System for calibration of a digital-to-analog converter

A self-calibrating digital-to-analog converter for converting a digital input signal to an analog output voltage, comprising:

• (a) a delta-sigma modulator having a digital input port receiving a digital representation of the digital input signal, an output producing a stream of digital pulses the density of which represents a value of the digital input signal;
• (b) a serial digital-to-analog converter coupled to the output of the delta-sigma modulator for converting the stream of digital pulses to a first analog signal;
• (c) a filter circuit coupled to filter the first analog signal to produce an analog output voltage;
• (d) an analog-to-digital converter coupled to convert the analog output signal to produce a digital feedback signal;
• (e) a successive approximation control circuit coupled to the analog-to-digital converter to produce a digital offset correction signal;
• (f) an offset register coupled to the successive approximation control circuit for storing the digital offset correction signal and having an output port coupled to the digital input port of the delta-sigma modulator, to provide self-calibration of an offset error of the self-calibrated digital-to-analog converter;
• and
• the filter circuit including a continuous time filter circuit including
• i. an operational amplifier having an input coupled to receive a pre-filtered portion of the first analog signal, the operational amplifier also having an output producing the analog output voltage, and
• ii. switching circuitry operative to isolate an output terminal on which the analog output voltage is produced from the output of the operational amplifier during the self-calibration operation of the self-calibrating digital-to-analog converter and to couple the output terminal to the output of the operational amplifier during normal digital-to-analog conversion operation of the self-calibrating digital-to-analog converter.

A self-calibrating digital-to-analog converter for converting a digital input signal to an analog output voltage, comprising:

• (a) a delta-sigma modulator having a first digital input receiving a digital representation of the digital input signal, an output producing a stream of digital pulses the density of which represents a value of the digital input signal, and an intermediate digital input port coupled to a digital summing element which is also coupled to an output of an integrator within the delta-sigma modulator;
• (b) a serial digital-to-analog converter coupled to the output of the delta-sigma modulator for converting the stream of digital pulses to a first analog signal;
• (c) a filter circuit coupled to filter the first analog signal to produce an analog output voltage;
• (d) an analog-to-digital converter coupled to convert the analog output signal to produce a digital feedback signal;
• (e) a successive approximation control circuit coupled to the analog-to-digital converter to produce a digital offset correction signal;
• (f) an offset register including an LSB portion coupled to the successive approximation control circuit for storing the digital offset correction signal and having an output port coupled to the intermediate digital input port of the delta-sigma modulator, to provide self-calibration of an offset error of the self-calibrated digital-to-analog converter without skewing an input range of the digital input signal and an MSB portion coupled to one input of a digital adder interposed between the first digital input and a digital input port of the digital-to-analog converter;
• and
• (g) an external offset input coupled to inputs of both the MSB and LSB portions of the offset register.

A self-calibrating digital-to-analog converter for converting a digital input signal to an analog output voltage, comprising:

• (a) a delta-sigma modulator having a first digital input receiving a digital representation of the digital input signal, an output producing a stream of digital pulses the density of which represents a value of the digital input signal, and an intermediate digital input port coupled to a digital summing element which is located within an integrating loop of the delta-sigma modulator and is also coupled to an output of an integrator within the delta-sigma modulator;
• (b) circuitry coupled to the output of the delta-sigma modulator for converting the stream of digital pulses to an analog output voltage;
• (c) an analog-to-digital converter coupled to convert the analog output signal to produce a digital feedback signal;
• (d) a successive approximation control circuit coupled to the analog-to-digital converter to produce a digital offset correction signal;
• and
• (e) an offset register coupled to the successive approximation control circuit for storing the digital offset correction signal and having an output port coupled to the intermediate digital input port of the delta-sigma modulator, to provide self-calibration of an offset error of the self-calibrated digital-to-analog converter without skewing an input range of the digital input signal.

A method of self-calibrating a digital-to-analog converter to compensate an offset voltage of the digital-to-analog converter without skewing an input range of a digital input signal, comprising:

• (a) providing (I) a delta-sigma modulator having a digital input receiving a digital representation of the digital input signal, an intermediate digital input port that is located within an integrating loop in the delta-sigma modulator, and an output, (ii) filter circuitry coupled to the output of the delta-sigma modulator for converting a stream of digital pulses thereon to an analog output voltage, (iii) an analog-to-digital converter coupled to convert the analog output voltage to produce a digital feedback signal, (iv) a successive approximation control circuit coupled to an output of the digital-to-analog converter, and (v) an offset register coupled to the successive approximation control circuit;
• (b) operating the digital-to-analog converter to produce a value of the analog output voltage representing the offset voltage;
• (c) operating the analog-to-digital converter and the successive approximation control circuit to convert the value to a digital offset number and storing the digital offset number in the offset register;
• and
• (d) applying the digital offset number to the intermediate digital input port during normal digital-to-analog conversion operation to compensate the offset.

A digital-to-analog converter for converting a digital input signal to an analog output voltage, comprising:

• (a) a delta-sigma modulator having a first digital input receiving a digital representation of the digital input signal, and an output producing a stream of digital pulses the density of which represents a value of the digital input signal;
• (b) circuitry coupled to the output of the delta-sigma modulator for converting the stream of digital pulses to an analog output voltage;
• (c) an analog-to-digital converter coupled to convert the analog output signal to produce a digital feedback signal;
• (d) a successive approximation control circuit coupled to the analog-to-digital converter to produce a digital offset correction signal;
• and
• (e) a digital step size indicator circuit coupled to the input of the delta-sigma modulator to measure the difference between the two most recent digital numbers input to the delta-sigma modulator and produce a bandwidth adjustment signal, and a switch control circuit responsive to the bandwidth adjustment signal to temporarily open or close at least one switch to modify the bandwidth of the continuous time filter circuit to reduce a settling time of a transition of the analog output voltage produced in response to the most recent digital number.