Time constant calibration circuit and method

A continuous-time sigma-delta analog-to-digital converter comprising:

• a time constant calibration circuit comprising:
• a resistor having a first terminal and a second terminal;
• a capacitor having a first terminal and a second terminal;
• an amplifier having a first input, a second input, and an output;
• a plurality of switches including a first switch and a second switch;
• and
• digital logic configured to control the plurality of switches and adjust resistances of the resistor and capacitances of the capacitor,
• wherein a resistance of the resistor and/or a capacitance of the capacitor is variable,
• wherein the first terminal of the resistor, the first terminal of the capacitor and the first input of the amplifier are coupled together at a common node,
• wherein the second input of the amplifier is coupleable to a reference voltage,
• wherein the output of the amplifier is coupled to the second terminal of the resistor and to the second terminal of the capacitor,
• wherein the first terminal of the resistor, the first terminal of the capacitor and the first input of the amplifier are coupleable to a reference current source at the common node, and
• wherein the time constant calibration circuit is operable to perform a time constant calibration process comprising one or more calibration cycles in which:
• the first switch and the second switch selectively route a reference current provided by the reference current source using the digital logic:
• through the resistor in a first phase of the, or each, calibration cycle;
• and
• through the capacitor in a second phase of the, or each, calibration cycle, and
• the resistance of the resistor and/or the capacitance of the capacitor is adjusted between each calibration cycle iteratively to determine an adjusted resistance of the resistor and/or an adjusted capacitance of the capacitor for producing said time constant.

A time constant calibration method comprising:

• providing a calibration circuit comprising:
• a resistor having a first terminal and a second terminal;
• a capacitor having a first terminal and a second terminal;
• an amplifier having a first input, a second input, and an output;
• a plurality of switches including a first switch and a second switch;
• and
• digital logic configured to control the plurality of switches and adjust resistances of the resistor and capacitances of the capacitor,
• wherein a resistance of the resistor and/or a capacitance of the capacitor is variable,
• wherein the first terminal of the resistor, the first terminal of the capacitor and the first input of the amplifier are coupled together at a common node,
• wherein the second input of the amplifier is coupled to a reference voltage,
• wherein the output of the amplifier is coupled to the second terminal of the resistor and to the second terminal of the capacitor,
• wherein the first terminal of the resistor, the first terminal of the capacitor and the first input of the amplifier are coupled to a reference current source at the common node;
• and
• performing a time constant calibration process comprising one or more calibration cycles in which:
• the first switch and the second switch selectively route a reference current provided by the reference current source using the digital logic:
• through the resistor in a first phase of the, or each, calibration cycle;
• and
• through the capacitor in a second phase of the, or each, calibration cycle, and
• the resistance of the resistor and/or the capacitance of the capacitor is adjusted, using the digital logic, between each calibration cycle iteratively to determine an adjusted resistance of the resistor and/or an adjusted capacitance of the capacitor for producing said time constant.