of:

• generating an analog voltage corresponding to the digital input word using a digital-to-analog converter (DAC) circuit which includes a sigma-delta modulator circuit configurable to operate as either a first order or second order modulator;
• and
• operating the sigma-delta modulator circuit as a second order modulator when the digital input word has a value within a middle portion of its full scale range, and operating it as a first order modulator otherwise;
• and
• coupling the analog voltage output of the DAC circuit to the varactor control signal.