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; a feed-back capacitor, having capacitance C farads, coupled between the inverting input terminal and the output terminal to accumulate charges Δ Q received from the input current source and to generate a feed-back voltage Δ V accordingly wherein the feed-back capacitor defines a negative feed-back loop of the voltage amplifier such that the output voltage is proportional to said accumulated charges Δ Q; characterised in that the capacitive trans-impedance amplifier comprises: a calibration unit including a calibration capacitor of capacitance C cal 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 Δ Q cal from the calibration capacitor of capacitance C cal to the feed-back capacitor; and an analyser unit arranged to determine a voltage Δ V cal generated across said feed-back capacitor while said calibration switch is in said calibration state and to determine the capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor according to the value of the generated voltage Δ V cal and the quantity of charge Δ Q cal; wherein the analyser unit is arranged to determine a rate of change of voltage ΔV Δt across the feed-back capacitor resulting from said accumulated charges Δ Q and to generate a value representing a current I received by the voltage amplifier from said input current source based on the rate of change of voltage ΔV Δt across the feed-back capacitor, using the determined capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor, as follows: ΔQ Δt = C ΔV Δt = I. 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; a feed-back capacitor, having capacitance C farads, coupled between the inverting input terminal and the output terminal to accumulate charges Δ Q received from the input current source and to generate a feed-back voltage Δ V accordingly wherein the feed-back capacitor defines a negative feed-back loop of the voltage amplifier such that the output voltage is proportional to said accumulated charges Δ Q; characterised in that the capacitive trans-impedance amplifier comprises: a calibration unit including a calibration capacitor of capacitance C cal 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 Δ Q cal from the calibration capacitor of capacitance C cal to the feed-back capacitor; and an analyser unit arranged to determine a voltage Δ V cal generated across said feed-back capacitor while said calibration switch is in said calibration state and to determine the capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor according to the value of the generated voltage Δ V cal and the quantity of charge Δ Q cal; wherein the analyser unit is arranged to determine a rate of change of voltage ΔV Δt across the feed-back capacitor resulting from said accumulated charges Δ Q and to generate a value representing a current I received by the voltage amplifier from said input current source based on the rate of change of voltage ΔV Δt across the feed-back capacitor, using the determined capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor, as follows: ΔQ Δt = C ΔV Δt = I. a calibration unit including a calibration capacitor of capacitance C cal 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 Δ Q cal from the calibration capacitor of capacitance C cal to the feed-back capacitor; and an analyser unit arranged to determine a voltage Δ V cal generated across said feed-back capacitor while said calibration switch is in said calibration state and to determine the capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor according to the value of the generated voltage Δ V cal and the quantity of charge Δ Q cal; wherein the analyser unit is arranged to determine a rate of change of voltage ΔV Δt across the feed-back capacitor resulting from said accumulated charges Δ Q and to generate a value representing a current I received by the voltage amplifier from said input current source based on the rate of change of voltage ΔV Δt across the feed-back capacitor, using the determined capacitance C = Δ Q cal /Δ V cal of the feed-back capacitor, as follows: ΔQ Δt = C ΔV Δt = I.