A time constant calibration circuit comprising: a resistor (R REF ) having a first terminal and a second terminal; a capacitor (C) having a first terminal and a second terminal; an amplifier having a first input, a second input and an output; and a plurality of switches including a first switch (SW1) and a second switch (SW2), wherein the resistance of the resistor and/or the 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 (V B ), 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 (I REF ) 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: 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 a resistance of the resistor and/or a capacitance of the capacitor for producing said time constant. a resistor (R REF ) having a first terminal and a second terminal; a capacitor (C) having a first terminal and a second terminal; an amplifier having a first input, a second input and an output; and a plurality of switches including a first switch (SW1) and a second switch (SW2), wherein the resistance of the resistor and/or the 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 (V B ), 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 (I REF ) 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: 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 a resistance of the resistor and/or a capacitance of the capacitor for producing said time constant. the first switch and the second switch selectively route a reference current provided by the reference current source: 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 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 a resistance of the resistor and/or a capacitance of the capacitor for producing said time constant. <EOS>