A capacitive trans-impedance amplifier comprising: a voltage amplifier having an inverting input terminal for connection to an input current source, and a non-inverting input terminal, the voltage amplifier being arranged to provide at an output terminal thereof an output voltage signal; a feed-back capacitor coupled between the inverting input terminal and the output terminal to accumulate charges received from the input current source and to generate a feed-back voltage accordingly wherein the capacitor defines a negative feed-back loop of the voltage amplifier such that the output voltage is proportional to said accumulation of said charges; a calibration unit including a calibration capacitor electrically coupled, via a calibration switch , to said inverting input terminal of said voltage amplifier and thereby electrically coupled to said feed-back capacitor , wherein the calibration unit is operable to switch the calibration switch to a calibration state permitting a discharge of a quantity of charge from the calibration capacitor to the feed-back capacitor ; wherein the capacitive trans-impedance amplifier is arranged to determine a voltage generated across said feed-back capacitor while said calibration switch is in said calibration state and to determine a capacitance value, C=Q/V, for the feed-back capacitor according to the value of the generated voltage, V, and the quantity of charge Q; characterised in that : said calibration switch comprises a photodiode operable to attain said calibration state in response to receipt of optical radiation sufficient to render the photodiode conductive. a voltage amplifier having an inverting input terminal for connection to an input current source, and a non-inverting input terminal, the voltage amplifier being arranged to provide at an output terminal thereof an output voltage signal; a feed-back capacitor coupled between the inverting input terminal and the output terminal to accumulate charges received from the input current source and to generate a feed-back voltage accordingly wherein the capacitor defines a negative feed-back loop of the voltage amplifier such that the output voltage is proportional to said accumulation of said charges; a calibration unit including a calibration capacitor electrically coupled, via a calibration switch , to said inverting input terminal of said voltage amplifier and thereby electrically coupled to said feed-back capacitor , wherein the calibration unit is operable to switch the calibration switch to a calibration state permitting a discharge of a quantity of charge from the calibration capacitor to the feed-back capacitor ; wherein the capacitive trans-impedance amplifier is arranged to determine a voltage generated across said feed-back capacitor while said calibration switch is in said calibration state and to determine a capacitance value, C=Q/V, for the feed-back capacitor according to the value of the generated voltage, V, and the quantity of charge Q; characterised in that : said calibration switch comprises a photodiode operable to attain said calibration state in response to receipt of optical radiation sufficient to render the photodiode conductive. <EOS>